Claire Mével

Julie Carreaud

Celine Caillaud

Francesco Bour

Gaelle Delaizir

Victor Castaing

Bruno Viana

Pierre Carles

Cécile François Brisset

Céline Caillaud

Gaëlle Delaizir

Cécile Genevois

Mathieu Allix

Sébastien Chenu
email: sebastien.chenu@univ-rennes1.fr

Sintering of lixiviated nano glass-ceramics: An original route to elaborate transparent ceramics

Keywords: Transparent ceramic, ZnGa2O4, glass ceramic, lixiviation, nanocrystals, spinel

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Transparent ceramics have raised significant attention during the last decades due to their promising and remarkable combination of optical, mechanical, dielectric and thermal properties [START_REF] Kong | Transparent Ceramics[END_REF][START_REF] Partridge | Inorganic materials IV. Transparent ceramics and glass-ceramics[END_REF][START_REF] Wang | Transparent ceramics: Processing, materials and applications[END_REF][START_REF] Jumpei | Yellow persistent luminescence in Ce 3+ Cr 3+ -codoped gadolinium aluminum gallium garnet transparent ceramics after blue-light excitation[END_REF][START_REF] Ikesue | Ceramic laser materials[END_REF][START_REF] Messing | Toward pore-free ceramics[END_REF][START_REF] Martin | Recent R&D Trends in Inorganic Single-Crystal Scintillator Materials for Radiation Detection[END_REF]. Transparent crystalline materials can be obtained through different synthesis routes using i) the Czochralski, Verneuil or flux growing methods for example to get high quality single crystals [START_REF] Dutta | 3.02 -Bulk Growth of Crystals of III V Compound Semiconductors[END_REF][START_REF] Galazka | MgGa2O4 as a new wide bandgap transparent semiconducting oxide: growth and properties of bulk single crystals[END_REF][START_REF] Galazka | Ultra-wide bandgap, conductive, high mobility, and high quality melt-grown bulk ZnGa2O4 single crystals[END_REF], ii) the sintering of nano-powders by vacuum sintering [START_REF] Epicier | Spatial distribution of the Yb 3+ rare earth ions in Y3Al5O12 and Y2O3 optical ceramics as analyzed by TEM[END_REF], hot uniaxial or isostatic pressing (HP/HIP) [START_REF] Wang | Optical transmission, dispersion, and transition behavior of ZnGa2O4 transparent ceramic[END_REF][START_REF] Chlique | Transparent ZnS Ceramics by Sintering of High Purity Monodisperse Nanopowders[END_REF] and iii) non-conventional sintering techniques like the Spark Plasma Sintering (SPS) [START_REF] Carreaud | Fabrication and optical properties of transparent fine-grained Zn1.1Ga1.8Ge0.1O4 and Ni 2+ (or Cr 3+ )-doped Zn1.1Ga1.8Ge0.1O4 spinel ceramics[END_REF][START_REF] Mével | First ZnGa2O4 Transparent Ceramics[END_REF].

Powder sintering ceramic process is challenging for several reasons. First, most light-scattering sources arise from the ceramic s microstructure, such as pores, as well as the presence of impurities, secondary phases, and birefringence effects. Indeed, the presence of residual porosity (i.e. source of light scattering) in the final ceramic is strongly deleterious for the transparency [START_REF] Apetz | Transparent alumina: A light-scattering model[END_REF]. In fact, it has been often reported that the volume of pores in such ceramics must remain smaller 100 vppm to reach photonic quality (i.e. with transmissions close to theoretical ones) [START_REF] Kong | Transparent Ceramics[END_REF][START_REF] Ikesue | Ceramic laser materials[END_REF][START_REF] Messing | Toward pore-free ceramics[END_REF]. Since 2012, an innovative process has been developed by M. Allix et al. [START_REF] Milisavljevic | Crystallization of glass materials into transparent optical ceramics[END_REF] to synthesize transparent oxide ceramics by complete crystallization of bulk glasses.

Nevertheless, this method requires the vitrification of the targeted ceramic composition which is not always possible for the ZnGa2O4 spinel. This cubic zinc gallate spinel has been widely studied for optical applications such as saturable absorbers [START_REF] Loiko | Saturable absorber: transparent glass-ceramics based on a mixture of Co:B-Zn2SiO4 and Co:ZnO nanocrystals[END_REF], in vivo bio-imaging applications [START_REF] Maldiney | The in vivo activation of persistent nanophosphors for optical imaging of vascularization, tumours and grafted cells[END_REF], thermal sensors [START_REF] Glais | Luminescence properties of ZnGa2O4:Cr 3+ ,Bi 3+ nanophosphors for thermometry applications[END_REF], X-ray dosimetry [START_REF] Luchechko | Afterglow, TL and OSL properties of Mn 2+ doped ZnGa2O4 phosphor[END_REF], optoelectronics [START_REF] Chikoidze | Type Ultrawide-Band-Gap Spinel ZnGa2O4: New Perspectives for Energy Electronics[END_REF], anticounterfeiting applications [START_REF] Ma | The Second Near-Infrared Window Persistent Luminescence for Anti-Counterfeiting Application[END_REF]. Recently, this spinel material has been elaborated as transparent ceramic by combining high energy milling and SPS [START_REF] Mével | First ZnGa2O4 Transparent Ceramics[END_REF] but also using coprecipitation method and HIP [START_REF] Wang | Optical transmission, dispersion, and transition behavior of ZnGa2O4 transparent ceramic[END_REF][START_REF] Liu | Fabrication and characterizations of Cr 3+ doped ZnGa2O4 transparent ceramics with persistent luminescence[END_REF]. Secondly, the use of nanometric precursor powders with controlled granulometry and high purity requires physicochemical synthesis routes that are often time-consuming, expensive and complex to implement. Here, an innovative approach has been developed, nanopowders of ZnGa2O4 ceramic could be obtained by the lixiviation of glassceramics.

In the present study, we report a novel synthesis route starting from a transparent nano glassceramic material that undergoes, after prior comminution, a selective lixiviation or dissolution process of the glass matrix leading to obtainment of fine particles carrying separated nanocrystals. Indeed, S. Chenu et al. [START_REF] Chenu | Long-lasting luminescent ZnGa2O4:Cr 3+ transparent glass-ceramics[END_REF] have shown that highly transparent ZnGa2O4 glassceramic materials are elaborated via a simple heat treatment of a 55 SiO2 -5 Na2O -17 ZnO -23 Ga2O3 parent glass composition, which presents nanoscale spinodal phase separation. This optimized glass-ceramic exhibits 50 %wt of ZnGa2O4 nanocrystals showing a homogeneous and tunable size until 1200°C. After lixiviation, these spinel crystals are then used as a powder media which is sintered into a transparent ceramic material by SPS. The scheme of this process is depicted in Figure 1. Glass crystallization is indeed a powerful and non-conventional elaboration process for stabilizing novel compounds with interesting functional properties and metastable phases such as BaAl4O7 [START_REF] Allix | Highly Transparent BaAl4O7 Polycrystalline Ceramic Obtained by Full Crystallization from Glass[END_REF], nonstoichiometric garnets [START_REF] Cao | Highly Nonstoichiometric YAG Ceramics with Modified Luminescence Properties[END_REF], or La2Ga3O7.5 [START_REF] Fan | La2Ga3O7.5: A Metastable Ternary Melilite with a Super-Excess of Interstitial Oxide Ions Synthesized by Direct Crystallization of the Melt[END_REF]. This innovative technique paves the way to new compositions of transparent ceramics that are difficult to obtain by others common synthesis routes. Glass dissolution (or lixiviation) is highly studied in nuclear waste [START_REF] Grambow | Nuclear waste glass dissolution: Mechanism, model and application[END_REF] or geological fields [START_REF] Brady | Controls on silicate dissolution rates in neutral and basic pH solutions at 25°C[END_REF][START_REF] Brady | Kinetics of quartz dissolution at low temperatures[END_REF] but to date no study has been reported in the field of optical materials. Influence parameters are linked to the medium used for lixiviation (composition, pH and stirring parameters) and to the composition and structure of the material to dissolve. The lixiviation is directly linked to the dissolution rate Ri (and to its efficiency), defined by the equation ( 1) [START_REF] Strachan | Glass dissolution asa function of pH and its implications for understanding mechanisms and future experiments[END_REF]:

The dissolution phenomenon thus depends on the activation energies Ea associated with the acid, water, or hydroxide activated reactions, the temperature T, the chemical activities ai and the order of the reaction related to the involved species H3O + , H2O and HO -. Ki is a constant depending on the dissolved composition. The ability of a material to be dissolved is related to i) its composition and structure (network) and ii) the environment of lixiviation i.e pH, temperature, stirring and renewal of the lixiviation solution.

As a proof-of-concept, a glass-ceramic from the SiO2 -Na2O -ZnO -Ga2O3 system has been chosen as it allows the crystallization at the nanometric scale of the well-known cubic spinel ZnGa2O4 [START_REF] Chenu | Long-lasting luminescent ZnGa2O4:Cr 3+ transparent glass-ceramics[END_REF]. ZnGa2O4 is a promising material for optical applications, due to its wide transparency window from ultraviolet to mid-IR wavelengths (0.2-9 µm) and its afterglow luminescence when doped with metal transition cations [START_REF] Chenu | Long-lasting luminescent ZnGa2O4:Cr 3+ transparent glass-ceramics[END_REF][START_REF] Zhang | Crystallization kinetics and optical properties of transparent glass-ceramics embedding ZnGa2O4 nanocrystals with enhanced defect luminescence[END_REF][START_REF] Chenu | Tuneable Nanostructuring of Highly Transparent Zinc Gallogermanate Glasses and Glass-Ceramics[END_REF][START_REF] Jin | A new persistent luminescence phosphor of ZnGa2O4:Ni 2+ for the second near-infrared transparency window[END_REF]. ZnGa2O4 crystallizes in a 2O4 a=8.3366 Å in which Zn 2+ ions occupy the tetrahedral A-sites and Ga 3+ the octahedral B-sites [START_REF] Hornstra | Oxygen parameter of spinel ZnGa2O4[END_REF]. This material can be synthetized as single crystal [START_REF] Galazka | Ultra-wide bandgap, conductive, high mobility, and high quality melt-grown bulk ZnGa2O4 single crystals[END_REF][START_REF] Boy | Charge carrier density, mobility, and Seebeck coefficient of melt-grown bulk ZnGa2O4 single crystals[END_REF], nanocrystals [START_REF] Duan | Structure and luminescent properties of Co 2+ /Cr 3+ co-doped ZnGa2O4 nanoparticles[END_REF][START_REF] Byun | Blue, green, and red emission from undoped and doped ZnGa2O4 colloidal nanocrystals[END_REF], micrometric ceramic powder [START_REF] Allix | Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa2O4[END_REF][START_REF] Bessiere | ZnGa2O4:Cr 3+ : a new red long-lasting phosphor with high brightness[END_REF], glass ceramics [START_REF] Chenu | Long-lasting luminescent ZnGa2O4:Cr 3+ transparent glass-ceramics[END_REF][START_REF] Chenu | Tuneable Nanostructuring of Highly Transparent Zinc Gallogermanate Glasses and Glass-Ceramics[END_REF][START_REF] Tanaka | Optical properties of transparent glass-ceramics containing ZnGa2O4-Cr 3+ microcrystal[END_REF], thin films [START_REF] Arroyo | Persistent luminescence of transparent ZnGa2O4:Cr 3+ thin films from colloidal nanoparticles of tunable size[END_REF][START_REF] Anoop | Influence of a dopant source on the structural and optical properties of Mn doped ZnGa2O4 thin films[END_REF] or even nanotubes [START_REF] Tien | Direct vapor transport synthesis of ZnGa2O4 nanowires with superior photocatalytic activity[END_REF] but also as a transparent ceramic [START_REF] Wang | Optical transmission, dispersion, and transition behavior of ZnGa2O4 transparent ceramic[END_REF][START_REF] Mével | First ZnGa2O4 Transparent Ceramics[END_REF][START_REF] Liu | Fabrication and characterizations of Cr 3+ doped ZnGa2O4 transparent ceramics with persistent luminescence[END_REF].

Here, we show that high crystallinity zinc gallate spinel nanocrystals could be synthesized and embedded in a glass-ceramic material. Contrary to common lixiviation studies in glass, the aim of this work is to favor the glass matrix dissolution of a nano glass-ceramic to isolate embedded nanocrystals (Figure 1). This process is considered as a new technique to obtain nanocrystals using the natural dispersion of crystals in a nano glass-ceramic. Furthermore, the nano-powders obtained are highly crystalline depending on the heat-treatment, which may not be the case when sol-gel technique is used. The dissolution of the glass matrix is studied to recover the nanocrystals which are further used in SPS sintering experiments in order to elaborate transparent ZnGa2O4 transparent ceramics. The structure, microstructure and optical properties of the obtained transparent ceramic are studied and compared to the literature.

Experimental procedures a. Synthesis

A glass composition of 55 SiO2 -5 Na2O -17 ZnO -23 Ga2O3 (% mol) was prepared by the conventional melt-quenching technique as mentioned in [START_REF] Chenu | Long-lasting luminescent ZnGa2O4:Cr 3+ transparent glass-ceramics[END_REF]. First, the precursors (SiO2 (Tekna), Na2CO3 (Aldrich, 99,95-100,05%), ZnO (Alfa Aesar, 99,99%) and Ga2O3 (Strem Chemicals, 99,998%)) were mixed in an agate mortar and decarbonated under air at 900°C for 6 hours. The mixture was then melted at 1600°C for 45 min under air in a platinum crucible and quenched in water. The resulting glass was crushed in an agate mortar until it passed through a 63 µm-mesh sieve and was further crystallized under air at 1100°C for 1 min in a platinum crucible. The expected crystalline fraction in these conditions is ~ 50%wt. Then, the glass-ceramic powder was lixiviated as followed [START_REF] Castaing | Research of new colours and morphologies in persistent luminescence materials[END_REF]: 4g of glass ceramic powder were introduced in a 150 mL of NaOH solution, concentrated at 4 mol.L -1 . The solution was maintained at 95°C for 24 hours to allow the dissolution of the glass matrix. The powder was then washed by centrifugation at 1000 rpm for 10 minutes with distilled water, until the pH remained neutral. The drying of nanocrystalline powder was performed at room temperature under air.

The densification of the nanocrystalline ZnGa2O4 powders was achieved by spark plasma sintering (SPS) using a Dr.Sinter 825 Syntex machine (Fuji FDC, Japan) to obtain a transparent ceramic. The powder was introduced in an 8 mm diameter graphite die and was consolidated under vacuum at 960°C under 100 MPa. The thermo-mechanical cycle is presented in Figure 2.

During the SPS process, the temperature was monitored by a thermocouple positioned through the die in the vicinity of the sample.

b. Structural characterizations

Powder X-Ray Diffraction (PXRD) data were recorded on a Bragg Brentano D8 Advance Bruker diffractometer (CuK radiation) equipped with a LynxEye XE detector. Short were perfor software [START_REF] Petricek | Crystallographic Computing System JANA2006: General features[END_REF].

Scanning Electron Microscopy observations were performed using a FEG Quanta FEI device, operating at 15 kV.

Electron backscatter diffraction (EBSD) maps were recorded on the ceramic after densification by SPS, using an OIM TSL/EDAX system mounted on a FEG-SEM (Zeiss SUPRA 55 VP) system. The SEM was set with the VP mode to avoid a carbon coating and the high voltage was 30 kV. It was used to evaluate the amorphous residual parts and to visualize the microstructure of sample without chemical or thermal etching which could modify grain sizes. Before the observations, the samples were optically mirror-polished.

Transmission Electron Microscopy (TEM) was used to characterize the microstructure of the lixiviated powder and the ceramics sintered by SPS. The lixiviated powder and glass-ceramics were analyzed using a Titan Themis (200 kV, probe-corrected, probe size 0.1 nm) by TEM imaging, selected-area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDS) analyses. The ceramics were characterized by High-Resolution TEM (HR-TEM) and scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) imaging modes, as well as by EDS elemental mapping, using a JEOL ARM 200CF (JEOL Ltd.) cold FEG operating at 200 kV, equipped with a double spherical aberration corrector and a JEOL SDD CENTURIO EDS system. In STEM-HAADF imaging mode, a 68-174.5 mrad inner-outer collection angle and a 0.1 nm probe size were used while in EDS mapping a probe size of 0.13 nm was chosen.

The ceramics were prepared prior to (S)TEM observations by focused ion beam (FIB, ZEISS Crossbeam 550).

Optical transmission measurements were carried out within the 300-3300 nm range, with the sample placed at normal incidence, using a Varian Cary 5000 spectrophotometer operated in a dual beam configuration and in the infrared range using a Thermo Scientific Nicolet 6700 FTIR spectrophotometer.

The density of the SPS bulk samples was determined through the Archimedean principle using a Kern balance with ethanol as buoyancy media.

Results and discussion

Here, the lixiviation process of spinel glass-ceramics (i.e. dissolution of the silicate glass matrix) is studied to recover the nanocrystals which are further used in SPS sintering experiments in order to elaborate transparent ZnGa2O4 ceramics. Before this lixiviation steps, an adequate nano glass-ceramic is synthesized. S. Chenu et al. [START_REF] Chenu | Long-lasting luminescent ZnGa2O4:Cr 3+ transparent glass-ceramics[END_REF] have previously reported that 55 SiO2 -5 Na2O -17 ZnO -23 Ga2O3 glass crystallizes with 50%wt ZnGa2O4 nanocrystals (crystallite size ~ 30 nm) using a heat treatment at 1000°C for 10 min. The process is the following; the glass exhibits a nanoscale spinodal phase separation with regions rich in i) SiO2 and ii) ZnO and Ga2O3. As the size of the phase separation can be tailored via the nominal glass composition, the control of the crystal size in the glass-ceramic material is made easier via designing the phase separation size in the parent glass. The later region will transform into crystalline ZnGa2O4 at higher temperature while the region with a high silica content will remain amorphous even at high temperatures (until 1200°C) and will limit the spinel crystals growth. By this method a fraction of about 50%wt of the glass crystallizes forming ZnGa2O4 nanocrystals as describe below.

Figure 3-a shows the XRD patterns of the glass-ceramic powder heat-treated at 1100°C for 30s to 1h. These temperature and duration have been chosen according to previous studies [START_REF] Chenu | Long-lasting luminescent ZnGa2O4:Cr 3+ transparent glass-ceramics[END_REF] as a good compromise to get high crystallized fraction, homogeneous and controlled crystallite size and high crystallinity degree. Even after only 30 s, some crystallization peaks appear in the diffractogram that well indexed by the spinel ZnGa2O4 phase which is in good agreement with the study of Chenu et al. [START_REF] Chenu | Long-lasting luminescent ZnGa2O4:Cr 3+ transparent glass-ceramics[END_REF].

SEM observations of the glass-ceramic powder heat-treated at 1100°C for 1 min reveal some heterogeneous agglomerated particles of few micrometers, each particle exhibiting a glass matrix with embedded ZnGa2O4 cubic crystals measuring few tens of nanometers as shown in The glass-ceramic powder was then lixiviated according to the protocol explained in [START_REF] Castaing | Research of new colours and morphologies in persistent luminescence materials[END_REF] and described in the experimental part to isolate ZnGa2O4 nano-crystals.

The microstructure of the lixiviated powder evidences the presence of grains that become porous after the lixiviation step as evidenced by SEM observations (Figure 4a andb), this is in favor of the dissolution of the glass matrix. These grains mainly consist of cubic nanoparticles of size between 20-60 nm (Figure 4-d ande). The inset Selected Area Electron Diffraction (SAED) pattern (Figure 4-e) evidenced bright spots which superimpose discrete diffraction rings matching the ZnGa2O4 spinel structure and attesting the good crystallinity of the material.

One can observe the presence of an amorphous shell around some ZnGa2O4 nanocrystals as shown by yellow arrows in Figure 4e. EDS-TEM and EDS-SEM analyzes (Table 1), made on lixiviated powder, highlight the presence of SiO2 in the sample (3.8% mol.) which corresponds to amorphous SiO2 at the surface of ZnGa2O4 nanocrystals (Table 1). The presence of this residual amorphous phase may not be detrimental to the later sintering process as it could act as sintering aid [START_REF] Yang | The effect of MgO and SiO2 codoping on the properties of Nd:YAG transparent ceramic[END_REF][START_REF] Basyrova | Effect of SiO2 addition on structural and optical properties of Yb:Lu3Al5O12 transparent ceramics based on laser ablated nanopowders[END_REF][START_REF] Kochawattana | Sintering and grain growth in SiO2 doped Nd:YAG[END_REF][51]. The particle size distribution of the lixiviated powder is shown in Figure 4-c. The distribution is multi-modes with a median diameter D50=1.18 µm. The first mode centered around 0.15 µm corresponds to single particles and small agglomerates of few particles while the 2 nd and 3 rd modes centered around 1.5 µm and 15 µm respectively correspond both to larger agglomerates that are in good agreement with SEM observations (Figure 4a). The use of sonification and dispersant are not efficient to break the agglomerates which means that the ZnGa2O4 nanoparticles may be still linked together by the residual glass matrix. EDS-SEM analyzes, made on SPS-sintered ceramic, reveal the presence of a small amount of silicon that could be found as SiO2 in the SPS sample (~ 0.8% at of Si) as depicted in Figure 5- c. One can note that the Ga2O3/ZnO ratio in ZnGa2O4 ceramic is equal to one while in the overall glass-ceramic material, it is note the case (ratio=1.35). As explained in [START_REF] Chenu | Long-lasting luminescent ZnGa2O4:Cr 3+ transparent glass-ceramics[END_REF], during the crystallization process, a phase separation with enriched amorphous gallium oxide appears in the residual glass and allows to obtain pure ZnGa2O4 glass-ceramic. The percentages of Zn and Ga in the SPS sample are consistent with the formation of the ZnGa2O4 cubic phase.

To have a better understanding of the microstructure of the SPS ZnGa2O4 ceramic, SEM measurements as well as EDS-SEM were acquired on a fresh fracture and a polished sample respectively. As presented in Figure 5-a, we can highlight the presence of a complex and heterogeneous microstructure with two different features. Grains of around 1 µm in size are surrounded by smaller ones as shown in Figure 6a. This Si quantity is small and the accuracy concerning this value is relatively high, nevertheless the presence of silicon is attested. SiO2 is a well-known sintering aid for the densification of transparent ceramic, YAG for example [51,[START_REF] Sallé | A First Approach of Silica Effect on the Sintering of Nd:YAG[END_REF]. Its addition usually promotes densification, elimination of pores and grain growth either through the presence of a liquid phase, or by other mechanisms by enhancing diffusion during thermal treatments. Interestingly, in our study, the SiO2-enriched domains are characterized by inhibited grain growth as shown in Figure 6a. This result could be explained by low sintering temperature (960°C) at which SiO2 is not liquid (in contrary to much higher sintering temperatures of YAG) and the low solubility of ZnGa2O4 into amorphous SiO2 at such temperature prevents grain coalescence and crystal growth.

EDS-SEM measurements (table in

Furthermore, Si is not uniformly distributed in the ceramic while Zn and Ga seem to be distributed homogeneously. Electron backscattering diffraction (EBSD) map was recorded on the ZnGa2O4 SPS ceramic (Figure 5d). The crystalline domains show random orientations as illustrated by pole figures (Figure 5e) and their size evolves from 100 to 600 nm, with an average size of 200 nm. The black (non-indexed) areas may attest the presence of glass phase at grain boundary and/or zones with very small grains that are not detectable. This point will be discussed in the following part of the paper. To go further in the understanding of the microstructure TEM observations of the SPS ZnGa2O4 ceramics were performed on a FIB-prepared thin foil. Low magnification observations reveal that the sample is dense as no porosity is observed (Figure SI 3). STEM-HAADF (Z-contrast)

imaging clearly shows a chemical contrast with two different phases: crystals with a bright contrast are surrounded by darker, i.e. a lighter phase at the grain boundaries (Figure 6c). It is noteworthy that no porosity is visible even at high magnification. The particle size distribution calculated on more than 500 grains is in the range 10-160 nm, the average size and the standard deviation are around 57 nm and 26 nm respectively (Figure 6b). STEM-EDS elemental map and punctual analyzes reveal that the lighter phase is only composed of SiO2 while the composition of the grains is ZnGa2O4 (Figure 6d). The HRTEM images taken on different location of the FIB prepared sample exhibit different types of grain boundaries. First, it is important to highlight that the SiO2 phase at the grain boundaries is amorphous. Indeed, the FFT indexations of the crystals and surrounded phase are consistent with both a cubic spinel phase structure and amorphous phase, respectively (Figure 6-d). The amorphous SiO2 phase that remains after incomplete lixiviation process of the glass-ceramic can be found as domains of few tens of nm that fill the porosities which is of great importance especially for optical properties (i.e.

diminution of light scattering). One also can note that the amorphous SiO2 phase is localized on thin edges between ZnGa2O4 grains (yellow arrows in Figure 6) which prevents from grain growth and enhances mechanical properties. Some thin boundaries (~ 2 nm) between two crystals and without any glass at the interface are also evidenced in the sample (Figure 6-f).

Indeed, at some point and when no amorphous phase is detected at the grain boundaries, the coalescence phenomenon leading to grain growth takes place as illustrated in Figure 6-g, where we can see on the HRTEM image and the associated FFT that the (111) plans of two grains are in the process of aligning with each other to become finally one grain. An attempt to estimate the amount of amorphous phase has been performed by assuming that (i) the amorphous phase is only made of pure SiO2 glass and (ii) the crystals are made of pure ZnGa2O4 with no Si. With such hypotheses, the amount of amorphous phase, µm² area, is estimated to ~ 6% mol which is in the same range of the content found by EDS-SEM (~ 5% mol i.e. ~ 2.3% wt). Finally, the transmittance of the SPS sintered sample was measured. The resulting ZnGa2O4 ceramic is translucent and exhibits a density of 5.93 g.cm -3 leading to a compactness of 99.8% considering a density of 6.15 g.cm -3 for ZnGa2O4 and the presence of ~ 5% mol of pure SiO2 as observed by EDS. This result is in good agreement with the TEM observations (Figure 6-a) where no porosity is detectable. One may confirm that a residual glass phase contributes to the density.

Some black spots that are characteristics of carbon contamination due to the use of graphite environment [START_REF] Mével | First ZnGa2O4 Transparent Ceramics[END_REF][START_REF] Morita | Distribution of carbon contamination in MgAl 2 O 4 spinel occurring during spark-plasma-sintering (SPS) processing: I Effect of heating rate and post-annealing[END_REF] are observable in the sample (Figure 7). This contamination has already been oberved in ZnGa2O4 samples and others transparent materials sintered by SPS [START_REF] Mével | First ZnGa2O4 Transparent Ceramics[END_REF][START_REF] Bertrand | A Comprehensive Study of the Carbon Contamination in Tellurite Glasses and Glass-Ceramics Sintered by SPS[END_REF][START_REF] Morita | Distribution of carbon contamination in oxide ceramics occurring during spark-plasma-sintering (SPS) processing: II -Effect of SPS and loading temperatures[END_REF]. In some cases, after this SPS step, a post annealing treatment in air was performed to remove carbon contamination [START_REF] Carreaud | Fabrication and optical properties of transparent fine-grained Zn1.1Ga1.8Ge0.1O4 and Ni 2+ (or Cr 3+ )-doped Zn1.1Ga1.8Ge0.1O4 spinel ceramics[END_REF][START_REF] Morita | Influence of pre-and post-annealing on discoloration of MgAl2O4 spinel fabricated by spark-plasma-sintering (SPS)[END_REF]. The transmission window extends from 0. This value is far from the theoretical value calculated on a ZnGa2O4 single crystal (82.5% at 2 µm) but this result can be explained by different parameters. First and even if the amount of amorphous silica is low and the size of the amorphous domains is small (from few nm to few tens of nm) and acts as a sintering aid by closing the intergranular positites, its refractive index (1.45 at 1µm) is very different from the one of ZnGa2O4 (1.97 at 1µm) and may induce some light scattering [START_REF] Apetz | Transparent alumina: A light-scattering model[END_REF]. Then the carbon contamination is also a source of scattering [START_REF] Morita | Distribution of carbon contamination in MgAl 2 O 4 spinel occurring during spark-plasma-sintering (SPS) processing: I Effect of heating rate and post-annealing[END_REF]. As the ZnGa2O4 spinel is a cubic structure, the grain size is not a detrimental parameter for transmittance but the starting powder is made of agglomerates with different sizes as shown in

Conclusion

We reported on a detailed investigation of the innovative synthesis, microstructure and optical properties of ZnGa2O4 translucent ceramics. Nanocrystalline zinc gallate spinel powder was obtained by an original process based on the glass-ceramic lixiviation. First, a parent glass of composition 55 SiO2 -5 Na2O -17 ZnO -23 Ga2O3 (%mol) was synthetized by conventional melt quenching technique and, after milling, was crystallized at 1100°C for 1 min. The resulting glass-ceramic with a ZnGa2O4 crystalline fraction of ~50%wt. was then lixiviated in a basic media to dissolve selectively the glass matrix. The ZnGa2O4 crystalline nano-powder (20-60 nm) was then recovered by centrifugation, dried and densified by Spark Plasma Sintering. We showed that some residual amorphous silica part that comes from the slightly incomplete lixiviation process, surrounds some ZnGa2O4 grains. A transmittance of 60% at 4.5µm is obtained in the best ceramic sample.

The use of glass-ceramic lixiviation to obtain a powder of high crystalline nanocrystals was never reported before in the literature. This innovative approach constitutes a real breakthrough in the field of nanometric powder preparation, as it allows a highly crystalline nano powder to be obtained, which is usually not the case with sol-gel technique for example.

Moreover, it is well-known that glass ceramics enable the stabilization of novel and metastable phases. Therefore, this technique could be used to further synthetize new metastable powders that cannot be obtained by common solid-state synthesis routes.

Figure 1 :

 1 Figure 1: Scheme of the principle of the innovative synthesis way developed during this study and based on lixiviation of a nano glass-ceramic material (dissolution of the glass matrix to obtain nanocrystals sintered into a ZnGa2O4 transparent ceramic).

Figure 2 :

 2 Figure 2: SPS thermo-mechanical cycle applied to sinter ZnGa2O4 nanocrystals powder (Ø 8 mm graphite die).

Figure 3 :

 3 Figure 3: (a) XRD patterns of glass-ceramics heat-treated under air at 1100°C for 30 seconds (black), 1 minute (red), 3 minutes (blue) and 1 hour (green) (purple tick marks represent ZnGa2O4, JCPDS 71-0643). (b) TEM picture of glass-ceramic powder heat-treated at 1100°C for 1 min under air.

Figure 3 -

 3 Figure 3-b. As the duration or temperature of crystallization increases, the size of crystals

Figure 4 :

 4 Figure 4: (a) and (b) Lixiviated ZnGa2O4 powder observed by SEM. (c) Particle size distribution of the lixiviated powder. (d) and (e) TEM and SAED pattern shown in insert

Figure SI 1 .

 1 A Rietveld refinement has been performed on the lixiviated powder (FigureSI 2). It shows that (i) no decomposition of ZnGa2O4 occurred and (ii) the average crystalline size, 56 nm, remains the same as before lixiviation. These results, in good agreement with TEM observations showing sharp edges (Figure4-d), demonstrate that the nanocrystals were not damaged during the lixiviation process. A quantification of the amorphous phase content has been attempted using ZnO as an internal standard (FigureSI 2). This led to almost no amorphous phase detection, which, considering the error on the quantification, shows that very few amorphous phase (SiO2) remains in the final nanoceramic.In order to obtain a dense ZnGa2O4 ceramic, the lixiviated powder was then sintered by SPS according to the thermo-mechanical cycle depicted in Figure2. XRD measurement confirms the conservation of the zinc gallate spinel phase after SPS sintering (FigureSI 2).

Figure 5 -

 5 c) reveal that the big grains show a ZnGa2O4 composition while the domains including the small grains are constituted by ZnGa2O4 associated with a small quantity of silicon (0.5 to 1%at Si).

Figure 5 :

 5 Figure 5: SEM micrographs of SPS fresh fracture at low magnification (a) and high magnification (b). (c) EDS-SEM measurement performed on the ZnGa2O4 transparent ceramic obtained by SPS densification of lixiviated zinc gallate spinel nanocrystals (standard deviation ± 1 at%). (d) EBSD-SEM orientation map of the ceramic showing crystalline domains. (e) Pole figures showing texture in ZnGa2O4 ceramic.

Figure 6 :

 6 Figure 6: ZnGa2O4 SPS ceramic (a) STEM-HAADF image of FIB lamella, (b) Grain-size distribution performed on more than 500 crystals, (c) STEM-EDS elemental maps (d) STEM-EDS (d), (e), (f), (g) (e), (f), (g), HRTEM images and associated FFT showing different types of grain boundaries.

7 µm to 8. 5 µm.

 5 Absorption bands at 2 µm, 3.1 µm, 5.5 µm and after 7.2 µm are observed on the different transmittance spectra. The broad band located around 3 µm is due to the presence of free hydroxyl groups probably exacerbated by the lixiviation media[START_REF] Abe | Determination of combined water in glasses by infrared spectroscopy[END_REF]. It is noteworthy that the other bands are absent in the study made by C. Mével et al.[START_REF] Mével | First ZnGa2O4 Transparent Ceramics[END_REF] who prepared transparent ZnGa2O4 ceramic by ball-milling from nanometric and micrometric precursors and subsequent SPS sintering. These extra bands can thus be associated to the presence of a small amount of silica in the ceramic sample[START_REF] Efimov | IR absorption spectra of vitreous silica and silicate glasses: The nature of bands in the 1300 to 5000 cm 1 region[END_REF]. This amorphous SiO2 residual part could be found around the nanocrystals of zinc gallate spinel as shown by TEM observations (Figure4-e) and may result from an incomplete lixiviation process. The maximum of transmittance is ~ 60% at 4.5 µm.

Figure 4 -

 4 Figure4-a that affects the sinterability and thus the optical properties of the final ceramic[59,

Figure 7 :

 7 Figure 7 : Transmittance spectra of as-SPS sample (thickness: 1.2 mm) and photograph of SPS ZnGa2O4 translucent ceramic (Ø 8mm).

Table 1 :

 1 EDS-SEM measurement performed on the glass-ceramic and the lixiviated powder (standard deviation ± 1 mol%).

		SiO2 (%mol)	ZnO (%mol)	Ga2O3 (%mol)
	Glass ceramic (nominal)	57.9	17.9	24.2
	Glass ceramic (experimental)	58.5	16.9	24.6
	Lixiviated powder	3.8	47.5	48.7
	ZnGa2O4 (theoretical)	-	50	50

As the glass matrix dissolution of the glass-ceramic could lead to the partial decomposition of the zinc gallate spinel nano-crystals, XRD measurements have been performed on the resulting crystals. No changes in the XRD patterns are discernable as shown in

Acknowledgements

This work was supported by institutional grants from LabEX SigmaLim (ANR-10-LABX-0074-01). This work has benefited from the electron microscopy facilities of the Platform MACLE-CVL, which was cofounded by the European Union and Centre-Val de Loire Region (FEDER). The authors would like to thank the NEMATUUM project (AAP NA 2017-1R50313) for its financial support.

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