PhD Ana Paula Furlan

Ph.D Andry Rico Razakamanantsoa

PhD Harifidy Ranaivomanana

PhD Ouali Amiri

PhD Daniel Levacher

PhD Dimitri Deneele

Rico Andry

Ph.D Razakamanantsoa

Effect of Fly Ash on microstructural and resistance characteristics of dredged sediment stabilized with lime and cement

Keywords: dredged sediment, Fly ash, Soil stabilization, mechanical resistance, microstructure dredged sediment, fly ash, soil stabilization, mechanical resistance, microstructure 29 30

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Background 31

Selection of pavement materials is based on mechanical properties such as strength and stiffness. This 32 procedure is adequate in case of raw and inert materials are used. In stabilized materials, physical 33 changes and chemical reactions take place [START_REF] Rajasekaran | Sulphate attack and ettringite formation in the lime and cement stabilized marine 541 clays[END_REF] [2]. Therefore, it is important to identify modifications 34 which occur at different levels, in order to determine how the action mechanisms of different additives 35 affect soil resistance. 36

Multiscale analysis leads to a comprehensive knowledge of improvement of stabilized soil using 37 hydraulic binders. In such a way that microscale analysis provides evidences of pozzolanic reactions, 38 cementing material formation and evolution of crystalline phases, and macroscale analysis explains 39 enhances in mechanical properties resulted from previous interaction highlighted at microscale level 40 [START_REF] Solanki | Microstructural and mineralogical characterization of clay stabilized using 545 calcium-based stabilizers[END_REF] [START_REF] Horpibulsuk | Strength and microstructure of cement stabilized clay[END_REF]. 41

At present time, Portland cement and lime are considered as the most convenient stabilizers for soil 42 [2]. In soils stabilized using Portland cement, soil gradation changes since cement grains may fill a little 43 portion of the soil voids [START_REF] Simoni | Contribution regarding the design and the mechanical behavior of soil-aggregate-550 cement mixture for pavement course[END_REF] [START_REF] Furlan | Shear strength 553 performance of marine sediments stabilized using cement[END_REF]. Hence, cement hydration is responsible for the significant strength gain 44 over time. The mechanism of interaction between the different components can be explained as 45 follows: Portland cement hydrates using available water in the soil and form a stone-like material 46 [START_REF] Solanki | Microstructural and mineralogical characterization of clay stabilized using 545 calcium-based stabilizers[END_REF] [START_REF] Chew | Physicochemical and engineering behavior of cement 556 treated clays[END_REF]. 47 Typical mineral components of Portland cement are calcium and silicon which are often in the form of 48 oxides, such as CaO, SiO2, Al2O3 and FeO2. The main clinker constituents are tricalcium silicate (C3S), 49 dicalcium silicate (C2S), tricalcium aluminate (C3A), and calcium ferroaluminate (C4AF). To control 50 reaction rate, gypsum (CSH2) might be added to cement. 51 It is known that the hydration of clinker constituents produces cementitious compounds such as 52 calcium silicate hydrates (C-S-H), calcium hydroxide (CH). In pre induction of hydration, cement reacts 53 with the available water and the most active phase C3S produces calcium ions and OH -, SO 2-4, K + , Na + . 54

In dormant stage, C3S hydration continues and C2S begins to be hydrated. CH separates from hydrolysis 55 of C3S and C2S and may precipitate into empty voids. Ettringite (Ett) also forms due to the reaction of 56 gypsum with C3S and C4AF [START_REF] Ye | Experimental study and numerical simulation of the development of the microstructure and 558 permeability if cementitious materials[END_REF]. 57

Over time (at early stage), C-S-H crystalline phases form an acicular morphology which branch, forming 58 a honeycomb-shape structure. CH crystallizes in large crystals (~40μm), presenting hexagonal plate-shape, depending on the produced lime amount in early stages and the available free space. Ettringite 60 crystalizes in a needle-shape, with length up to 10μm and diameter of 0.25 μm, which does not branch. 61 All aforementioned phases and processes have been observed in cement-based materials, included in 62 soil-cement mixtures [START_REF] Solanki | Microstructural and mineralogical characterization of clay stabilized using 545 calcium-based stabilizers[END_REF][7] [START_REF] Franus | SEM Investigation of Microstructures in Hydration Products of 560 Portland Cement[END_REF]. 63 At microscale level, when cement is mixed with soil, the soil-cement mixture presents a significant gain 64 of mechanical strength. Soil-cement strength results from cementation bonds and pore space 65 reduction [START_REF] Lemaire | Effects of lime and cement treatment on the 563 physicochemical, microstructural and mechanical characteristics of a plastic silt[END_REF] [START_REF] Ranaivomanana | Effects of Cement Treatment on Microstructural, 566 Hydraulic, and Mechanical Properties of Compacted Soils: Characterization and Modeling[END_REF]. Accordingly, cementitious compounds fill up the pores and connect cement grains 66 resulting in the increase in intra-aggregate pore volume [START_REF] Horpibulsuk | Role of fly ash on strength and microstructure development 569 in blended cement stabilized silty clay[END_REF]. 67

When lime is used in soil stabilization, soil fabric changes since lime addition induces cation exchanges 68 and generates flocculation/agglomeration mechanism, in short term. In other words, water dissolves 69 some constituents of lime (CaO, CaSO4, MgO and quartz) that react with soil and reduce double diffuse 70 layer (DDL), resulting in flocculation (agglomeration). This process reduces soil plasticity and improves 71 workability [START_REF] Consoli | Key parameters for strength control of lime stabilized soils[END_REF]. Over time, pozzolanic reactions take place at alkaline environment (pH=12), forming 72 cementitious compounds i.e. C-S-H and C-A-S-H, that are responsible for increasing of long-term 73 strength [3][10]. 74 At microscale level, soil-lime strength gain is also explained based on the formation of cementitious 75 compounds from pozzolanic reactions. However, the increase of soil resistance generated by lime 76 addition is less significant than that observed in mixtures using cement; and the use of lime is 77 preferably recommended for clayey soils, in order to improve soil fabric, plasticity and workability [START_REF] Rajasekaran | Sulphate attack and ettringite formation in the lime and cement stabilized marine 541 clays[END_REF] 78 [2] [START_REF] Consoli | Key parameters for strength control of lime stabilized soils[END_REF]. It is also worth noting that the mechanical properties of lime-treated soils are affected by the 79 curing temperature. By studying the stiffness evolution of a silt soil stabilized with quicklime cured at 80 30 °C, Silva et al. [START_REF] Silva | Two staged kinetics of moduli evolution with 574 time of a lime treated soil under different curing temperatures[END_REF] observed two different stages on the stiffness evolution suggesting the existence 81 of two different chemical phenomena involved. Evolution in the first stage seems to be mostly related 82 to the formation of calcium aluminate hydrates (CAH). However, the evolution in the second stage can 83 be more related to a structural rearrangement of CAH and the formation of calcium silicate hydrates existence of two apparent activation energies (one for each process). 86

To maximize the benefits of soil stabilization, a binary or ternary combination of hydraulic binder is 87 proposed. Most of the time, lime and cement combination is recommended. Lime-cement stabilization 88 combines workability enhancement from lime addition and resistance gain from cement addition. Both 89 additives may be mixed without disturbing their own action mechanism. However, multicomponent 90 mixed materials show differences in physical and chemical properties due to coexistence of cement 91 hydration and mineral admixtures, changing hydration kinetics process and microstructure formation 92 mechanisms [START_REF] Yin | Investigation of the hydration kinetics and 577 microstructure formation mechanism of fresh fly ash cemented filling materials based on hydration heat and 578 volume resistivity characteristics[END_REF]. According to [START_REF] Consoli | Water content, porosity and cement content as 580 parameters controlling strength of artificially cemented silty soil[END_REF] [START_REF] Ribeiro | Influence of water content in the UCS of soil-cement mixtures for 583 different cement dosages[END_REF], the dosage of cement and the water content have a 93 significant impact on unconfined compression strength (UCS) values. By testing soil-cement mixtures 94 with dosage of cement between 10 and 13%, Ribeiro et al. [START_REF] Ribeiro | Influence of water content in the UCS of soil-cement mixtures for 583 different cement dosages[END_REF] observed that UCS is always larger for 95 highest dosage of cement independently from the water to cement ratio, whereas independently from 96 the dosage of cement, there is a clear optimum water content providing the maximum UCS value. 97 Finally, compressive strength develops faster with time when larger water and cement ratios are 98 adopted. For treatment using lime, the effects of molding water content on UCS values seems to be 99 insignificant independently from the dosage of lime [START_REF] Consoli | Control factors for the long term compressive 586 strength of lime treated sandy clay soil[END_REF]. 100

For sustainable development purpose, the use of local soil, waste and industrial byproduct are 101 encouraged to supply earthwork and earthen structure [START_REF] Furlan | Shear strength 553 performance of marine sediments stabilized using cement[END_REF][19] [START_REF] Phetchuay | Strength 591 development in soft marine clay stabilized by fly ash and calcium carbide residue based geopolymer[END_REF]. Two well-known industrial 102 byproducts are often cited: bottom ash and fly ash. The benefits of use of bottom ash for road 103 construction was been recently studied by different authors [START_REF] Le | Evaluation of 594 physicochemical and hydromechanical properties of MSWI bottom ash for road construction[END_REF], [START_REF] Minh Nguyen | [END_REF], [START_REF] Yoobanpot | Multiscale laboratory 600 investigation of the mechanical and microstructural properties of dredged sediments stabilized with cement and 601 fly ash[END_REF]. On the other hand, the 104 use of fly ash still requires further investigation because different mechanisms of interaction need to 105 be clarified. 106

Researches in concrete technology has demonstrated that fly ash (FA) improves mechanical properties 107 of concrete, reduces the costs of production and is ecologically beneficial. FA addition may enhance 108 durability of the matrix and reduces the loss of heat energy during cement hydration [START_REF] Yin | Investigation of the hydration kinetics and 577 microstructure formation mechanism of fresh fly ash cemented filling materials based on hydration heat and 578 volume resistivity characteristics[END_REF] [START_REF] Lee | Strength and microstructural characteristics of chemically activated fly 603 ash-cement systems[END_REF]. stabilization [START_REF] Furlan | Shear strength 553 performance of marine sediments stabilized using cement[END_REF][12] [START_REF] Phetchuay | Strength 591 development in soft marine clay stabilized by fly ash and calcium carbide residue based geopolymer[END_REF]. Commonly, FA is composed by calcium, silicon and aluminium. The main 111 oxides are CaO, SiO2 and Al2O3. Class C Fly ash contains about 20% of CaO which might induce cation 112 exchanges, flocculation and pozzolanic reactions, resulting in strength gain. Aluminum rich 113 composition of FA might promote a specific cementitious compounds formation, such as CAH and 114 CASH [START_REF] Solanki | Microstructural and mineralogical characterization of clay stabilized using 545 calcium-based stabilizers[END_REF][7] [START_REF] Fernandez | Formation of C-A-S-H phases from the interaction between 605 concrete or cement and bentonite[END_REF]. 115

In concrete, FA addition stimulates reaction rate of cement hydration, promotes nucleation and 116 growth of cementitious compounds. However, FA may also retard the onset of acceleration (Stage III 117 of hydration) because of (i) inhibition of CH precipitation due to the formation of water containing 118 organic species and (b) slows formation of Ca rich surface layers on clinker phases, in the case of 119 aluminum rich FA [START_REF] Biernacki | Kinetics of reaction of calcium hydroxide and fly ash[END_REF]. 120

The effects of FA action on soils are then physical and chemical. At physical point of view, finer particles 121 of FA would fill voids of soil particles. Chemically, pozzolanic products induced by FA presence would 122 fill pores. Both effects reduce porosity so that microstructure becomes denser. As results, strength and 123 stiffness are increased and compressibility is reduced [START_REF] Kang | Chemically Stabilized Soft Clays for Road-Base Construction[END_REF]. 124

Research findings suggest that FA disperses clusters of soil and strength development is controlled by 125 FA hydration [START_REF] Phetchuay | Strength 591 development in soft marine clay stabilized by fly ash and calcium carbide residue based geopolymer[END_REF][28] [START_REF] Sivapullaiah | Gypsum induced strength behavour of fly ash-lime stabilized expansive 614 soil[END_REF], leading to an analogy with the aforementioned processes in cement pastes. 126

It is worth noting that water content of the fly ash stabilized soil mixture affects the strength [START_REF]Fly ash facts for highway engineers[END_REF]. The 127 maximum strength reached in soil-fly ash mixtures generally occurs at moisture contents below 128 optimum moisture content for density. For silt and clay soils the optimum moisture content for 129 strength is generally four to eight percent below optimum for maximum density while for granular 130 soils the optimum moisture content for maximum strength is generally one to three percent below 131 optimum moisture for density. Therefore, it is crucial that moisture content be controlled during 132 construction. Initial water content significantly affects the efficiency of soil stabilization. allowable levels, then a combination with other additives is required [START_REF] Sivapullaiah | Gypsum induced strength behavour of fly ash-lime stabilized expansive 614 soil[END_REF]. Particular attention must be 135 paid when FA is added to soil-cement mixture. FA may compete with cement for the water available 136 in the soil. This competition may be detrimental to cement hydration and it is more problematic in soil-137 cement because of the typical low water and cement ratio (w/c). 138 Interestingly, in binary combination involving lime and FA, stabilized soils present some benefits 139 resulting in strength gain. Notwithstanding, Sivapullaiah and Jha [START_REF] Sivapullaiah | Gypsum induced strength behavour of fly ash-lime stabilized expansive 614 soil[END_REF] indicate that there may be 140 particular FA contents to find greater resistance in the short and long term, respectively. Low contents 141 of FA increase strength in 28 days, whereas high contents of FA increase strength in 4-7 days. Generally, 142 FA in soil-lime changes the rate of strength gain and not the strength itself. Therefore, FA content up 143 to 15% is recommended. The effects of initial water content on the compressibility, strength, 144 microstructure, and composition of a lean clay soil stabilized by compound calcium-based stabilizer 145 composed of cement, lime and fly ash was investigated by Yin et al. [START_REF] Yin | Effects of Initial Water Content on Microstructure and 618 Mechanical Properties of Lean Clay Soil Stabilized by Compound Calcium-Based Stabilizer[END_REF]. It was observed that as the 146 initial water content increases in the range studied (from 11 to 19%), both the compaction energy and 147 the maximum compaction force decrease linearly and there are less soil aggregates or agglomerations, 148 and a smaller proportion of large pores in the compacted mixture structure. In addition, for specimens 149 cured with or without external water supply and under different compaction degrees, the variation 150 law of the unconfined compressive strength with initial water content is different and the highest 151 strength value is obtained at various initial water contents. Finally, with the increase of initial water 152 content, the percentage of the oxygen element tends to increase in the reaction products of the 153 calcium-based stabilizer, whereas the primary mineral composition of the soil-stabilizer mixture did 154 not change notably. 155

Regarding the microstructure organization, Furlan et al. [START_REF] Furlan | Shear strength 553 performance of marine sediments stabilized using cement[END_REF] demonstrate that FA addition refines the 156 pore structure prior the development of pozzolanic reaction. This previous study demonstrates that 157 FA can be used as soil stabilization additive. However, some question related to the reactivity of FA 158 combined with other chemical additive requires further investigation. This paper aims to investigate the contribution of FA in soil stabilization using hydraulic binders, with the purpose of finding: (i) 160 technically compatible material options, (ii) rational use of waste and byproducts and (iii) optimized 161 mix dosage. 162

Experimental program 163

Chemical stabilization of soils induces physicochemical changes of soils characteristics due to chemical 164 interactions between additives and soil. These changes imply improvements in the mechanical 165 behavior of the soil. Therefore, a multiscale analysis was selected in order to provide a holistic 166 knowledge of the stabilization mechanisms. 167

If, on the one hand, microscopic analysis can show the new cementing products resulting from 168 chemical reactions, on the other hand, it is worth quantifying their impacts on mechanical 169 performance improvement of stabilized soils. Then, measurements of design and mechanical 170 properties are necessary. Moreover, the combined use of these techniques allows to relate the 171 changes in different scales, in order to highlight advantages and disadvantages of chemical 172 stabilization. 173 Thus, the present experimental program aims to respond these questions, demonstrating the 174 beneficial of using fly ash as a complementary additive in soil stabilization, indeed, when combined 175 with conventional hydraulic binders. 176

In this study, dredged sediment from La Baule Le Pouliguen Harbor, France, and two cementitious 177 additives were used. Fly ash was added to mixtures containing lime, cement and lime and cement. 178

Only chemical stabilization was made (so without gradation correction) in order to find the best 179 combination of additives to improve the characteristics of this dredged soil considered as waste, 180 regarding economic and environmental aspects. 181

Materials 182

The geotechnical properties of the sediment were carried out in accordance with GTR guide [START_REF]Réalisation des remblais et des couches de forme[END_REF] 183 dedicated for embankment and pavement applications. Proviacial® ST quicklime was added to the sediment soil. The quicklime was provided by LHOIST from 188 Dugny-sur-Meuse, in Lorraine, France. This lime contains at least 90% of calcium oxide and at 189 maximum 2% of magnesium oxide. The lime content added to soil was 2%. 190 Portland cement was a CEM II/B-LL 32,5R CE CP2 (French Standard) whose short-term resistance is 191 32.5 MPa. Clinker content is between 65 and 79%, being its chemical constituents: tricalcium silicate 192 (66%), dicalcium silicate (10%) and tricalcium aluminate (7%). Limestone is the main natural 193 component of this cement, presenting a total organic material less than 0.20% in mass. The cement 194 content added to soil was 7% of the dry mass of the sediment. 195 Sodeline ® Fly ash is manufactured and was provided by the central Emile Huchet in Saint Avold, France. 196 The main constituents of this fly ash are silicon dioxide (47.36%), aluminum oxide (21.63%) and calcium 197 oxide (8.52%). It is important to point out that sulfur content (4.02%) is higher than the conventional 198 ones. The fly ash content added to soil was 9%. 199

The particle size distribution of sediment, hydraulic binders and fly ash are summarized in Figure 1. 200

The particle size distributions help to understand the contribution of binders on the sediment particle 201 arrangement. The particle size distributions were carried out with Malvern mastersizer. 202 Comparing the curves from Figure 1, it can be observed that the sediment is composed with coarsest 205 material (D90 =150 μm), with particle diameters between 500 and 1 μm; and cement is the finest binder 206 compared to lime and FA (D90 =15 μm), with particle diameters between 10 and 2 μm, that is also the 207 most uniform distribution. Particle-size distributions of lime and FA are quite similar (D90 =90 μm), with 208 particle diameters between 200 and 0.8 μm. 209

Procedures of specimen preparation 210

Raw sediments have been oven-dried at 50°C for 48h prior to the treatment process. The sediment 211 mixtures considered in this study are respectively formulated with 2% lime (S2L), 7% cement (S7C) or 212 2% lime and 7% cement (S2L7C). The percentages considered here correspond to the dry mass of 213 sediment. A binary or ternary binder mixture has also been proposed for the mixture including fly ash 214 (FA). The rate of fly ash addition, which is equal to 9%, represents the sum of the standard binder rates 215 used, i.e. 2% lime and 7% cement: S2L9FA, S7C9FA, S2L7C9FA. 216

Proctor tests were performed according to French standard (NF P94-093) to get the optimum 217 parameters of the mixtures, that is optimum water content and maximum dry density. All mixtures 218 were prepared based on several precedent dosages [START_REF] Liang | Co-valorisation de sédiments et de sols fins par apport de liants et de fibres[END_REF]. In addition to mixture design, shrinkage tests of cement and pozzolanic reactions but also to characterize sediment matrix. The XRD analyses were 245 performed on powder using a Brucker diffractometer with a detector over the range 5°80°2θ using Cu-246 Kα1 radiation (1.55Å). Diffractometer operated with input voltage of 30 kV and current of 10mA. 247 Crystallography Open Database (COD) was used to identify constituents and new crystalline phases in 248 the patterns. 249 SEM images were used to investigate the mineral phases resulting from the interaction between the 250 sediment and the binder components. SEM observations were carried out using a FEI Inspect F-50 SEM 251 instrument coupled to an energy dispersive X-ray analyzer (EDX). To improve the image quality, 252 samples are coated with gold. 253

The impact of the mineral compound development on the sediment structure resulting from hydration 254 mechanism of the binder mixture was followed by the pore distribution. The pore size distributions 255 were plotted from MIP data obtained with Micromeritics Autopore IV. 256

Results 257

Microstructure investigation 258

Microstructure investigation by XRD 259

The main strength gain in chemical stabilized soil is mostly attributed to cementitious compounds 260 formed from cement hydration and/or pozzolanic reactions. Figure 2 shows XRD patterns of all 261 mixtures at 7 and 28 days. These patterns compare mixtures with original soil, in order to identify new 262 crystalline phases addressed to chemical reactions. Different curing times are considered in attempt 263 to evidence the formation and growth of cementitious compounds. New peaks in patterns confirmed 264 not only the formation of cementitious compounds but also indicated the presence of non-hydrated 265 materials. 266

In Figure 2, it is observed that untreated sediment presents quartz (Q), feldspar (F), calcium carbonate 267 (C), Illite (I) and kaolinite (K) in its composition. Samples at 7 days (Figure 2a) showed the presence of insufficiency. This might be related to (i) lime addition, because the change of texture (agglomeration) 290 would interfere with the amount of available water for hydration, since water may be trapped in the 291 soil cluster; and/or (ii) FA addition, because water demand for FA is generally important due to its 292 appreciable porosity, it might create a competition for available water. Besides, it is important to 293 remember that cementitious compounds in early ages are poorly crystalized and they are not well 294 detected by diffractometer. 295

Regarding cementitious compounds, one may observe that all treated samples had produced them at 296 28 days, unlike what happened at 7 days. Curing time was preponderant to development of 297 cementitious compounds, since time allows binders to hydrate according to their own reaction rates 298 (pozzolanic reactions) and allows also the transport of water necessary to the reactions of anhydrous 299 grains surrounded by hydration products. This implies that it would be recommended at least 28 days 300 to verify strength gains, at macroscale. 301 Hydrated silicates exhibit different morphologies and it might be more problematic for chemically 302 stabilized soil due to the several factors, such as the type of additive, the low additive contents, the 303 low water content, the complexity of hydration mechanisms and cementitious material formation of 304 multicomponent mixed materials [START_REF] Yin | Investigation of the hydration kinetics and 577 microstructure formation mechanism of fresh fly ash cemented filling materials based on hydration heat and 578 volume resistivity characteristics[END_REF] [START_REF] Fernandez | Formation of C-A-S-H phases from the interaction between 605 concrete or cement and bentonite[END_REF]. 305

In other words, since C-S-H are amorphous and/or poorly crystallized, it may be advisable to be careful 306 when reading XRD patterns because of the disturbing factors (producing a heterogeneous sample) and 307 to combine other microstructural data sources to help to support the results analyses. 308

Microstructure observation by Scanning Electron Microscopy (SEM) 309

SEM images are often used to provide evidences of cementitious compounds development and 310 mixture changes. Firstly, Figure 3 attempts to demonstrate visually constituents and compounds, 311 which were previously identified in the mixtures using XRD tests (Figures 2 and3). SEM Images are 312 presented in appropriate magnificence to highlight details of the materials. As it can be seen in figure CaCO3 (found in sediment and in mixtures with cement), hydrated silicates and FA as well. 315 316 The observation of cementation products and the resulting block-like structure lead to infer some 334 increase in strength at macroscale, for instance. Again, that is in accordance with the assumption that 335 cementation enhances the mixture cohesion and controls strength gain of mixtures using cement [START_REF] Horpibulsuk | Role of fly ash on strength and microstructure development 569 in blended cement stabilized silty clay[END_REF]. 336 Figure 4(b) presents images of binary and ternary mixtures using FA, in attempt to observe its effect 337 when combined with conventional binders. Generally, mixtures using FA revealed significant 338 occurrence of pozzolanic and hydration products. These phenomena are more effective surrounding 339 FA spheres. This finding is consistent to nucleation process and reaction stimulation reported in 340 technical literature [START_REF] Solanki | Microstructural and mineralogical characterization of clay stabilized using 545 calcium-based stabilizers[END_REF][12] [START_REF] Fernandez | Formation of C-A-S-H phases from the interaction between 605 concrete or cement and bentonite[END_REF]. 341

Concerning visual aspect, at 7 days, it can be observed that mixture with lime and FA shows a flaked-342 like structure. At 28 days, mixture revealed a blocky structure. The change in structure is remarkable 343 and is attributed to the development of cementation between the soil clusters and also covering FA, 344 as seen in S2L9FA mixture at 28 days. 345

For mixture with cement and FA (S7C9FA), blocky structure is observed at 7 and 28 days. C-S-H gel 346 covers the surface of soil and FA and shrinkage fissures were identified. For the mixture with lime, 347 cement and FA (S2L9FA), once again the flocculated structure was observed (at 7 days) as well as its 348 evolution to a blockier structure (at 28 days). 349 Cementitious compounds and hydration fissures seem to occur more frequently in mixtures using FA 350 for both curing time. The increase of CaO content due to addition of FA as long as the nucleation 351 process would promote formation of cementitious materials [START_REF] Solanki | Microstructural and mineralogical characterization of clay stabilized using 545 calcium-based stabilizers[END_REF][4] [START_REF] Biernacki | Kinetics of reaction of calcium hydroxide and fly ash[END_REF]. 352

MIP results 353

The XRD patterns (Figure 2) showed the occurrence of cementation from new crystalline phases in the 354 mixtures such as hydrated silicates. The SEM images (Figures 4) confirmed the presence of the 355 cementitious compound and qualified the changes of the structure caused by the additives. MIP tests 356 were carried out, in attempt to clarify the mechanism of interaction between FA and conventional 357 binder which govern the alteration of sediment fabric.

Figure 5 presents cumulative intrusion versus pore diameter curves. In general, additives reduced the 359 pore diameter of original soil. In mixtures S2L and S2L9FA, for large pores (from 10 to 0.1 µm), it is 360 observed that there are reductions of diameter of pore. Whereas, for small pores (from 0.1 to 0.001 361 µm) there is an increase of cumulative intrusion, i.e. an increase of percentage of small pores. Curves 362 are approximately parallels, but it can be seen that FA tends to change further large pores. 363 364 365

Figure 5: Pore distributions of all mixtures (7 days) 366

Mixtures S7C and S7C9FA presented significant changes mostly in large pores. This finding suggests 367 that changes in the pore distribution depend on the type and amount of binder added, and particularly 368 the gradation of binders. 369

Pore distributions revealed the soil fabric changes resulting from addition of binders. For instance, at 370 7 days, it is expected that: 371 (i) Lime generates a formation of aggregates/flocculates with finer pore width resulted from the 372 exothermic reaction which take place during lime hydration [START_REF] Lemaire | Effects of lime and cement treatment on the 563 physicochemical, microstructural and mechanical characteristics of a plastic silt[END_REF] [START_REF] Cuisinier | Microstructure and hydraulic conductivity of a 625 compacted lime-treated soil[END_REF]. 373 (ii) Cement hydration produces cementitious compounds that fill large pores and also causes 374 shrinkage reducing original soil pores diameter. No significant evolution of small pores domain is 375 observed for both treatments using lime and cement. This is due to the fact that small pores are 376 mainly associated with intra-aggregate porosity which is less affected by the treatment. 377 (iii) both lime and cement combine these two mentioned effects, and (iv) in addition to the role of filler played by FA, it also promotes nucleation and formation of 379 cementitious compounds around FA, increasing the percentage of small pores of the original soil 380 [START_REF] Horpibulsuk | Role of fly ash on strength and microstructure development 569 in blended cement stabilized silty clay[END_REF]. 381 Since total pore volume (i.e. the highest cumulative intrusion value) is an indicator of material 382 densification, one could assume that, at macroscale, the resistance of mixtures using FA would be 383 higher than that of mixtures without FA. Nevertheless, accordingly [START_REF] Horpibulsuk | Analysis of strength 611 development in cementstabilized silty clay form microstructural considerations[END_REF], cementation controls 384 predominantly the resistance development of soil-cement mixtures. 385

Macroscale analysis 386

Macroscale analysis was done in two steps. The first consisted of immediate property assessment of 387 the mixtures based on Proctor, shrinkage, and UCS test results. The latter was based on UCS results 388 throughout curing time. First and second steps focused on distinguishing the filler effect from curing 389 effect, respectively, in an attempt to remark the most adequate stabilization process, in terms of 390 additive content and strength gain. 391

Design parameters and immediate strength 392

Binder addition and content may change soil gradation. Since additives are finer than the soil (Figure 393 1), they shall fill partially empty voids of soil and increase the initial strength values, due to the filler 394 effect. Table 2 present design parameters and immediate strength. Comparing to untreated sediment, 395 it is observed that binder addition promoted (a) the reduction of maximum dry densities (ρdmax) and 396 the increase of optimum water content (wopt), (b) the reduction of the volumetric variation (dV) and 397 (c) the increase of immediate strength (UCS0d). Nevertheless, the intensity of changes depends on the 398 type and the content of binder. 399 To illustrate filler effect, Figure 6 shows correlations between Proctor and shrinkage parameters, and 402 immediate UCS values in function of additive content. Figure 6(a) shows that the reduction of ρdmax is 403 a result of the increase of binder content, but this behavior depends on the type of binder considered. 404

It is observed that the evolution of the ρdmax follow a linear regression law with high degree of 405 accuracy (R 2 ). R 2 values for mixtures containing lime (L) and cement (C) are 0.81 and 0.88, respectively. 406 FA addition reduced ρdmax in mixtures with cement. In mixtures with lime this behavior was not 407 observed. 408 Linear law demonstrates that cement addition reduces maximum dry density less than lime addition. 409

Both curves converge to the ρdmax value of the SLCFA mixture, which was the minimum one. 410 Volumetric deformation is often consequence of shrinkage which occurs in cemented materials due to 411 the water consumption for clinker component, generating fissures. Shrinkage test reports the variation 412 of volume (dV) that a soil-cement may present and gives an idea about material durability in dosage 413 phase. Figure 6 It would be reasonable to indicate the addition of inactive fillers in order to reduce shrinkage in soil-419 cement type mixtures. Figure 6(b) led to infer that FA plays this role in this phase, because its addition 420 in the mixtures further reduced dV. In mixtures without FA, the reduction of dV was approximately 421 equals to 30% in relation to untreated soil. When FA was added, this reduction was about 40% for SL 422 and SC mixtures, and 55% for SLC mixture. 423

Regarding the filler effect on immediate strength, Figure 6(c) shows a good correlation (R 2 =0.79) 424 between additive content and UCS of the mixtures, where the higher the additive content the greater 425 the UCS0d. It is important to point out that immediate UCS0d of SL mixture was lower than the 426 untreated sediment. This behavior is attributed to flocculation reactions of lime that increase of voids 427 in the mixture, decreasing ρdmax and increasing water content (as seen in Table 2). 428

It is worth emphasizing that the combination of additives changes gradation of untreated sediment. 429

As additives are finer than the sediment soil, they may fill empty voids, increasing the sediment 430 strength. As a rule, the initial strength values gradually improve as binder percentages increase. 431

In spite of the good correlation between immediate UCS and additive content, the strength gain may 432 be also related to the type of additive, or combination between them, since the greatest immediate 433 UCS were observed in mixtures SLC, SCFA and SLCFA. The common point of these mixtures is the 434 presence of cement associated to other(s) additive(s). It seems to be a contribution of additive 435 gradations to the particle size arrangement of mixtures, resulting in the increase of their immediate 436 UCS. Immediate UCS is a reference point that might lead to dosage optimization attempts. Accordingly, 437 from this point on, it shall be observed the strength gain over time due to chemical reactions. 438

UCS and water content results over time 439

In order to highlight the effects of additives on the mechanical resistance of mixtures, Figure 7 presents 440 the kinetics of the UCS over time. Regarding compressive strength values (UCS), one may observe that required more time to present significant strength gains, such as mixture S2L that presented reduction 443 of UCS value at 7 days. All other additives combination increased UCS values and these increases are 444 more expressive when FA is added. 445 From 0 to 7 days, the strength gain of S2L and S7C was about 0.05 MPa that represents respectively 446 20% and 16% of their initial UCS values (0.21 and 0.31 MPa). For conventional binders, the mixture 447 with lime and cement (S2L7C) had the higher strength gain (~0.15 MPa), that is 40% of its initial 448 strength (0.37 MPa). 449

For mixtures containing FA, strength gains were in average of 0.15 MPa. It is worth emphasizing the 450 strength gain of the mixture S2L9FA that was about 70%. In contrast, the strength gain at 7 days of the 451 mixture S2L7C9FA was the lowest one (0.10MPa) that represents about 20% of initial UCS value. 452 At 28 days, the UCS value of S2L was 0.31 MPa, S7C was 0.40 MPa, and S2L7C was 0.87 MPa. From 7 453 to 28 days, the gain of UCS remains about 0.05 MPa for S2L and S7C. Interestingly, FA addiction 454 doubled the strength gains in these mixtures. The faster strength gain was also observed by Kang et al. 455 [START_REF] Kang | Chemically Stabilized Soft Clays for Road-Base Construction[END_REF]. 456 457 Figure 7: UCS over time 458

For the mixture S2L7C9, the strength gain was 0.35 MPa. In this case, FA addition did significantly not 459 increase strength gain of SLC mixture. In general, mixtures with FA presented the higher UCS values. 4). However, for intermediary age (28 days), these changes evidence the reduction 487 of water content, the average reduction was 5%. For longer period (90 days), the average reduction of 488 water content is 12%, except for SLC, which was 31%. 489 FA addition also alters hydric conditions of stabilized mixtures over time but at lower intensity. As a 490 rule, one may suppose that the greater the decrease in moisture content the greater the gain in 491 resistance. This hypothesis is true since water is consumed in formation and/or crystallization of 492 cementing products, becoming the structure denser and more resistant than the original soil. 493

Discussion 494

The decision for a given product takes into account technical, economic and environmental 495 parameters. This study has shown that combining FA with stabilized soil with lime and cement (or both) 496 is promising because of the FA action mechanisms promoted an acceleration of strength gain 497 regardless of the curing time. Besides, mixtures using FA have always shown higher resistance, either 498 through physical interactions or chemical reactions. These findings support technically the choice for 499 combining FA with chemical soil stabilization using conventional binders. 500 For instance, UCS value at 90 days of mixture using cement (S7C) is compatible with that of the mixture 501 using cement and FA (S7C9FA) at 28 days, or with mixture using lime and FA (S2L9FA) at 90 days. These 502 mechanical compatibilities indicate alternative material options that might save time and economy, since it is possible to select mixtures that present greater resistance ahead of time, use cheaper 504 additives or smaller quantities of conventional binders. 505 These benefits have a positive impact on environmental indicators because they promote the rational 506 use of waste and byproducts (marine sediment and fly ash). In this sense, it is worth mentioning that 507 the mixture with lime, cement and FA (S2L7C9FA) presented resistance compatible with the mixture 508 without FA (SLC), leading to question the advantages of this mixture (S2L7C9FA) as well as to think 509 alternatively about the possibility of reducing the cement content in order to optimize the mixture 510 design in terms of economic and environmental. 511

Conclusions 512

The effects of FA addition on microstructural and resistance characteristics of mixtures stabilized with 513 conventional binders were investigated. From this study the conclusions are made focusing the 514 interrelations between microstructural and macrostructural analyses. 515

Regarding microstructural analyses, it was identified new crystalline phases in DRX patterns. 516

Cementitious materials, such as ettringite, portlandite and hydrated silicates, are produced over time 517 by cement hydration and pozzolanic reactions. The occurrence of these products depends on the 518 curing time due to particular binder reaction rate. These findings are supported by SEM images, that 519 confirmed the presence of hydrated silicates in different morphologies (gel, acicular C-S-H for 520 instance), mainly in mixtures using FA. The SEM images also show physical modification of the soil 521 fabric over time, which was also confirmed by analysis of pore distributions. The results of the pore 522 distribution measurements show a structural densification of the sediment matrix at early age, 523 demonstrating the contribution of finer gradations of the additives on the sediment pore 524 rearrangement, i.e. filler effect. 525 Macroscale analyses led also to observe a prevalent filler effect on gain of immediate resistance and 526 reduction of volumetric variation by shrinkage (dV). These physical responses are especially 527 remarkable in mixtures using FA. These findings led to recommend more studies in dosage parameters

Figure 1 :

 1 Figure 1: Grain size distribution of materials 204

Figure 2 :

 2 Figure 2: XRD results of mixtures at 7 days (a) and 28 days (b) 286

Figure 3 :Figure 4 :

 34 Figure 3: Identification of constituents and cementitious compounds of mixtures from (a)S2L9FA-7d, 317 (b) S7C, (c)S2L7C9FA -7d, (d) S7L9FA -7d, (e)S2L7C9FA-7d,(f) S2L9FA-28d 318 Secondly, Figures 4 shows SEM images obtained from sediment mixtures without and with FA (4a and 319

  (b) shows a strong correlation between dV and additive content (R 2 =0.93), indicating 414 that dV decreases with increasing additive content. In other words, partial filling of the voids would 415 inhibit shrinkage. 416

Figure 6 :

 6 Figure 6: Immediate Properties and characteristics of mixtures 418

Figure 8 :

 8 Figure 8: Variation of water content over time 485

Figure 1 Figure 2

 12 Figure 1

Figure 3

 3 Figure 3

Figure 4

 4 Figure 4

Figure 5

 5 Figure 5

Figure 6

 6 Figure 6

Figure 7

 7 Figure 7

Figure 8

 8 Figure 8

Table 1 .

 1 Table 1 shows results of soil characterization 184 based on results of maximum specific gravity, and organic material, pH and carbonates contents. 185 Geotechnical properties of sediment 186

	Parameters	Value
	Sand fraction > 63µm (%)	23
	Silt fraction -2 to 63µm (%)	41
	Clay fraction < 2µm (%)	36
	Specific gravity (g/cm 3 )	2,66
	Plasticity limit (%)	36.06
	Liquidity Limit (%)	54.54
	Plasticity Index	18.48
	Organic matter content (%)	10.97
	Carbonates content (%)	22.21
	pH	8.5
	187

Table 2 .

 2 Design parameters and immediate strength 400

	Proctor
	Parameters	Shrinkage Parameters

Table 1 :

 1 Geotechnical properties of sediment

	Parameters			Value
	Sand fraction > 63µm (%)		23
	Silt fraction -2 to 63µm(%)	41
	Clay fraction < 2µm (%)		36
	Specific gravity (g/cm 3 )		2.66
	Plasticity limit (%)			36.06
	Liquidity Limit (%)			54.54
	Plasticity Index			18.48
	Organic matter content (%)	10.97
	Carbonates content (%)		22.21
	pH				8.5
				Proctor
				Parameters	Shrinkage Parameters
	Type of mixture	Legend	% filler	wopt (%)	dmax (g/cm 3 ) (days) Time	dv (%)	dv/T (%/hours) (MPa) UCS0d
	Soil	S	0,0	27.8	1.48	3,0	27.0	0.44	0.28
	Soil+ 2% lime	S2L	2.0	32.2	1.38	2.6	20.5	0.50	0.21
	Soil+ 2% lime + 9% fly ash	S2L9FA	11.0 32.0	1.37	2.8	17.0	0.39	0.30
	Soil+ 7% cement	S7C	7.0	28.1	1.46	2.4	19.0	0.32	0.31
	Soil+ 7% cement + 9% FA	S7C9FA	16.0 30.3	1.41	3.0	15.5	0.33	0.37
	Soil+ 2% lime+ 7% cement	S2L7C	9.0	30.7	1.43	2.2	18.0	0.25	0.37
	Soil+ 2% lime+ 7% cement+9%
	FA	S2L7C9FA 18.0 31.6	1.37	2.0	12.5	0.17	0.49

Table 2 :

 2 Design parameters and immediate strength

Table

  Click here to access/download;Table;tables on table 1 and 2.docx

additives increased initial property of untreated sediment (0,27 MPa), even though some mixtures

Microanalysis Congress. Springer Proceedings in Physics, vol 164, Springer, Cham.


Cindy Cordelle

Dominique Agustin
email: dominique.agustin@iut-tlse3.fr

Jean-Claude Daran

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr

Oxo-bridged bis oxo-vanadium(V) complexes with tridentate Schiff base ligands (VOL)2O (L= SAE, SAMP, SAP): synthesis, structure and epoxidation catalysis under solvent-free conditions

Keywords: Vanadium, vanadyl complexes, Schiff base ligands, epoxidation, homogeneous catalysis

    Oxo-bridged bis oxo-vanadium(V) complexes with tridentate Schiff base ligands (VOL)2O (L=SAE, SAMP, SAP): Synthesis, structure and epoxidation catalysis under solvent-free conditions

Introduction

High-oxidation state metal oxido complexes are among the most interesting oxidation catalysts [1,2]. Among them, vanadium complexes are used in processes of industrial interest [START_REF] Celaya Sanfiz | Recent patents concerning vanadium oxides containing processes (a)[END_REF][START_REF] Ivars | Recent articles concerning vanadium oxides containing processes[END_REF] and are present in several biological systems [5], for instance in bromoperoxidase [6,7].

The advantage of vanadium species is their ability to activate smooth oxidants. Indeed, a variety of (ep)oxidation reactions with smooth oxidants, including dioxygen, have been reported with use of "V(O)L(OR)" complexes [8]. We have recently become interested in oxidative processes catalyzed by oxomolybdenum complexes [9,10,11,12]. In particular, we have reported that the dioxomolybdic complexes [MoO2L]2 and MoO2L(MeOH) (L= tridentate Schiff base ligand) catalyze the epoxidation of cyclooctene by aqueous TBHP with better selectivities than the reference compound MoO2(acac)2 under the same experimental conditions [12]. However, we found that some of these systems are limited by hydrolytic stability. We have therefore set out to examine the behaviour of the related oxovanadium(V) systems.

Mimoun has has already reported on the excellent catalytic activity of mononuclear oxovanadium(V) complexes containing tridentate ligands, outlining the role played by peroxo-substituted intermediates [13]. Dinuclear complexes with a [V2O3] 2+ core and tridentate ONO ligands have apparently been scarcely explored in catalysis. Some articles relate highly enantioselective oxidative couplings of 2-naphthols [14,15,16,17], others epoxidation or sulfoxidation catalysis [18,19], but the majority of experiments have been performed using organic chlorinated solvents. We have then decided to use these species under solvent-free conditions.

Results and discussion

Synthesis and characterization of (VOL)2O complexes

The LH 2 ligands salicylideneaminoethanol (SAEH 2 ), salicylideneaminomethylpropanol (SAMPH2) and salicylideneaminophenol , SAPH2) have been obtained as described recently using salicylaldehyde and the corresponding aminoalcohols in water [10]. The syntheses of the dinuclear vanadium complexes occur in a "one pot" procedure, through two consecutive reactions steps (see Scheme 1).

O H N V O O O H N V O O O (VOL) 2 O OH H N OH + O=V(acac) 2 -2 acacH O H N V O O O H N V O O air solvent (VOL) 2 L = SAE L = SAP N O N O N O = L = SAMP N O LH 2
Scheme 1 -Synthetic procedure for the synthesis of (VOL)2O complexes.

In the first reaction step, V(O)(acac)2 and one equivalent of the tridentate LH2 ligand lead to a [V IV (O)L]2 intermediate complex as described elsewhere for a procedure carried out under anaerobic conditions [20,21]. Transformation into the corresponding dinuclear [LV(O)]2O complex occurred by simple air oxidation. The compounds gave dark solids, in sufficiently good crystalline state for 1 and 2 to perform an X-ray structural determination (see below).

The IR spectra of these species show absorption bands around 980 cm -1 , attributable to V=O vibrations, one broad band around 760 cm -1 for the V-O-V moiety, as well as the imino vibration around 1630 cm -1 (see Table 1) [22,23,24 25,26].

X-ray characterization of (VOL)2O complexes

Single crystals suitable for X-ray analysis could be obtained for compounds 1 and 2. The structure of compound 2 (and also that of 3) had already been reported in the literature [20],

but was determined at a higher level of precision in our case, certainly due to the lowtemperature measurement. A view of the molecular geometry is shown in Figure 1, with selected distances and angles listed in Table 3. The molecular unit is composed of two 107.62( 7)

N(11)-V(1)-V(2) 120.99(7) O(3)-V(1)-V(2)
34.90( 6) O( 23)-V(1)-V(2) 39.90(5)

DFT calculations for complex 1

The structure of 1 was optimized by DFT using the experimentally determined structure as input and included the calculation of the normal mode frequencies on the geometry minimum. The agreement between experimental and calculated structure is excellent (see Table 4). Note that the computed V=O bonds are shorter relative to the experiment, whereas the V-N and the loose V•••O bonds are longer. The use of a better basis set for the V atom yields a slightly better agreement fore the covalent V-O bonds but a greeter disagreement for the loose V•••O bond. A comparison between computed and observed IR absorptions is shown in Figure 2. The three most relevant vibrations are those associated with the C=N, V=O and asymmetric V-O-V stretching motions. As expected, all computed frequencies are slightly shifted to higher frequencies relative to those observed experimentally (see Figure 2).

The use of the better basis set for the V atom causes little changes to the calculated spectrum.

Greater shifts are noted for the vibrations of doubly bonded moieties, whereas a closer agreement is found for the V-O-V vibration. The same trend has been observed for higher nuclearity tungsten and molybdenum polyoxometalates [30]. This phenomenon is probably related to the greater discrepancy between calculated and observed bond distances for the bonds of higher multiplicity.

Epoxidation of cyclooctene with TBHP in water using [VO(SAP)]2O complex.

Complex 3 has been tested as cyclooctene epoxidation pre-catalyst using aqueous TBHP or aqueous H2O2 (30% wt) as oxidant, without organic solvents, at 80°C (3/oxidant/cyclooctene = 1/200/100). Under these conditions, 94% of the cyclooctene was converted in 5.5 h with an epoxide selectivity of 83% when using TBHP as oxidant, whereas no reaction occurred with H2O2. This may be related to the inability of the H2O2 reagent to mix with the organic substrate phase. A control experiment carried out under the same condition with the equivalent amount of H2SAP ligand instead of complex 3 gave no conversion, confirming the catalytic role of the vanadium atom. In another control reaction, the oxidation was carried out under the same conditions with the precursor complex VO(acac)2, without the SAP ligand. The conversion was still quite high (74% after 5.5 h), but the selectivity was completely different (only 13% of epoxide). This result confirmed the important role of the ligand SAP associated to vanadium. Finally, the same experiment was carried out in the presence of TEMPO in order to test whether radicals may be involved, yielding identical results. Compared to previous studies, carried out in solution of organic solvents ranging from aromatic to chlorinated ones, the activity is very good as well as the selectivity. Bellemin et al. indicated a conversion of just 50% of 1-hexene to the corresponding using TBHP and the [(2,6-Bis{(-)-menthyl}pyridine)VO]2O dimeric complex [19] as pre-catalyst, the nature of which is similar to that of compound 3. Other contributions relate the epoxidation of cyclooctene with V(IV) complexes in a N2O2 tetradentate coordination environment. A few of these systems displayed moderate cyclooctene epoxidation activity with TBHP in acetonitrile (88-96% selectivity but only 46-75 % conversion in 6 h and at reflux [31]. The solvent influence (dichloromethane, acetonitrile or chloroform) was investigated, the highest conversion (94%) being obtained with VO{salnptn(3-OMe)2} and VO{hnaphnptn} in chloroform after 6 h [32]. Similar complexes have been used in combination with greener oxidants, i.e. dioxygen, again for cyclooctene epoxidation by THBP, leading to conversion (selectivity) in the range 25-91 % (30-60%) in acetonitrile or in DMF [33,34]. Another [VOL]2 complex containing vanadium(V) atoms and NNO Schiff base ligands has shown good epoxidation activity (94% after 4 h) but in chloroform [35]. Although all these systems are catalytically efficient, organic solvents were needed in the experimental procedures. We have shown here for the first time that a vanadyl complex is also catalytically efficient in the absence of organic solvent and by using an aqueous oxidant.

Conclusion

The synthesis of bis(vanadyl) complexes of general formula [VO(L)]2O with tridentate ligands (L= SAE, SAMP, SAMP) has been revisited here. The synthetic procedure makes use of easily accessible ligands. The crystallographic structure of [VO(SAE)]2O has been determined here for the first time. Compound [VO(SAP)]2O exhibited good catalytic activity in cyclooctene epoxidation using aqueous TBHP as oxidant and no organic solvent. These results are of interest for the development of greener oxidation protocols. Further mechanistic and experimental studies under solvent-free conditions are in progress and will be described in due course.

Experimental section

Compound VO(acac)2 (Aldrich), MeOH and acetonitrile (Aldrich, technical grade) were used as received. The ligands SAEH2, SAMPH2 and SAPH2 were synthesized using previously described methods [10]. Infrared spectra were recorded on KBr pellets at room temperature with a Mattson Genesis II FTIR spectrometer. Thermogravimetric analyses were performed with a SETARAM TGA 92-16.18 thermal analyzer. The samples were placed into nickel/platinum alloy crucibles and heated at 0.83 K•s -1 in reconstituted air flow from 20°C to 650°C. An empty crucible was used as a reference. 1 H and 13 C{ 1 H} NMR measurements were carried out with a Bruker Avance DPX 200 spectrometer.

Synthesis of [VO(SAE)]2O (1)

. This procedure follows previously reported protocols using ethanol instead of dichloromethane [20] or acetone [36]. In a 100 mL flask, SAEH2 (497.6 mg, 3.01 mmol) was dissolved in methanol (10 mL) and a MeOH solution of VO(acac)2 (795.6 mg, 3.00 mmol, in 15 mL) was then added dropwise. The mixture was then refluxed for four hours. Volatiles were eliminated by evaporation and the pasty brown residue was recrystallized from hot acetonitrile to afford dark crystals suitable for X-ray crystallography, which were finally washed with a minimum of diethyl ether (yield 330 mg, 48.6 %). 1

Synthesis of [VO(SAMP)]2O, (2)

. This procedure follows previously reported protocols using ethanol instead of dichloromethane [20] or acetone [36]. To a solution of SAMPH2 (583 mg, 3.02 mmol) in methanol (10 mL) was added a methanol solution of VO(acac)2 (796 mg, 3.00 mmol, in 15 ml). The mixture was then refluxed for four hours. The solution was then concentrated under vacuum to a dark pasty residue. Addition of acetonitrile led to a red-blood solution, which yielded a few crystals upon standing for three days. The partial evaporation of the solution led to a solid has been separated from the solution by filtration and washed with diethylether. The solid was recrystallized by dissolution in the minimum amount of acetonitrile and dark crystals suitable for X-ray crystallography were obtained after slow evaporation (yield 0.520 g, 68 %). 1 H NMR (DMSO-d6) 1.00-1.80 (m, 6H), 4.51 (s, 2H), 6.4-8.0 (m, 8H), 8.89 (s, 1H). 13

Synthesis of [VO(SAP)]2O (3)

. This procedure follows previously reported protocols using ethanol instead of dichloromethane [20] or acetone [36]. To a solution of SAPH2 (641.7 mg, 3.0mmol) in MeOH (10 mL) was added dropwise a MeOH solution of VO(acac)2 (795.6 mg, 2.36 mmol, in 15 mL). The resulting mixture was refluxed for four hours and left cooling at room temperature, leading to crystals after one night. The rest of the pastry solution was slowly evaporated, washed with diethyl ether. (Yield 715 mg 84%). 1 H NMR (DMSO-d6): 6.4-8.0 (m, 8H), 9.40 (s, 1H). 13 C{ 1 H} NMR (DMSO-d6): 118.2, 120.7, 123.3, 130.6, 133.6, 137.7, 149.9, 161.9, 193.1. UV-Vis (DMF; nm ( in mol -1 L cm -1 )) 275 nm (46000), 382 (15500) 427 (20500). TGA: Exp (theor) mass loss 68.3% (68.2) -2 (L-O).

Catalytic tests. In a round flask was added 3 (52.4 mg, 0.091 mmol), dodecane (internal reference, 42.0 mg, 0.246 mmol) and cyclooctene (1.07 g, 9.68 mmol). The magnetically stirred mixture was heated to 80°C and the reaction was started by the addition of aqueous TBHP (2.5 mL, 70% wt, 18.06 mmol). Stirring was stopped after 5.5 h, leading to the appearance of two phases. A 0.1 ml sample of the organic phase was withdrawn, mixed with Et2O (2 mL) and a small quantity of MnO2 to destroy the residual peroxide. The mixture was then filtered through silica and analyzed by GC equipped with FID. GC parameters were quantified with authentic samples of products prior to the analysis. Conversion of ciscyclooct-1-ene and the formation of the cyclooctene oxide were calculated from calculation curves (r 2 = 0.999) relatively to dodecane prior to the reaction. Other reactions were carried out in an NMR tube and analyzed by 1 H NMR using acetophenone as internal standard.

X-ray crystallography. Single crystals of compounds 1 and 2 were mounted under inert perfluoropolyether on the tip of a glass fibre and cooled in the cryostream of a Bruker APEXII-II diffractometer. Data were collected using the monochromatic MoK radiation (= 0.71073). The structures were solved by direct methods (SIR97) [37] and refined by leastsquares procedures on F 2 using SHELXL-97 [START_REF] Sheldrick | SHELXL97. Program for Crystal Structure refinement[END_REF]. All H atoms attached to carbon were introduced in idealised positions and treated as riding models in the calculations. The drawing of the molecule was realised with the help of ORTEP3 [START_REF] Farrugia | [END_REF]. Crystal data and refinement parameters for 1 and 2 are shown in Table 5. Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 775912. and CCDC 775913. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk)

[

  (SAE)V=O] moieties asymmetrically linked together through the bridging O(3) atom [V-O bond lengths of 1.770(2) and 1.866(2) Å for V(2)-O(3) and V(1)-O(3), respectively]. The V=O bonds lengths are in the range 1.59-1.60 Å, conform to those found in the literature for related terminal V V =O bonds. Each vanadium atom is located at the center of a O3N base in a square pyramidal geometry, a terminal oxido ligand being situated at the top of the pyramid.The structures of 1, 2, and that previously reported for 3[20], are very close to each other in terms of length of the covalent and coordinative bonds and coordination geometries (bond angles). An interesting feature that has been observed earlier in other related structures is the presence of a loose intramolecular O•••V interaction between an oxygen atom of one [OV(SAE)] unit and the vanadium center of the second [OV(SAE)] unit (V(1)-O(23), 2.401(2) Å). This distance is at the short end of the range observed for this class of compounds, the shortest being 2.380(2) Å for compound 2 (our low-temperature structure; 2.404(2) Å in the literature report[20]). Others are 2.403(9) Å for compound 3[27], 2.459 Å for [(ONpAE)VO]2O [ONpAE = N-(2-oxyethyl)-2-oxidonaphthaliden-iminato] [22] and the longest one is 2.684 Å for [(SABH)VO]2O SABH = salicylaldehydebenzoylhydrazonato [28]. However, the dioxane adduct of compound 3 does not show this type of interaction [29]. The V(2)•••O(11) distance is even longer (> 2.8 Å), making the coordination geometry around V(2) more typical of a square pyramid, but the geometry is strongly distorted in both cases [displacements from the mean plane of atoms O(12), O(21), O(23) and N(21): V(2), 0.469(1) Å; O(11), 2.322(2) Å; displacements from the mean plane of atoms O(11), O(12), O(13) and N(11): V(1), 0.365(1) Å; O(23), 2.015(2) Å].

Figure 1 - 1 Table 3 .

 113 Figure 1 -Molecular structure of [VO(SAE)]2O, 1

Figure 2 -

 2 Figure 2 -Comparison between experimental and calculated vibrational spectra of compound 1: bottom: experimental; middle: computed with the 6-31g basis set for all atoms; top: computed with the SDD basis set for the V atom.

  H NMR (DMSO-d6): 3.66 (m, 2H), 3.76 (m, 2H), 4.72 (s, 2H), 6.4-8.0 (m, 4H), 8.44 (s, 1H). 13 C{ 1 H} NMR (DMSO-d6): 59.0 (CH2N), 67.8 (CH2O), 118.4 (CAr), 120.4 (CAr), 120.5 (CAr), 135.4 (CAr), 136.3 (CAr), 165.4 (CAr-O) 168.7 (CH=N). UV-Vis (DMSO; nm ( in mol -1 L cm -1 )): 262 nm (32000), 355 (8500). TGA: Exp (theor) mass loss of 61.4% (61.8) -2 (L-O).

  C{ 1 H} NMR (DMSO-d6): 26.6, 76.4, 89.1, 118.1, 120.6, 121.5, 135.2, 137.2, 163.4, 163.9. UV-Vis (DMSO; nm ( in mol -1 L cm -1 )): 264 nm (39000), 356 (7900). TGA: Exp (theor) mass loss of 66.2% (65.8) -2 (L-O).

Table 1 .

 1 Main IR vibration for compounds 1

-3.

	(LVO) 2 O	 -2 L + 2 O	V 2 O 5
	Scheme 2 -General scheme of the thermal degradation behaviour
	Compound	V=O V-O-V CH=N
	[(SAE)VO]2O (1)	979	756	1630
	[(SAMP)VO] 2 O (2)	977	757	1628
	[(SAP)VO]2O (3)	990	753	1602
	The thermal behaviour in the 20-650°C range is similar to those observed with
	[MoO 2 L] 2 complexes [10]. Ligand loss and concurrent oxygen uptake occur to yield the final
	vanadium oxide V2O5 (see Scheme 2 and Table 2). The thermograms of the three compounds
	are shown in the Supporting Information.

Table 2 .

 2 Experimental

	and theoretical mass losses observed in TG analysis for complexes 1-3
	within the range 20-650°C.
	Compound	Experimental loss Theoretical
	[(SAE)VO]2O (1)	61.4	61.8
	[(SAMP)VO]2O (2)	66.2	65.8
	[(SAP)VO]2O (3)	68.3	68.2

Table 4 .

 4 Comparison of experimental and theoretical bond lengths (Å) for compound 1.

		Observed	Calculated	Calculated
			(B3LYP/6-31g)	B3LYP/6-31g + SDD for V
	V1-O11	1.899(2)	1.908	1.892
	V1-O12	1.866(2)	1.810	1.801
	V1-O13	1.811(2)	1.811	1.809
	V1-O14	1.590(2)	1.564	1.570
	V1-N11	2.101(2)	2.167	2.165
	V1-O23	2.401(2)	2.662	2.847
	V2-O12	1.770(2)	1.759	1.767

Table 5 .

 5 Crystal data and structure refinement for 1

		1 -[VO(SAE)]2O	2 -[VO(SAMP)]2O
	Empirical formula	C 18 H 18 N 2 O 7 V 2	C 22 H 26 N 2 O 7 V 2
	Formula weight	476.22	532.33
	Temperature (K)	110(2)	293(2)
	Wavelength (Å)(MoK)	0.71073	0.71073
	Crystal system	Monoclinic	Monoclinic
	Space group	P21/n	C2/c
	Unit cell dimensions	a = 6.6299(3) Å	a = 33.758(2)
		b = 16.9926(8) Å	b = 7.1777(6)

Acknowledgment

We thank the CNRS and the Université Paul Sabatier (IUT A, Chemistry Department, Castres) for support, and the Centre Informatique National de l'Enseignement Supérieur (CINES, Montpellier) for free computational time.

 [40]. The calculations used the standard B3LYP three-parameter functional [41,42,43] in conjunction with the 6-31G** basis set, either for all atoms, or for the light atoms (O, N, C, H) plus the SDD set for the V atom, which includes a pseudopotential, augmented by an f polarization function with the optimized [44] 1.715 coefficient. The optimized geometry was confirmed to be a local minimum by the frequencies analysis. The calculated IR spectrum shown in Figure 2 was generated from the DFT-generated frequencies and intensities by applying Lorentzian functions and adjusting the line width to best fit the experimental spectra.


Laurent Lermusiaux

Véronique Many

Philippe Barois

Virginie Ponsinet

Serge Ravaine

Etienne Dugue

Mona Tréguer-Delapierre

Alexandre Baron

Toward Huygens' sources with dodecahedral plasmonic clusters

The design and chemical synthesis of plasmonic nanoresonators exhibiting a strong magnetic response in the visible is a key requirement to the realization of efficient functional and self-assembled metamaterials. However, novel applications like Huygens' metasurfaces or mu-near-zero materials require stronger magnetic responses than those currently reported. Our numerical simulations demonstrate that the specific dodecahedral morphology, whereby 12 silver satellites are located on the faces of a nanosized dielectric dodecahedron, provides sufficiently large electric and magnetic dipolar and quadrupolar responses that interfere to produce so-called generalized Huygens' sources, fulfilling the generalized Kerker condition. Using a multistep colloidal engineering approach, we synthesize highly symmetric plasmonic nanoclusters with a controlled silver satellite size and show that they exhibit a strong forward scattering that may be used in various applications such as metasurfaces or perfect absorbers.

The generation of artificial optical magnetism is a classical goal of the field of metamaterials. The vanishing of the magnetic susceptibility of natural materials for frequencies above gigahertz is well-known. [START_REF] Landau | Electrodynamics of Continuous Media[END_REF] Nevertheless, it was a success of the emerging field of metamaterials to show that a strong local magnetization could be induced by an electromagnetic wave in artificial composites by an appropriate design of conducting elements embedded in an insulating dielectric matrix. The trick was to engineer subwavelength resonant inductor-capacitor circuits in which strong induced circular currents generate a magnetic dipole. Strictly speaking, the magnetization is induced by the electric field E (or more precisely by its spatial variations as the curl of E) unlike natural magnetic materials like paramagnets in which magnetic dipoles (spins) truly couple to the magnetic field. At microwave frequencies, the split-ring resonator design enabled the generation of a negative effective magnetic permeability. [START_REF] Pendry | Magnetism from Conductors and Enhanced Nonlinear Phenomena[END_REF][START_REF] Smith | Composite Medium with Simultaneously Negative Permeability and Permittivity[END_REF] At higher frequencies of visible light, the size of the optical resonators should not exceed a few tens of nanometers, and plasmonic nanoparticles are good candidates for the realization of resonant optical devices. Several models of clusters of plasmonic nanoparticles have indeed been proposed [START_REF] Alù | Negative Effective Permeability and Left-Handed Materials at Optical Frequencies[END_REF][START_REF] Alù | The Quest for Magnetic Plasmons at Optical Frequencies[END_REF][START_REF] Rockstuhl | Design of an Artificial Three-Dimensional Composite Metamaterial with Magnetic Resonances in the Visible Range of the Electromagnetic Spectrum[END_REF][START_REF] Simovski | Model of Isotropic Resonant Magnetism in the Visible Range Based on Core-Shell Clusters[END_REF] for the generation of an intense magnetic polarization in response to visible light. Experimentally, the bottom-up approach has been successfully used to synthesize several types of magnetic clusters. [START_REF] Mühlig | Self-Assembled Plasmonic Core-Shell Clusters with an Isotropic Magnetic Dipole Response in the Visible Range[END_REF](9)(10)[START_REF] Qian | Raspberry-like Metamolecules Exhibiting Strong Magnetic Resonances[END_REF][START_REF] Ponsinet | Resonant Isotropic Optical Magnetism of Plasmonic Nanoclusters in Visible Light[END_REF][START_REF] Gomez-Graña | Hierarchical Self-Assembly of a Bulk Metamaterial Enables Isotropic Magnetic Permeability at Optical Frequencies[END_REF] Among them, the so-called "plasmonic raspberries" based on the Simovsky-Tretyakov model of magnetic nanoclusters [START_REF] Simovski | Model of Isotropic Resonant Magnetism in the Visible Range Based on Core-Shell Clusters[END_REF] have been extensively studied. Plasmonic raspberries consist of a corona of metallic nanospheres (gold or silver) evenly but randomly distributed at the surface of a dielectric core. The electric and magnetic contributions to their optical response have been unambiguously measured by light scattering experiments. (10,[START_REF] Ponsinet | Resonant Isotropic Optical Magnetism of Plasmonic Nanoclusters in Visible Light[END_REF][START_REF] Gomez-Graña | Hierarchical Self-Assembly of a Bulk Metamaterial Enables Isotropic Magnetic Permeability at Optical Frequencies[END_REF] It was found that the electric and magnetic dipolar resonances occur in the same frequency range and that the ratio of the two polarizations RM/E of the magnetic to electric dipolar scattering cross sections reaches 4.5% [START_REF] Ponsinet | Resonant Isotropic Optical Magnetism of Plasmonic Nanoclusters in Visible Light[END_REF] and 28% [START_REF] Gomez-Graña | Hierarchical Self-Assembly of a Bulk Metamaterial Enables Isotropic Magnetic Permeability at Optical Frequencies[END_REF] at resonance for gold and silver satellites, respectively. Moreover, a bulk metamaterial formed by a dense assembly of such silver raspberries exhibits non-natural values of the effective magnetic permeability ranging from 0.8 to 1.45 μ0. [START_REF] Gomez-Graña | Hierarchical Self-Assembly of a Bulk Metamaterial Enables Isotropic Magnetic Permeability at Optical Frequencies[END_REF] Novel applications like Huygens' metasurfaces [START_REF] Pfeiffer | Metamaterial Huygens' Surfaces: Tailoring Wave Fronts with Reflectionless Sheets[END_REF] or mu-near-zero materials [START_REF] Mahmoud | Wave-Matter Interactions in Epsilon-and-Mu-near-Zero Structures[END_REF] require, however, a stronger magnetic response. For example, in the dipolar regime, the first Kerker condition for zero-backscattering requires equal values of the electric and magnetic dipole response (RM/E = 1). [START_REF] Kerker | Electromagnetic Scattering by Magnetic Spheres[END_REF][START_REF] Dezert | Isotropic Huygens Dipoles and Multipoles with Colloidal Particles[END_REF] In a recent publication, Elancheliyan et al. demonstrated Huygens' scattering in the dipolar regime with spherical ensembles of large amounts of gold nanoinclusions arranged in emulsion droplets. [START_REF] Elancheliyan | Tailored Self-Assembled Nanocolloidal Huygens Scatterers in the Visible[END_REF] In the multipolar regime, the so-called generalized Kerker (GK) condition is achieved when the superposition of symmetric (or even) multipoles is equal to that of odd multipoles. (19) The odd/even parity is defined with respect to the radiated electric field distribution. Nanoresonators that reach the GK condition are also referred to as generalized Huygens' sources and constitute key building blocks toward the realization of efficient functional metasurfaces such as metalenses, (20) beam deflectors, [START_REF] Yu | High-Transmission Dielectric Metasurface with 2π Phase Control at Visible Wavelengths[END_REF] vortex plates, [START_REF] Lin | Dielectric Gradient Metasurface Optical Elements[END_REF] holograms, (23) or perfect absorbers. (24) In an attempt to increase the magnetic polarizability, a more elaborated design was proposed in which a welldefined number of metallic satellites are located at the faces of platonic solids. (25) In a recent paper, we showed numerically that the dodecahedral morphology whereby 12 gold satellites are distributed on the faces of a nanosized dielectric dodecahedron is the most favorable, and the experimental study showed that RM/E is indeed increased by a factor 3 or more, compared to gold raspberries (from 4.5% to 14.5%). (26) This significant gain suggests that the dodecahedral morphology should be applied to silver satellites in order to surpass the silver-raspberry value of 28%. The goal of the present paper is to present the effect of the insertion of silver in the satellites of these highly symmetric dodecahedral clusters and demonstrate that they act as efficient forward scatterers.

The inset in Figure 1a is a typical rendition of the designed dodecahedral nanoclusters. It consists of a silica core of 45 nm in radius surrounded by silver satellites of 31.4 nm in radius positioned at the center of the 12 pentagonal faces of a dodecahedron (or equivalently on the vertices of a regular icosahedron) and spaced 0.5 nm from the silica core. The nanocluster is designed to have a broad-band overlap of the scattering cross section of even and odd multipoles in order to fulfill the GK condition as much as possible when it is immersed in water (Figure 1a). The spectrum of the efficiency for backscattering (EFB) is represented in Figure 1a and is equal to zero, whenever the GK condition is reached (see the Supporting Information for a definition of the EFB). We see that it reaches a minimum at three different positions, two of which reach the GK. As can be seen from the radiation diagrams taken at four specific wavelengths (Figure 1b), the scattering is predominantly forward, and the backscattering is nearly suppressed at the three wavelengths for which the EFB is minimal. At 425 and 615 nm, the superposition of odd and even multipoles is exact, and the GK condition is fulfilled. At 500 nm, even though the superposition is not exact, it is close enough that the GK condition is nearly reached. At 800 nm, however, the scattering cross section is dominated by the dipolar radiation of the nanocluster, and the radiation diagram is a torus centered around the direction of the electric dipole moment; and as a result, both forward and backward radiation occur. Thanks to the multipolar superposition, we further see in Figure 1c that the fraction of energy scattered in the forward direction is close to 1 over a broad range of wavelengths across the visible range, and following that metric, it does better than a high-index silicon nanosphere with a diameter equal to the external diameter of the dodecahedral cluster. cross section of the particle: πR [START_REF] Pendry | Magnetism from Conductors and Enhanced Nonlinear Phenomena[END_REF] , where R is the sum of the core radius, the satellite diameter, and the 0.5 nm spacer layer between the core and the satellites. The gray dashed curve is the efficiency for backscattering (EFB) spectrum. The inset is a 3D rendition of the designed dodecahedral nanocluster composed of a silica core surrounded by 12 silver satellites.

b) Radiation diagrams of the modulus of the far field taken at the four spectral positions shown in panel a). The nanocluster is excited by a plane wave propagating in the direction of the propagation constant k (black arrow). The polarization directions of both the electric E (red arrow) and magnetic field H (white arrow) are shown for every diagram. c) Fraction of energy scattered in the forward direction as a function of wavelength for the designed dodecahedral nanocluster (continuous line)

and for a silicon nanoparticle of radius equal to R.

We synthesized silver dodecapods by applying the principles of the fabrication of gold dodecahedral clusters presented in detail elsewhere. (25,26) Briefly, a multistep synthetic process-which includes the fabrication of silica/polystyrene dodecahedral templates, [START_REF] Many | High Optical Magnetism of Dodecahedral Plasmonic Meta-Atoms[END_REF][START_REF] Désert | High-Yield Preparation of Polystyrene/Silica Clusters of Controlled Morphology[END_REF] the overgrowth of the silica core, the removal of the polystyrene satellites, and the regioselective functionalization of the concave dimples (29)(30)[START_REF] Chomette | Templated Growth of Gold Satellites on Dimpled Silica Cores[END_REF] -enables the formation of silica particles with dodecahedrally coordinated concave dimples as shown in Figure 2a. The center of each dimple possesses an aminated polystyrene residue allowing attachment of 2-3 nm gold seeds (structure A0). We developed a following one-step synthesis leading, in a few seconds, to the growth of silver directly on the dimpled templates. This synthesis protocol consists on successively adding a specific amount of silver nitrate, formaldehyde, and a solution of diluted ammonium hydroxide dropwise (detailed protocols in the Supporting Information). It is adapted from a previous protocol used to synthesize Ag nanoshells from silica particles decorated with gold seeds. [START_REF] Jackson | Nanoshells: Variations in Morphologies and Optical Properties[END_REF] Formaldehyde starts a slow reduction of the silver onto the gold seeds attached in the dimples, and the pH increase, upon injection of sodium hydroxide, results in the Ag + ions reduction and their deposition onto the forming satellites. We synthesized batches with increasing satellite size (A1 to A12 in Figures 2b andS1) by using different initial silver precursor amounts (× 1.5-fold between consecutive samples). The symmetrical spatial positioning of the 12 individual satellites onto the silica particles is confirmed by TEM images of well-orientated dodecapods (Figures 2a andS2). For low silver concentration (A1 to A3 in Figure 2b), each dimple is still composed of small individual Ag clusters. For higher concentration, the rather fast silver deposition provokes all the growing seeds to rapidly merge and form a single spheroidal Ag nanoparticle but still with traces of Au from the seeds at the bottom of the dimples (Figure 2c). The approach presented here offers several advantages compared to the synthesis of Au dodecapods. (25,26) In those, gold satellites were grown by iteration directly on the dimpled templates, hence slower kinetics of the seed regrowth (hours compared to seconds). After the regrowth, each dimple was still filled with several individual gold nanoparticles. Therefore, a subsequent annealing step, after transfer into an organic solvent, was required to condense them into a larger single bulk Au nanoparticle which exhibited a lenslike shape. (25,26) An additional oxidative etching of the gold could then be performed to make them spheroidal. (26) For silver, the growth conducted in one fast step avoids the dissolution of the fragile silica template in basic medium. Finally, as we shall see, the optical properties of the silver dodecahedra largely surpass those of gold, and the synthesis protocol offers unprecedented tunability of the multipolar response of odd parity modes with respect to even parity modes in the visible. We also observed that injecting the ammonium hydroxide directly, instead of dropwise, produced less spherical silver satellites resulting in weaker optical properties.

This approach takes advantage of several high-yield synthetic steps and results in the production of the plasmonic dodecapods, at the gram scale, with a yield as high as 70% (Figure 2d). The principal yield-limiting step is the synthesis of silica decorated with dodecahedrally coordinated polystyrene satellites, precursor of the silica particles with 12 dimples. We estimated the average satellite diameters of our samples by analyzing TEM images of the sample A6. It possesses individual Ag particles small enough to be well separated and therefore not overlapping on the EM images. Because only two or three pods can be precisely measured on a single dodecapod, the satellite size distribution obtained takes into account both the intra-and intersatellites variations. We found for A6 an average radius of 27.4 ± 5.3 nm (Figure S3). Diameters of the other samples A1 to A12 can be estimated from this single value because the silver amount increase of 1.5 is equivalent to a satellite diameter increase of 1.5 1/3 (Table S1). The scattering properties of the dodecapods were studied using a polarization and angle-resolved static light scattering setup (SLS) described elsewhere. [START_REF] Ponsinet | Resonant Isotropic Optical Magnetism of Plasmonic Nanoclusters in Visible Light[END_REF][START_REF] Gomez-Graña | Hierarchical Self-Assembly of a Bulk Metamaterial Enables Isotropic Magnetic Permeability at Optical Frequencies[END_REF] The linearly polarized white beam of a supercontinuous laser is shone onto a dilute suspension of dodecapods in water, and the scattered signal is collected and analyzed in polarization and energy by a microspectrophotometer. The scattering angle Scat can be set between 20 and 140° by a goniometric platform (Figure S4).

We first record the differential scattering at Scat = 90°along the two output polarization channels parallel (σ∥) and perpendicular (σ⊥) to the scattering plane. The reason is that the scattering multipoles of order n split into two separate series when Scat = 90°; σ∥(90°) collects the contributions of the modes of odd spatial parity, namely the magnetic and electric multipoles of odd and even order n, respectively, while σ⊥(90°) collects the other modes of even spatial symmetry. Note that the forward scattering condition reads σ∥(180°) = σ⊥(180°) = 0. At dipolar order (n = 1) which may be a realistic approximation at the long-wavelength side of the resonance spectrum, σ∥(90°) and σ⊥(90°) measure the scattering from the magnetic (odd symmetry) and electric (even symmetry) dipoles, respectively, so that the first Kerker condition for forward scattering simply reads σ∥(90°)/σ⊥(90°) = RM/E = 1. At shorter wavelengths, when higher order multipoles are excited and may overlap, the GK condition is more demanding as it involves all multipoles. The strict equality of the odd and even multipoles an = bn at each order n fully satisfies both the GK condition and the experimental equality σ∥(90°)/σ⊥(90°) = 1. (24) The GK condition is indeed less strict as it only requires the sum of the odd and even terms to be equal. Consequently, the ratio σ∥(90°)/σ⊥(90°) may differ from 1 at the GK condition. However, we point out that this difference is expected to be weak, at least in the spectral region where the dipolar scattering is present. In other words, reaching values of the ratio σ∥(90°)/σ⊥(90°) close to 1 at optical resonances augurs well for the production of intense Huygens' sources. Up to now, the highest reported RM/E value for spherical scatterers is about 0.28 for plasmonic Ag raspberries. [START_REF] Gomez-Graña | Hierarchical Self-Assembly of a Bulk Metamaterial Enables Isotropic Magnetic Permeability at Optical Frequencies[END_REF] The parallel-to-transverse ratio σ∥(90°)/σ⊥(90°) is plotted in Figure 3a across the visible spectrum.

Figure 3. Optical properties of silver dodecapods. a) Spectral variations of σ∥(90°)/σ⊥(90°) for samples within A1 to A10. b) Spectral variations of σ∥(90°) in red and σ⊥(90°) in blue for sample A7. c) Simulated spectral variations of σ⊥(90°) = |S1(λ)| 2 /k 2 (solid blue line) for the ideal dodecapod with dimensions corresponding to those of sample A7. Also represented are the contributions of the electric dipole (|a1| 2 ), the magnetic quadrupole (|b2| 2 ), and the interference term between the electric dipole and magnetic quadrupole (Re(a1b2 * )). d) Simulated spectral variations of σ∥(90°) = |S2(λ)| 2 /k 2 (solid red line) for the ideal dodecapod with dimensions corresponding to those of sample A7. Also represented are the contributions of the magnetic dipole (|b1| 2 ), the electric quadrupole (|a2| 2 ), and the interference term between the magnetic dipole and electric quadrupole (Re(b1a2 * )).

A series of broad bell-shaped curves are observed with a maximum that increases and red-shifts upon increasing the size of the silver satellites. The ratio σ∥(90°)/σ⊥(90°) reaches 1 for A7 and largely exceeds unity for A10, hence surpassing previously reported values in a spectacular way.

P a g e | 6

The differential scattering cross sections σ∥(90°) and σ⊥(90°) can be extracted after proper normalization by a reference sample made of a suspension of calibrated polystyrene spheres (see the SI for details). The two components are shown in Figure 3b for sample A7. The transverse signal σ⊥(90°) shows a broad resonance at 600 nm, whereas the parallel signal σ∥(90°) displays two maxima peaking at 500 and 775 nm and a clear dip is observed near 600 nm. At this stage, it is hard to assign these resonances to a particular set of scattering multipoles. We resort to numerical simulations of an ideal dodecapod with dimensions corresponding to those of sample A7 to determine the multipolar contributions to the spectral behavior of σ⊥(90°) and σ∥(90°). σ⊥(90°) and σ∥(90°) are proportional to |S1(90°)| 2 /k 2 and |S2(90°)| 2 /k 2 , respectively, where S1(90°) and S2(90°) are the scattering matrix amplitude coefficients at 90° (see the SI). Expressing these two quantities up to the quadrupolar multipolar terms enlightens the scattering behavior :

where a1 (respectively b1) is the electric (respectively magnetic) dipole coefficient, and a2 (respectively b2) is the electric (respectively magnetic) quadrupolar coefficient.

represents the multipolar terms of order higher than 2, which are fully considered in the numerical simulations. In this decomposition, a1 and b2 are the coefficients of the even multipoles, while b1 and a2 are the coefficients of the odd multipoles. Both |S1(90°)| 2 /k 2 and |S2(90°)| 2 /k 2 display the sum of three terms that contain the main contribution to the scattering: a dipole (a1 or b1), a quadrupole (a2 or b2), and an interference term between a dipole and a quadrupole (a1 with b2 or a2 with b1). The simulated spectral variations of |S1(90°)| 2 /k 2 and |S2(90°)| 2 /k 2 are represented on Figure 3c,d, along with the three leading terms contributing to the scattering. Both simulated curves reproduce the main features observed experimentally on the curves of σ∥(90°) and σ⊥(90°), albeit at different wavelengths, attributed to the polydisperse nature of the sample. We see that the dip observed for σ∥(90°) is also present in |S1(90°)| 2 /k 2 (pointed by a black arrow in Figure 3c) and is due to the interference between the electric dipole and magnetic quadrupole. We also see that |S2(90°)| 2 /k 2 has a main resonance peak near the dip of |S1(90°)| 2 /k 2 and is composed of a superposition of a magnetic dipole resonance and an electric quadrupole resonance. The interference term between these two multipoles reduces the amplitude of scattering at wavelengths below and above the main resonance peak of |S2(90°)| 2 /k 2 . These observations are a clear indication that the optical response of Ag dodecapods sits in an interference regime between odd and even multipoles. Experiments and simulations exhibit similar trends in terms of amplitude and frequency shift. Indeed, the amplitude of the ratio increases and red-shifts when the satellite size is increased. However, the resonance wavelengths are slightly different, and experimental data are systematically red-shifted compared to numerical simulations, suggesting a higher electromagnetic coupling between satellites in the real system. We assign this difference to the imperfect shape of the real satellites, which may grow bigger than the perfect spheres of the model while still not coalescing. Furthermore, the TEM images of Figure 2b reveal the presence of facets on the satellites, which, in turn, implies the presence of hot spots at the vertices. Both effects, bigger size and presence of hot spots, do enhance the electromagnetic coupling (as confirmed by UV-vis spectroscopy in Figure S5).

The equality of the parallel σ∥(90°) and transverse σ⊥(90°) scattering cross sections in sample A7 together with the associated numerical simulations suggests that the optical response of these dodecapods may be close to the GK condition of forward-only scattering. In order to verify the directionality of the scattered light, we recorded the scattering across a broad range of angles Scat from 20° to 140°. Normalization with a reference suspension of polystyrene beads was used again for calibration. Figure 5a is a color plot of the measured total scattering (parallel + transverse) from sample A7 as a function of wavelength and scattering angle. We see that the scattering is predominantly forward over a broad range of frequencies spanning almost all across the visible. Figures 5b are polar plots of the same data taken at three specific wavelengths (500 nm, 615 and 800 nm). The scattering is strongly forward for all three wavelengths. Moreover, the scattering strength and the forward directionality noticeably increased upon decreasing wavelength. Figure 5c shows the corresponding numerical simulations of the polar plots expected for the A7 dodecapod system at the same three wavelengths, accounting for the polydispersity of the sample as for the simulations of Figure 4. The simulations reproduce remarkably well the experimental observation of the forward scattering increase both in strength and directionality upon decreasing wavelength. Interestingly, the experimental forward scattering appears to be squeezed in a narrower angular range than the simulated one. Once again, we ascribe this quantitative difference to the simplified description of the satellite shape and their polydispersity, particularly as it excludes dodecapods with too large silver satellites. In this work, we successfully synthesized dodecapods made of 12 silver satellites located at the faces of a dodecahedron. We demonstrated that, when the satellites are separated by nanoscale gaps, these finely engineered nanoclusters behave as powerful Huygens' sources in visible light: their scattering is strongly oriented toward the forward direction with a high scattering efficiency. Numerical simulations and experimental observations reveal that the scattering is not limited to dipolar order but results instead from the contribution of a series of electric and magnetic multipoles approaching the generalized Kerker condition that interfere. Our bottomup approach enables to tune finely the size of the silver satellites and to produce large amounts of dodecapods with a high degree of control over morphology (with a yield of the same morphology as high as 70%). Our synthesis approach done in bulk can be scaled up, and the procedure could produce clusters at gram scales or more. This offers the possibility to make optical materials such as metasurfaces by depositing the structures on a bare or patterned substrate. This may contribute positively to the rationale behind metasurface fabrication because the individual Huygens' sources are designed and assembled separately from any substrate or array. In contrast to many top-down lithography techniques, they come in very large amounts, in the form of colloidal inks, which could be subsequently deposited or coated onto a surface to add functionality. Furthermore, since they are isotropic and resonant, the nanocolloidal platform could increase the potential of 3D metamaterials using hierarchical selfassembly. [START_REF] Gomez-Graña | Hierarchical Self-Assembly of a Bulk Metamaterial Enables Isotropic Magnetic Permeability at Optical Frequencies[END_REF] The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.0c04666.

 Materials and methods, electron microscopy, optical characterization, theoretical approach, and numerical simulations (PDF)

o :

The authors declare no competing financial interest.

Figure 1 .

 1 Figure 1. Designed dodecahedral nanocluster and scattering behavior. a) Scattering cross-section efficiency of the dodecahedral nanocluster as a function of wavelength. The red and blue lines evidence the contribution of the odd and even modes to the total scattering (black). The efficiency is defined as the scattering cross section divided by the outer geometrical

Figure 2 .

 2 Figure 2. Silver dodecapods. a) Synthetic scheme of plasmonic clusters from a silica template with dodecahedrally coordinated dimples. Inset: bright-field TEM image exhibiting distinctively the 12 silver satellites. b) TEM close-ups of gold-seeded (A0) and silver (A1 to A12) dodecapods with increasing silver satellites size. Scale bar = 100 nm. c) Widefield SEM image of silver dodecapods A7. Scale bar = 1 μm. d) STEM-EDX elemental analysis revealing silicon (top left), gold (bottom left), silver (bottom right), and overlaid view (top right) of the dodecahedral morphology.

Figure 4 .

 4 Figure 4. Comparison between simulations and experiment for samples A6, A7, and A10. Solid curves are the experimental spectral variations of σ∥(90°)/σ⊥(90°) for samples A6 (red), A7 (blue), and A10 (black). The dashed curves are the simulated |S2(λ)| 2 /|S1(λ)| 2 for the corresponding dodecapod geometries taking the experimental polydispersity into account.

Figure 4

 4 Figure4compares the experimental ratio σ∥(90°)/σ⊥(90°) for samples A6, A7, and A10 to the simulated ratios |S2(λ)| 2 /|S1(λ)| 2 for corresponding dodecapod dimensions and by considering the experimental polydispersity. The polydispersity for a given sample consists in a Gaussian distribution of the satellite size that follows the histogram of experimentally retrieved satellite sizes (see the SI for details). Experiments and simulations exhibit similar trends in terms of amplitude and frequency shift. Indeed, the amplitude of the ratio increases and red-shifts when the satellite size is increased. However, the resonance wavelengths are slightly different, and experimental data are systematically red-shifted compared to numerical simulations, suggesting a higher electromagnetic coupling between satellites in the real system. We assign this difference to the imperfect shape of the real satellites, which may grow bigger than the perfect spheres of the model while still not coalescing. Furthermore, the TEM images of Figure2breveal the presence of facets on the satellites, which, in turn, implies the presence of hot spots at the vertices. Both effects, bigger size and presence of hot spots, do enhance the electromagnetic coupling (as confirmed by UV-vis spectroscopy in FigureS5).

Figure 5 .

 5 Figure 5. Ag dodecapod forward scattering. a) Color plot showing the measured scattered intensity as a function of the scattering angle and wavelength. The data are normalized to the maximum value. (b and c): Experimental a) and simulated b) scattering polar plots shown for sample A7 at three wavelengths: 500 nm (green) 615 nm (red), and 800 nm (brown). The dashed lines on the color plot of panel a) show the wavelengths for which the polar plots are represented on panel b). Symbols show actual data. The dotted lines are guides for the eyes computed as polynomials in μ = cos  (see the SI for details).

  Véronique Many -Université de Bordeaux, CNRS, ICMCB, Bordeaux INP, UMR 5026, Pessac 33600, France; Université de Bordeaux, CNRS, CRPP, UMR 5031, Pessac 33600, France o Philippe Barois -Université de Bordeaux, CNRS, CRPP, UMR 5031, Pessac 33600, France o Virginie Ponsinet -Université de Bordeaux, CNRS, CRPP, UMR 5031, Pessac 33600, France o Serge Ravaine -Université de Bordeaux, CNRS, CRPP, UMR 5031, Pessac 33600, France o Etienne Duguet -Université de Bordeaux, CNRS, ICMCB, Bordeaux INP, UMR 5026, Pessac 33600, France; http://orcid.org/0000-0002-0675-5987 Funding : This work was supported by the LabEx AMADEus (ANR-10-LABX-42) and IdEx Bordeaux (ANR-10-IDEX-03-02), i.e., the Investissements d'Avenir program of the French government managed by the Agence Nationale de la Recherche.

Acknowledgments : TEM experiments were performed at the Plateforme de Caractérisation des Matériaux of the University of Bordeaux (UMS CNRS 3626). STEM and STEM-EDX experiments were performed at the Centre de Caractérisation R. Castaing of the University of Toulouse (UMS CNRS 3623).

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