Norio Saito

Pierric Lemoine

Stéphane Cordier

Yoshiki Wada

Takeo Ohsawa

Noriko Saito

Fabien Grasset

Jeffrey Scott Cross

Naoki Ohashi

Norio Saito

Noriko Saito

Solvent-mediated purification of hexa-molybdenum cluster halide, Cs-2[Mo6Cl14] for enhanced optical properties

come

Introduction

The presence of impurities, such as water molecules and hydroxyl groups, is a possible cause of serious performance degradation and lifetime limitations of electrical and optical materials and structures. In particular, materials with strong ionicities often degrade through the incorporation of water molecules and/or hydroxyl groups. For instance, a number of papers have been published on the effects of moisture on the performance and lifetimes of solar cells involving perovskitetype lead halides. [START_REF] Christians | Transformation of the Excited State and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite upon Controlled Exposure to Humidified Air[END_REF]2 The use of a vacuum chamber at low water vapor pressure is sometimes insufficient to prevent the insertion of water-based impurities in solids, [START_REF] Miyase | Roles of Hydrogen in Amorphous Oxide Semiconductor In-Ga-Zn-O: Comparison of Conventional and Ultra-High-Vacuum Sputtering[END_REF] and heat treatment at high temperature, e.g., 1000 °C, is unable to remove hydroxyl groups from some crystalline solids. [START_REF] Watanabe | Hydrogen in Tin Dioxide Films and Bulk Ceramics: An Attempt to Identify the Most Hidden Impurity[END_REF] In general, heating is a common and efficient way of dehydrating thermally stable materials. However, materials with poor heat resistances, such as organic/inorganic hybrid systems, decompose even at relatively low temperatures, which are insufficient for dehydration. Hence, it is necessary to consider chemical approaches to prevent water and/or hydroxyl insertion into materials with poor heat resistances. In the present study, we have focused on improving the purities of metal-clustercomplex-based compounds by applying solvent-mediated dehydration techniques in order to observe the intrinsic properties of these compounds and to enhance their optical properties.

The hexamolybdenum cluster halide complexes ([Mo6X i 8X a 6] 2-, X = halogen; i = inner; a = apical), which are typical metal-cluster-complex-based compounds, have been widely utilized as building blocks not only for solid-state inorganic compounds, [START_REF] Simon | Clusters of Valence Electron Poor Metals-Structure, Bonding, and Properties[END_REF][START_REF] Perrin | Octahedral Clusters in Molybdenum(II) and Rhenium(III) Chalcohalide Chemistry[END_REF][START_REF] Perrin | The Molybdenum and Rhenium Octahedral Cluster Chalcohalides in Solid State Chemistry: From Condensed to Discrete Cluster Units[END_REF] but also supramolecular assemblies [START_REF] Kim | Novel Compounds Based on [Re6Q8(L)6] 4-(Q = S, Se, Te; L = CN, OH) and Their Applications[END_REF][START_REF] Ababou-Girard | Assembly of Hexamolybdenum Metallic Clusters on Silicon Surfaces[END_REF] and nanostructures, particularly organic/inorganic hybrid systems. [START_REF] Molard | Sensitization of Er 3+ Infrared Photoluminescence Embedded in a Hybrid Organic-Inorganic Copolymer Containing Octahedral Molybdenum Clusters[END_REF][START_REF] Amela-Cortes | Deep Red Luminescent Hybrid Copolymer Materials with High Transition Metal Cluster Content[END_REF] Since the size of a [Mo6X i 8X a 6] 2-complex is much larger (~1 nm) than an ordinary complex, and these complexes possess good solubilities in a variety of organic solvents, crystallization of [Mo6X i 8X a 6] 2-complexes involving organic counter cations [START_REF] Preetz | Synthesis, Structure and Properties of the Cluster Anions [(Mo6Cl i 8)X a 6] 2with X a ≡ F, Cl, Br, I[END_REF][START_REF] Preetz | Darstellung, Kristallstruktur und spektroskopische Eigenschaften der Clusteranionen[END_REF][START_REF] Brückner | Darstellung, Kristallstrukturen, NMR-, Schwingungsspektren und Normalkoordinatenanalyse der Clusteranionen [(Mo6I i 8)[END_REF] have been widely studied with the aim of functionalizing these compounds. One of the notable functions of a [Mo6X i 8X a 6] 2-complex is broadband red luminescence under excitation with blue or UV illumination. [START_REF] Maverick | Spectroscopic, Electrochemical, and Photochemical Properties of Molybdenum(II) and Tungsten(II) Halide Clusters[END_REF][START_REF] Maverick | Luminescence and Redox Photochemistry of the Molybdenum(II) Cluster Mo6Cl14 2[END_REF][START_REF] Mikhailov | Synthetic Tuning of Redox, Spectroscopic, and Photophysical Properties of {Mo6I8} 4+ Core Cluster Complexes by Terminal Carboxylate Ligands[END_REF] As such, the luminescence properties of these complexes are compatible with blue-LED-based solid-state lighting devices; however, their luminescence efficiencies as well as stabilities need to be improved before they can be used in practical devices.

As [Mo6X i 8X a 6] 2--complex-based crystals have large interstitial spaces in their lattices, the insertion of impurity molecules into these lattices is possible. Hence, the intercalation of impurity molecules is a potential cause of the degradation of the physical properties and stabilities of these materials, although it has not been well evaluated. While the trigonal form of 31 P c symmetry (Fig. 1(a)) is the stable form of Cs2[Mo6X14] at room temperature, [START_REF] Saito | Theoretical and Experimental Determination of the Crystal Structures of Cesium-Molybdenum Chloride[END_REF][START_REF] Saito | From Cs2Mo6Cl14 to Cs2Mo6Cl14•H2O and Vice Versa: Crystal Chemistry Investigations[END_REF][START_REF] Kirakci | Synthesis and Characterization of Cs2Mo6X14 (X = Br or I) Hexamolybdenum Cluster Halides: Efficient Mo6 Cluster Precursors for Solution Chemistry Syntheses[END_REF] recrystallization of Cs2[Mo6X14] in solvents such as acetone often results in the formation of the monoclinic form rather than the trigonal form. [START_REF] Saito | Theoretical and Experimental Determination of the Crystal Structures of Cesium-Molybdenum Chloride[END_REF][START_REF] Saito | From Cs2Mo6Cl14 to Cs2Mo6Cl14•H2O and Vice Versa: Crystal Chemistry Investigations[END_REF] The formation of the monoclinic form has been attributed to the intercalation of water molecules and the monoclinic phase was recently identified to be Cs2[Mo6Cl14]•H2O and belongs to the C2/c space group (Fig. 1(b)). [START_REF] Saito | From Cs2Mo6Cl14 to Cs2Mo6Cl14•H2O and Vice Versa: Crystal Chemistry Investigations[END_REF] It is also notable that the lattice parameters of trigonal Cs2[Mo6Cl14] vary with preparation conditions, [START_REF] Saito | Theoretical and Experimental Determination of the Crystal Structures of Cesium-Molybdenum Chloride[END_REF][START_REF] Healy | Crystal Structure of Dicaesium Octa-µ3-Chloro-Hexachloro-Octahedro-Hexa-Tungstate(II) and -Molybdate(II) Complexes[END_REF][START_REF] Potel | New Families of Ternary Molybdenum (II) Chlorides with Octahedral Mo6 Clusters[END_REF] and such variations in lattice parameters are likely due to the insertion of water molecules at the interstitial positions of the trigonal lattice. [START_REF] Saito | Theoretical and Experimental Determination of the Crystal Structures of Cesium-Molybdenum Chloride[END_REF] As disorder in a crystalline lattice potentially causes This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins the degradation of its luminescence properties, it is necessary to prepare cluster-complex compounds of very high purity, i.e., low concentrations of water impurities, and to investigate the effects of purity on their luminescence properties in order to further improve luminescence efficiency.

In this context, we realize that the synthesis of pure Cs2[Mo6X i 8X a 6] is of great importance for the research and development of these compounds. Hence, in this study, recrystallization experiments that explored efficient ways of obtaining high-purity Cs2[Mo6Cl14], free from water impurities, have been performed. In particular, three alcohols, methanol (MeOH), ethanol (EtOH), and 1-propanol (1-PrOH) were used as solvents, and the recrystallization behavior of Cs2[Mo6Cl14] in these solvents was investigated. Here, we assume that the strong hydrophilicity of the alcohol traps water molecules in the solvent and induces the crystallization of water-free Cs2[Mo6Cl14]. In order to provide evidence of the watertrapping ability of these alcohols, we used two types of precursors for the crystallization of Cs2[Mo6Cl14]. One is trigonal Cs2[Mo6Cl14] and is of relatively low water content, and the other is monoclinic Cs2[Mo6Cl14]•H2O and contains water molecules in its crystal structure. As a result, we found that trigonal Cs2[Mo6Cl14], devoid of water contamination, can be obtained using EtOH or 1-PrOH as the recrystallization solvent. The purification mechanism will be discussed in light of thermal and optical characterizations of compounds crystallized in different solvents.

Experimental

Precursors

Two kinds of [Mo6Cl14] 2--complex-based compound, Cs2[Mo6Cl14]•H2O and Cs2[Mo6Cl14] were prepared as precursors for the recrystallization experiments. Here, monoclinic Cs2[Mo6Cl14]•H2O was selected as the watercontaining precursor in order to examine the effect of solvent on the preparation of trigonal water-free Cs2[Mo6Cl14]. Preparation of trigonal Cs2[Mo6Cl14] and monoclinic Cs2[Mo6Cl14]•H2O was conducted as described previously. [START_REF] Saito | From Cs2Mo6Cl14 to Cs2Mo6Cl14•H2O and Vice Versa: Crystal Chemistry Investigations[END_REF][START_REF] Potel | New Families of Ternary Molybdenum (II) Chlorides with Octahedral Mo6 Clusters[END_REF][START_REF] Koknat | Convenient Synthesis of the Hexanuclear Molybdenum(II) Halides Mo6Cl12 and Mo6Br12•2H2O[END_REF] Preparation methods of the precursors are described in the supporting information.

Hereafter, crystalline phases having structures similar to Cs2[Mo6Cl14]•H2O and belonging to the monoclinic C2/c space group are referred to as the 'monoclinic phase' or 'monoclinic form', while phases of structure similar to Cs2[Mo6Cl14] belonging to the trigonal 31 P c space group are abbreviated to 'trigonal phase' or 'trigonal form' for convenience.

Recrystallization experiments

Purification of trigonal Cs2[Mo6Cl14] by removal of residual water was examined as follows. Firstly, we prepared suspensions of trigonal Cs2[Mo6Cl14] or monoclinic Cs2[Mo6Cl14]•H2O in the alcohols. Accordingly, the precursor powder, Cs2[Mo6Cl14] or Cs2[Mo6Cl14]•H2O (80 mg), was dispersed in 1.0 mL of methanol (MeOH), ethanol (EtOH), or 1-propanol (1-PrOH) in glass vessels. As mentioned below, while some of each precursor dissolved in the solvent, a residual amount of solid remained in the solvent, which is why the term "suspension" is used in this report. Each suspension was stirred at 200 rpm at 25 °C in a glove box filled with dry N2 gas to prevent unintentional incorporation of water molecules from the environment. After stirring for 10 min, 30 min, 2 h, or 12 h, the suspensions were centrifuged at 5000 rpm for 3 min to separate the remaining solids in the suspensions. Unless otherwise stated, the results detailed below refer to samples collected after stirring for 12 h. Each solid obtained in this manner was dried under vacuum in a desiccator for subsequent characterization.

During stirring for 12 h, the state of each suspension was monitored by measuring the time evolution of [Mo6Cl14] 2-complex solubility in every solvent. The supernatant liquid (20 μL) was collected from each suspension for optical absorption measurements. The collected solvents were diluted with EtOH and the concentration of the [Mo6Cl14] 2-complex was determined by ultraviolet-visible absorbance as described in the supporting information (Fig. S1).

Characterizations and analyses

Crystalline phases in the powders before and after recrystallization were identified by powder X-ray diffraction (XRD). The observed profiles were indexed by comparison with simulated profiles generated by the RIETAN-FP program [START_REF] Izumi | A Rietveld-Analysis Program RIETAN-98 and its Applications to Zeolites[END_REF] on the basis of the structural parameters of Cs2[Mo6Cl14] in its trigonal form, [START_REF] Saito | Theoretical and Experimental Determination of the Crystal Structures of Cesium-Molybdenum Chloride[END_REF][START_REF] Saito | From Cs2Mo6Cl14 to Cs2Mo6Cl14•H2O and Vice Versa: Crystal Chemistry Investigations[END_REF] and Cs2[Mo6Cl14]•H2O in its monoclinic form. [START_REF] Saito | From Cs2Mo6Cl14 to Cs2Mo6Cl14•H2O and Vice Versa: Crystal Chemistry Investigations[END_REF] The parameters used for these simulations are summarized in Tables S1 andS2 in the supporting information. In order to identify the molecules incorporated into the samples, thermal desorption spectroscopy (TDS) and Fourier transform infrared spectroscopy (FT-IR) were conducted on each sample. In addition, optical properties, photoluminescence (PL) and time-resolved-photoluminescence (TRPL) spectra, were acquired in order to determine the effect of purification on physical properties. The observed TRPL profiles were fitted to exponential decay functions to evaluate luminescence lifetimes. Details of these characterizations can be referred in the supporting information.

Calculations

The electronic structures of the trigonal and monoclinic phases were studied by density functional theory (DFT) calculations. In particular, a revision of the generalized gradient approximation, PBE-Sol, [START_REF] Perdew | Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces[END_REF] implemented in a plane-wave based pseudo-potential calculation code, the CASTEP code, [START_REF] Clark | First Principles Methods Using CASTEP[END_REF] was selected for the simulation study. The calculation conditions are similar to those used in our previous study; [START_REF] Saito | Lattice and Valence Electronic Structures of Crystalline Octahedral Molybdenum Halide Clusters-Based Compounds[END_REF] however, in the present study, relativistic effects were included through the use of pseudo-potentials generated by a procedure developed by Koelling and Harmon. [START_REF] Koelling | A Technique for Relativistic Spin-Polarised Calculations[END_REF] Stable lattice parameters were obtained by geometry optimization calculations and, subsequently, energy band structures (E-k dispersions), densities-of-states (DOSs), and projected-densities-of-states (PDOSs) of the relaxed structures were calculated. Details of the methods used This journal is © The Royal Society of Chemistry 20xx

CrystEngComm | 3

Please do not adjust margins Please do not adjust margins during the DFT simulations are described in the supporting information.

Results and discussion

Identification of reaction products

Precursors

The monoclinic-phase precursor was clearly obtained as a single phase. On the other hand, the powder prepared with the aim of obtaining the trigonal phase contained significant amounts of the monoclinic phase. Subsequent heat treatment at 150 °C induced transformation of the residual monoclinic phase to the trigonal phase but very small amount of the monoclinic phase was remained. As perfect removal of monoclinic phase was not achieved in this study, the trigonal phase powder heated at 150 °C is regarded as a precursor of the trigonal phase in which the concentration of water is clearly lower than the precursor of the monoclinic phase. The detailed results of phase identification for the prepared precursor compounds are described in the supporting information (see Fig. S2).

As previously reported, it is noteworthy that the monoclinic phase decomposed to the trigonal phase during storage. [START_REF] Saito | From Cs2Mo6Cl14 to Cs2Mo6Cl14•H2O and Vice Versa: Crystal Chemistry Investigations[END_REF] The XRD pattern shown in Fig. S2(c) was obtained from a sample of the prepared monoclinic-phase powder that was kept for more than 4 weeks in a dry atmosphere; it is clear that the sample had evolved into the trigonal form, Cs2[Mo6Cl14]. This indicates that the monoclinic phase, Cs2[Mo6Cl14]•H2O, is not very stable in a dry atmosphere. However, the complete decomposition of the monoclinic phase to pure Cs2[Mo6Cl14] by thermal treatment was not successful, as evidenced by the XRD pattern shown in Fig. S2(e). Hence, these results are good examples that demonstrate the difficulty in dehydrating Cs2[Mo6Cl14].

Solids collected from the suspensions

Here, we discuss the powder collected from suspensions of the [Mo6Cl14] 2-complexes in alcohol solvents after stirring for 12 h. It is noteworthy that the amount of precursor dispersed in each solvent was enough to maintain solids in the suspension; in other words the amount of precursor dispersed in the solvent exceeded its solubility. The XRD pattern of the powder collected from each suspension is shown in Fig. 2, which compares the XRD patterns acquired from powders obtained using different precursors. For the suspension in EtOH, the collected powder was identified to be the trigonal phase regardless of the precursor dispersed in the solvent, as shown in Fig. 2(a). It is interesting to note that the trigonal phase, i.e., Cs2[Mo6Cl14], crystallized in the suspension even when the monoclinic phase, i.e., Cs2[Mo6Cl14]•H2O, was used. This means that water molecules are removed from the crystal structure leading to its transformation from the monoclinic phase to the trigonal phase in the EtOH suspension. Similar behavior was observed for the 1-PrOH suspension. In fact, the powder collected from the 1-PrOH suspension was uniquely trigonal phase regardless of the crystalline form of the precursor, as shown in Fig. 2(b). It should be noted that reproducibility was not very high when 1-PrOH was used as the solvent. Indeed, in some experiments, the transformation from Cs2[Mo6Cl14]•H2O to Cs2[Mo6Cl14] was not complete after stirring for 12 h, suggestive of a kinetic effect. This kinetics issue will be discussed in the latter part of this section.

On the other hand, the results for the MeOH suspensions are opposite to those involving EtOH and 1-PrOH; the solid collected from the MeOH suspension was identified to be the monoclinic phase, as seen in Fig. 2(c). From the XRD patterns in this figure, the presence of a trace amount of the trigonal phase, 2θ = 12.12°, is present in the sample collected from the suspension prepared with the trigonal-phase precursor (Fig. 2(c)). It is interesting to note that the use of acetone for purification resulted in the formation of a mixed phase, containing both monoclinic and trigonal phases, while the use of the alcohol solvents in this study enabled single phases, i.e., trigonal using EtOH or 1-PrOH, and monoclinic using MeOH, to be obtained. These results indicate that the selective growth of either single trigonal or monoclinic-phase samples can be achieved using an appropriate recrystallization solvent.

The lattice parameters of the recrystallized trigonal phases determined from the XRD results by the least square method (Table 1) indicate that the unit cell volumes of the trigonal phases recrystallized from EtOH and 1-PrOH are smaller than that of the trigonal-phase precursor. The smaller unit cell volume is due to the relatively small a-axis length. In particular, the lattice parameters of the precipitates collected from the 1-PrOH suspensions were almost identical irrespective of whether the monoclinic or trigonal-phase precursor was used. On the other hand, the unit cell volume of the trigonal phase obtained from the EtOH suspensions is slightly, but evidently, larger than that obtained from the trigonal phase collected from the 1-PrOH suspensions. In addition, the unit cell volume of the trigonal phase recrystallized from EtOH depended on the precursor. In fact, the unit cell volume of the sample collected from the EtOH suspension was larger when crystallized using Cs2[Mo6Cl14]•H2O as the precursor. As mentioned in our previous report, [START_REF] Saito | Theoretical and Experimental Determination of the Crystal Structures of Cesium-Molybdenum Chloride[END_REF] expansion of the trigonal-phase lattice is attributable to the insertion of impurities, such as water molecules. Hence, we presume that crystallization from 1-PrOH as the solvent is the appropriate way of achieving a highpurity trigonal phase. The use of EtOH as solvent is also a possible way of obtaining a single-phase powder of the trigonal phase, however the slightly larger lattice parameters observed for the sample recrystallized from EtOH suggests that the use of 1-PrOH as the solvent has advantages during the purification of the trigonal phase. The following discussion on the TDS and FT-IR results supports this hypothesis; particularly how the behavior of the lattice parameters correlates with the presence of water molecules inserted into the lattice of trigonal phase. As the signal at m/z = 12, corresponding to the [ 12 C] fragment, had no peak at 236 °C, it is presumed that the m/z = 16 peak at 236 °C does not correspond to [CH4], but rather to the [O] fragment. With these assignments, the peaks at 132 °C and 236 °C are attributed to the desorption of water; it is likely that the peak at 132 °C corresponds to surface adsorbates, and that at 236 °C to water molecules inserted into the lattice of the trigonal phase. This assignment is consistent to the XRD results. Indeed, the lattice parameters of the trigonal phase collected from EtOH vary slightly with preparation conditions, as indicated in Table 1, and the origin of this variation is attributed to lattice expansion resulting from the insertion of water molecules.

The TDS profile of the trigonal-phase powder collected from 1-PrOH also exhibits three peaks, as shown in Fig. S3(a) in the supporting information, however this profile differs from that of the sample prepared using EtOH. The assignment of those desorption peaks (see supporting information) indicated that the trigonal-phase powder collected from 1-PrOH does not show any trace of water desorption from its crystalline lattice, although the sample prepared with EtOH showed evidence of water desorption, as described above; this assignment is also consistent with the XRD results. Although signal for m/z = 18 fragment was found in TDS spectra of the sample prepared in 1-PrOH suspension, that was identified to be a fragment caused by decomposition of alcohol molecule [START_REF] Munson | Chemical Ionization Mass Spectrometry. I. General Introduction[END_REF] not to be a fragment from water molecule. Indeed, the trigonal phase collected from 1-PrOH exhibits the smallest lattice parameters, which is attributable to very low concentrations of intercalated water. Hence, we presume that water intercalation into the trigonal form is preferentially suppressed when 1-PrOH is used as the recrystallization solvent.

The TDS spectra of the monoclinic phase prepared using MeOH and assignments for the observed desorption peaks are also shown in Fig. S3(b) in the supporting information. As this sample was identified to be Cs2[Mo6Cl14]•H2O, we expected to see a significant desorption of water molecules. However, the m/z = 18 signal intensity for this material was much lower than that observed for the trigonal phase prepared using EtOH as described in the supporting information. With these assignments in mind, it is very likely that the water molecules in Cs2[Mo6Cl14]•H2O are not removed during this TDS analysis; we presume that the emission of water from solids collected from the various suspensions are only visible for the trigonal phase collected from EtOH. Very week desorption signal at m/z = 18 for the sample prepared in MeOH suspension will be discussed in relation with the results of FT-IR measurements. Summarizing the results of TDS measurement, it was confirmed that water concentration in the sample prepared in 1-PrOH suspension was very low compared to that prepared in the EtOH suspension.

Figure 4 summarizes the FT-IR spectra for the samples prepared from the different precursors in the different solvents. We first focus on the absorptions observed around 3350-3700 cm -1 shown in Fig. 4(a); these bands are attributed to various O-H stretching (OH-s) modes. [START_REF] Lutz | X = Cl, Br, I). A New Interpretation of the Frequency Shiftings of OH Stretching Modes in Solid Hydrates[END_REF][START_REF] Kubas | Dihydrogen: A Better Ligand Than Water? IR and X-ray Evidence for Aquo Coordination in W(CO)3(PR3)2(H2O), Thermodynamics of H2O Binding versus .eta. 2-H2 Binding and H2O/D2 Isotopic Exchange. Implications for the Biological Activation of Hydrogen[END_REF][START_REF] Plyler | Infrared Spectra of Methanol, Ethanol, and n-Propanol[END_REF][START_REF] Inumaru | Molybdenum Cluster Halide Compound Mo6Cl12(OH2)2 with Six-Handed Linkage Hydrogen Bonding[END_REF] In this figure, it is evident that the monoclinic phase prepared from MeOH exhibits a sharp and intense peak centered at 3572 cm -1 . This peak appears to be associated with shoulders at around 3621 and 3492 cm -1 . According to previous reports, [START_REF] Inumaru | Molybdenum Cluster Halide Compound Mo6Cl12(OH2)2 with Six-Handed Linkage Hydrogen Bonding[END_REF][START_REF] Larsen | Hydrogen-Bonded OH Stretching Modes of Methanol Clusters: A Combined IR and Raman Isotopomer Study[END_REF][START_REF] Watanabe | Isotope Tracer Investigation and Ab-initio Simulation of Anisotropic Hydrogen Transport and Possible Multi-Hydrogen Centers in Tin Dioxide[END_REF] the OH-s mode frequency is sensitive to structural restrictions and tends to decease with increasing restrictive forces. Hence, the set of three peaks at 3621, 3572, and 3492 cm -1 are attributable to water molecules incorporated into the monoclinic phase.

Interestingly, the OH-s band at 3640 cm -1 is relatively pronounced and the other two peaks at around 3572 and 3492 cm -1 are relatively suppressed in the spectrum of the trigonal precursor crystalized using acetone as solvent, which corresponds to the sample described in the supporting information (Fig. S2(d)) in which the monoclinic phase was detected as an impurity. As the OH-s mode frequency tends to decease with increasing restrictive forces, [START_REF] Inumaru | Molybdenum Cluster Halide Compound Mo6Cl12(OH2)2 with Six-Handed Linkage Hydrogen Bonding[END_REF][START_REF] Larsen | Hydrogen-Bonded OH Stretching Modes of Methanol Clusters: A Combined IR and Raman Isotopomer Study[END_REF][START_REF] Watanabe | Isotope Tracer Investigation and Ab-initio Simulation of Anisotropic Hydrogen Transport and Possible Multi-Hydrogen Centers in Tin Dioxide[END_REF] we conclude that water molecules inserted into the trigonal phase are rather free, while hydrogen bonding between Cl and water is likely to be stronger in the monoclinic phase.

On the other hand, the FT-IR profiles of the OH-s spectral regions of the trigonal phase prepared with EtOH and 1-PrOH were weak and broadened. Indeed, the two broad peaks found at around 3290-3500 cm -1 are characteristic of the trigonal phase prepared with EtOH and 1-PrOH. [START_REF] Plyler | Infrared Spectra of Methanol, Ethanol, and n-Propanol[END_REF][START_REF] Larsen | Hydrogen-Bonded OH Stretching Modes of Methanol Clusters: A Combined IR and Raman Isotopomer Study[END_REF] The FT-IR spectra of trigonal-phase precursor recrystallized from acetone possess between the monoclinic phase prepared using MeOH, and the trigonal phase prepared with EtOH or 1-PrOH. This means that while the set of peaks at 3640, 3572, and 3492 cm -1 was observed in the spectrum of the precursor prepared with acetone, and which dominates the spectrum of the trigonal phase, the two peaks at around 3290-3500 cm -1 are absent in the monoclinic phase prepared using MeOH.

Similar behavior was observed in the O-H bending (OH-b) mode region, as shown in Fig. 4(b). According to the literature, peaks at around 1600 cm -1 are assignable to OH-b modes. [START_REF] Lutz | X = Cl, Br, I). A New Interpretation of the Frequency Shiftings of OH Stretching Modes in Solid Hydrates[END_REF][START_REF] Inumaru | Molybdenum Cluster Halide Compound Mo6Cl12(OH2)2 with Six-Handed Linkage Hydrogen Bonding[END_REF] A sharp peak at 1612 cm -1 is observed in the spectrum of the monoclinic phase prepared using MeOH, while peaks at 1595-1601 cm -1 are observed in the spectrum of the trigonal phase prepared using EtOH or 1-PrOH. Looking at the FT-IR peak profile for the trigonal precursor prepared with acetone, a broad peak with long tails to both lower and higher frequencies is observed. This feature is likely to be due to the presence of the monoclinic phase as a minor impurity, and the presence of water incorporated into the trigonal phase. Summarizing the FT-IR observations in the OH-s and OH-b ranges, it is clear that the states of the OH groups, likely to be associated with water molecules, are different in the trigonal and monoclinic phases and, interestingly, the trigonal-phase precursor containing a minor amount of the monoclinic phase exhibited behavior intermediate between pure monoclinic and trigonal phases.

Figure 4(b) also compares the FT-IR spectra of the OH-b mode ranges for samples before and after the TDS experiments. Firstly, it is interesting to note that the FT-IR spectrum of the monoclinic phase prepared using MeOH exhibited no obvious change after heating under vacuum during the TDS analysis. This indicates that water molecules associated with the Please do not adjust margins Please do not adjust margins monoclinic phase were not removed by heating under vacuum. Indeed, the position of the peak at 1612 cm -1 was unchanged after TDS. The XRD profile of the monoclinic-phase powder after TDS could not be assigned because the pattern was completely different to that from either the monoclinic or trigonal phase. We conclude that the monoclinic phase is transformed into another crystal structure that retained water molecules in its lattice even after TDS, and is consistent with TDS results that show insignificant water desorption.

On the other hand, the spectral profile of the OH-b mode of the trigonal phase prepared with 1-PrOH had almost disappeared after heating under vacuum. This is different from the behavior of the trigonal phase crystallized from EtOH, in which the peak at 1601 cm -1 was observed to decrease upon heating under vacuum, but was still significantly present. As peaks around 1595-1612 cm -1 are commonly found in both monoclinic and trigonal phases, we attribute the peak at around 1610 cm -1 to water molecules inserted in the [Mo6Cl14] 2-complex lattice. Hence, the presence of a small water-related peak in the spectrum of the trigonal phase prepared with EtOH is consistent with the fact that water desorption was observed from this sample in the TDS experiment presented in Fig. 3. Here, the FT-IR results support those obtained by XRD. As mentioned, the trigonal phase prepared with EtOH and 1-PrOH exhibited smaller lattice parameters that were dependent on the precursor compound. On the other hand, the trigonal precursor prepared using acetone exhibits a relatively large unit cell, with cell parameters that depended on the preparation conditions. [START_REF] Saito | Theoretical and Experimental Determination of the Crystal Structures of Cesium-Molybdenum Chloride[END_REF] The FT-IR results reveal that OH-s and OH-b modes were pronounced in the samples prepared with acetone or MeOH as solvent, but were minor in the samples prepared with EtOH or 1-PrOH. Hence, we attribute these variations in the lattice parameters of the trigonal phase to variation in the amount of water incorporated into the trigonal-phase lattice. As the TDS experiments also revealed the absence of water desorption from the trigonal Cs2[Mo6Cl14] prepared with 1-PrOH, we can safely conclude that the preparation of a high-purity trigonal phase devoid of inserted water molecules is favoured by 1-PrOH as solvent.

Presence of residual alcohol related molecule is another important issue. As described in the supporting information (Fig. S4), extra peaks relating to residual alcohol related modes were clearly seen in the spectra for the sample collected from EtOH and 1-PrOH suspension. Those results supports identification of TDS profile shown above.

Kinetics and mechanism of recrystallization and dehydration

The results discussed above indicate that solvent-mediated purification, i.e., the formation of a single-phase powder of the dehydrated trigonal phase, is possible by the appropriate choice of recrystallization solvent. We now discuss the kinetics and mechanism of the purification process, through time-evolution experiments. Figure 5 Here, we attribute the transient solubility behavior to the dependency of the partition coefficient between water and the solvent, and the solid in suspension. It is likely that water molecules are more stable when dissolved in 1-PrOH or EtOH than when inserted into solids, while we assume the water molecules inserted into solids are more stable than those dissolved in MeOH. Hence, the transient solubility behavior is attributed to the recovery from an unstable state to stable state by removal of water molecules from the solid, or by intercalating water molecules into the solid. In the case of MeOH, once trigonal-phase [Mo6Cl14] 2-is dissolved in the solvent it enables crystallization of the monoclinic phase by trapping residual water molecules from the solvent. On the other hand, in the case of EtOH and 1-PrOH, once the monoclinic-phase [Mo6Cl14] 2-is dissolved in the solvent it enables crystallization of the trigonal phase by releasing water This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins molecules into the solvent. This explains the observed transient solubility behavior.

For 1-PrOH in particular, the very long time taken for the solubility to converge is attributed to a very slow recrystallization rate. The solid phase collected from the Cs2[Mo6Cl14]•H2O/1-PrOH suspension after stirring for 12 h was sometimes pure-trigonal, while a mixture of the trigonal and monoclinic phases was obtained at other times. Such relatively poor reproducibility is due to the incomplete convergence of the [Mo6Cl14] 2-solubility when 1-PrOH was used as solvent. As [Mo6Cl14] 2-solubility decreases in moving from MeOH to 1-PrOH, it is likely that the slow recrystallization rate observed for the Cs2[Mo6Cl14]•H2O/1-PrOH suspension is due to the slow rate of dissolution of Cs2[Mo6Cl14]•H2O into 1-PrOH. Hence, the evolution of [Mo6Cl14] 2-solubility corresponds to the recovery from the nonequilibrium state to the equilibrium state in terms of partition coefficient, presumably because the rate of recrystallization is limited by the diffusion of [Mo6Cl14] 2-in the solvent.

From the viewport of utilizing this process for practical use, it is worthwhile that the supernatant solution obtained in the recrystallization experiment is reusable. Repeatability depends on the partition coefficient between water, solvents and Cs2[Mo6Cl14] solids in the suspension. Since the concentration of water in the saturated solution becomes higher every reuse, repetition will be possible until the concentration of water in the reused solution becomes over a limit that can separate pure Cs2[Mo6Cl14] solids and water. Considering the solvent dependency of the partition coefficient, 1-PrOH will perform better than EtOH. As amount of water molecule is sensitive to water concentration in alcohol as descried in a literature, [START_REF] Flemström | The structures of X2[(Mo6Cl8)Cl6]•nH2O, X=NH4, K, Rb, Cs[END_REF][START_REF] Flemström | Effects from hydrogen bonds on water structure in (H3O)2[Mo6Cl8X6]•yH2O X=Cl (y=7), Br (y=6), or I[END_REF] we have to pay strong attention to water concentration in alcohol when considering reuse of the solvent for purification.

Here, we provide an explanation for why the monoclinic phase crystallizes from MeOH even when the trigonal-phase precursor is used; we assume that residual water molecules in the MeOH are trapped in the solid phase to form the monoclinic phase when the solvent dependency of the partition coefficient is considered. The transformation of 80 mg of Cs2[Mo6Cl14] into Cs2[Mo6Cl14]•H2O requires approximately 0.06 mmol of water. Considering the solubility of [Mo6Cl14] 2-in MeOH, more than 0.04 mmol of water is necessary to obtain pure Cs2[Mo6Cl14]•H2O in the suspension made from the Cs2[Mo6Cl14] precursor and MeOH. Hence, the MeOH used in this study (1.0 mL in volume) must contain 0.04 mmol or more of water, as pure Cs2[Mo6Cl14]•H2O was obtained from the Cs2[Mo6Cl14] precursor. As the residual concentration of water in commercially available MeOH is more than this assumed value. [START_REF] Burfield | Desiccant Efficiency in Solvent and Reagent Drying. 7. Alcohols[END_REF] Hence, it is reasonable to attribute the crystallization of Cs2[Mo6Cl14]•H2O from MeOH to the presence of residual water in the MeOH. In addition, we should mention a possibility that some MeOH molecules are inserted to the monoclinic phase. Indeed, as indicated in Table S3, the refined lattice parameters of the monoclinic phases collected from MeOH were likely to be slightly larger than those we previously studied by single-crystal XRD. [START_REF] Saito | From Cs2Mo6Cl14 to Cs2Mo6Cl14•H2O and Vice Versa: Crystal Chemistry Investigations[END_REF] It is an implication that a part of water molecule of Cs2[Mo6Cl14]•H2O was substituted by MeOH. Thus, the monoclinic phase collected from the MeOH is considered a water/MeOH mixed solvate.

Luminescence properties

On the basis of the discussion presented above, recrystallization from EtOH or 1-PrOH inhibits the intercalation of water molecules in the Cs2[Mo6Cl14] lattice even when precursors containing small quantities of water impurities are used, which means that the solvent-mediated purification of Cs2[Mo6Cl14] can be achieved by employing EtOH or 1-PrOH as the solvent. Subsequent PL and TRPL measurements revealed that the luminescence efficiency of Cs2[Mo6Cl14] was significantly improved by this purification process, although the PL spectra were almost unchanged by the presence of water molecules. Figure 6(a) compares the PL spectra of the trigonal and monoclinic phases recrystallized from EtOH and MeOH, respectively, which exhibit peaks centered at λ = 764 nm for the trigonal phase, and λ = 768 nm for the monoclinic phase; these spectral features were retained irrespective of the presence of water molecules in the crystal structure. On the other hand, the TRPL profiles recorded over the 740-800 nm range (Fig. 6(b)) suggest that the trigonal phases recrystallized from EtOH and 1-PrOH have very similar decay profiles, with relatively longer lifetimes than that of the monoclinic phase recrystallized from MeOH. In addition, the TRPL profile of the trigonal-phase precursor obtained with acetone exhibits relatively fast decay behavior similar to that of the monoclinic phase recrystallized from MeOH. For a quantitative comparison, the decay constants (τPL) estimated by exponential curve fitting of the TRPL profiles are summarized in Table 2. The τPL values of the trigonal forms recrystallized from EtOH and 1-PrOH are ~310 μs. In contrast, the τPL value of the monoclinic phase recrystallized from MeOH is lower, at 184 μs, and this value is close to that of the trigonal-phase precursor, namely 195 μs.

It is interesting to note that the monoclinic phase prepared with MeOH and the trigonal-phase precursor prepared with acetone exhibit very similar behavior and their luminescence lifetimes are much shorter than those of the trigonal phases prepared with EtOH or 1-PrOH. As mentioned, the trigonal precursor has relatively larger unit cell parameters (see Table 1), and this expansion of the trigonal-phase lattice has been attributed to water molecule insertion, as discussed above. Hence, we can safely conclude that the association of water molecule into these Cs2[Mo6Cl14]-based compounds reduces their luminescence lifetimes, and that the enhanced luminescence lifetimes of the trigonal phases obtained with EtOH or 1-PrOH are due to the elimination of inserted water through solvent-mediated purification. In other words, the solvent-mediated purification of Cs2[Mo6Cl14] via recrystallization from EtOH or 1-PrOH is an appropriate method for improving luminescence efficiency.

Electronic structure

The DFT simulations of both trigonal and monoclinic phases were well converged and the optimized lattice structures of these two compounds are listed in Table S4. Comparisons of Please do not adjust margins Please do not adjust margins the data in Tables S1, S2, and S4 reveal that the lattice structures optimized by DFT calculations are comparable to the experimental structures. Hence, these simulations appear to provide sufficient accuracy and are reliable enough for structure-property-relationship discussions. It is notable that the insertion of a water molecule into trigonal Cs2[Mo6Cl14], to form Cs2[Mo6Cl14]•H2O, is calculated to be an exothermic reaction (16.5 kcal/mol) at sufficiently high water vapor pressure, as evidenced by the difference in the total energies of the trigonal and monoclinic phases.

Figure 7 shows the E-k dispersion of each phase, and the PDOSs corresponding to each band structure is depicted in Fig. S5. The energy band structures shows very small dispersions in E-k space, and the band structures of the trigonal and monoclinic forms are nearly identical, indicating that the electronic state of the Mo6 cluster in the [Mo6Cl14] 2-complex, which is considered to play the main role during luminescence, [START_REF] Costuas | Combined Theoretical and Time-Resolved Photoluminescence Investigations of [Mo6Br i 8Br a 6] 2-Metal Cluster Units: Evidence of Dual Emission[END_REF] is highly localized; consequently the presence of water molecules do not cause significant changes in the electronic structure of the Mo6 cluster. As the Mo6 cluster is composed of a cage of fourteen Cl -ions, the electronic structure of the cluster is well screened by the cage. Indeed, the PL spectra of the trigonal and monoclinic phases are very similar to each other, despite the fact that the monoclinic phase contains water molecules in its crystal structure. When we examine the electronic-structural details, there is a clear difference between the trigonal and monoclinic phases, as indicated by the arrows in Fig. 7. It appears that the energy bands near the valence band maximum (VBM) are more localized in the monoclinic phase containing water. This feature is attributed to charge transfer from the water molecule to the Mo6 cluster. The population analysis data listed in Table 3 indicate that the water molecule in the monoclinic phase is slightly negatively charged and this negative charge is compensated for by a decrease in the negative charge on the [Mo6Cl14] 2-complex. Hence, the slight differences in the luminescence spectra of the monoclinic and trigonal phases result from slight changes in the electronic structure of the [Mo6Cl14] 2-complex that originates from the charge of the water molecule inserted there.

Regarding the TRPL behavior, the calculated results indicate that the presence of water may not be responsible for the observed significant reduction in τPL. Indeed, as shown in Fig. 8, the interatomic distance between H and Cl in the assumed structure is 0.23 nm and the overlap of the Cl2p and H1s orbitals is very small, which means that the probability of electron transfer between H and Cl is negligible. However, it should be remembered that thermal vibrations were not considered in the DFT simulations performed in this study. As the TRPL measurements were performed at room temperature, there is a probability that H2O interacts with Cl through thermal vibrations. As the difference between the values of τPL for the water-containing and dehydrated samples is less than a factor of two, this thermal-vibration assumption for the interaction between the [Mo6Cl14] 2-complex and water is likely to sufficiently explain the change in τPL observed upon insertion of a water molecule.

At the present time, the non-stoichiometries of these metalcluster-complex-based compounds have not been well investigated. For instance, the substitution of H3O + ions for Cs + ions may also possibly degrade luminescence properties. Hence, further detailed characterization of the materials, as well as further simulations that predict possible alternative mechanisms that govern luminescence properties, need to be considered in order to obtain a deeper understanding of the physical and chemical properties of these metal-cluster-complex based compounds. As the highly screened electronic structures of their metal-cluster cores are significant advantages that make these complexes useful in optical applications, further studies that reveal the intrinsic properties of related metal-cluster compounds will provide inspiration for practical engineering applications of these compounds.

Conclusions

The solvent-mediated purification of trigonal Cs2[Mo6Cl14] was examined with the aim of obtaining high luminescence efficiency. In particular, we used the hydrophilicities of alcoholic solvents to purify Cs2[Mo6Cl14] by dehydration. The use of EtOH and 1-PrOH as the recrystallization solvent resulted in the crystallization of pure trigonal Cs2[Mo6Cl14], while the use of MeOH induced the formation of monoclinic Cs2[Mo6Cl14]•H2O. This crystallization behavior is explained by assuming different partition coefficients between water and the various alcohols. We demonstrated that EtOH and 1-PrOH, particularly 1-PrOH, behave as dehydrating agents by trapping water molecules during the recrystallization process. On the other hand, MeOH works as a hydrating agent by providing residual water that intercalates into Cs2[Mo6Cl14]. Characterization of the resultant powders by XRD, TDS and FT-IR techniques reveal that recrystallization from 1-PrOH is an efficient way of purifying the trigonal phase. Finally, improvements in luminescence efficiency through dehydration were confirmed by TRPL experiments in which the trigonal phase purified in EtOH or 1-PrOH exhibited much longer luminescence lifetimes than the compounds contaminated by water. Hence, we conclude that 1-PrOH is a very useful solvent for the recrystallization of high purity Cs2[Mo6Cl14]. The enhancement in luminescence efficiency through the purification procedure demonstrated in this study encourages the further development of metal-cluster-complex-based compounds for optoelectronic applications. This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins would also like to thank Prof. Junzo Tanaka of Tokyo Tech for support and encouragement.

The study was financially supported by Saint-Gobain Co. Ltd. (Paris, France), NIMS, CNRS, Université de Rennes 1, Region Bretagne (France), and Fondation Langlois. Please do not adjust margins Please do not adjust margins Please do not adjust margins Please do not adjust margins Please do not adjust margins Please do not adjust margins

Figure 3

 3 depicts the normalized TDS profile of the trigonalphase sample collected from EtOH, which exhibits three major desorption peaks. Peaks centered at 183 °C are associated with fragments with m/z = 15, which corresponds to [ 12 C 1 H3], and m/z = 46, which corresponds [ 12 C2 1 H6 16 O]. Hence, the peak at 183 °C is attributed to the desorption of EtOH. On the other hand, the peaks observed at 132 °C and 236 °C are dominated This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins by fragments with m/z = 18, corresponding to [ 1 H2 16 O], and m/z = 16, corresponding to [ 16 O] and/or [ 12 C 1 H4].

  shows the evolution of the [Mo6Cl14] 2- concentration in the supernatants of the suspensions with time, in which six curves with different precursor/solvent combinations are depicted. First, we focus on the results for MeOH as the solvent. The [Mo6Cl14] 2-concentration in the supernatant of the Cs2[Mo6Cl14]/MeOH combination decreased with time, while that for the Cs2[Mo6Cl14]•H2O/MeOH combination remained constant during the stirring time. After stirring for 5 h, the [Mo6Cl14] 2-concentration for the Cs2[Mo6Cl14]/MeOH combination converged to that of the Cs2[Mo6Cl14]•H2O/MeOH combination. In other words, transient solubility behavior was observed for the Cs2[Mo6Cl14]/MeOH combination, where the solid phase in the suspension transformed from the monoclinic phase to the trigonal phase. On the other hand, the opposite behavior was observed when EtOH was used as solvent. Indeed, the concentration of [Mo6Cl14] 2-in the supernatant of the Cs2[Mo6Cl14]•H2O/EtOH combination decreased with time, while that for the Cs2[Mo6Cl14]/EtOH combination remained constant, irrespective of stirring time. The [Mo6Cl14] ]•H2O/EtOH combination converged to the value for the Cs2[Mo6Cl14]/EtOH combination after stirring for 12 h. The observed transient behavior in the Cs2[Mo6Cl14]•H2O/EtOH combination corresponds to the transformation from the monoclinic phase to the trigonal phase. The evolution of solubility in the 1-PrOH solvent with time was similar to that observed for the EtOH solvent. Indeed, the solubility of [Mo6Cl14] 2-in the supernatant of the Cs2[Mo6Cl14]•H2O/1-PrOH combination gradually decreased with time, in a similar fashion to that observed for the Cs2[Mo6Cl14]•H2O/EtOH combination. The solubility of the Cs2[Mo6Cl14]/1-PrOH combination appears to be not very stable and remained lower than that of the Cs2[Mo6Cl14]•H2O/1-PrOH combination. Scatter in the [Mo6Cl14] 2-solubility data for the Cs2[Mo6Cl14]/1-PrOH combination with time was due to technical difficulties, as the very low solubility of [Mo6Cl14] 2- in the 1-PrOH solvent causes results in a low signal/noise ratio upon measurement. While the [Mo6Cl14] 2-solubility of the Cs2[Mo6Cl14]•H2O/EtOH combination converged to that for the Cs2[Mo6Cl14]/EtOH combination, as mentioned above, the [Mo6Cl14] 2-solubility for the Cs2[Mo6Cl14]•H2O/1-PrOH combination remained higher than that for the Cs2[Mo6Cl14]/1-PrOH combination, even after stirring for 12 h.

FiguresFigure 1

 1 Figures

Figure 2

 2 Figure 2 XRD patterns of the powder collected from the suspensions prepared using two different precursors, i.e., monoclinic-phase Cs2[Mo6Cl14]•H2O and trigonal-phase Cs2[Mo6Cl14], and three different solvents: (a) EtOH, (b) 1-PrOH, and (c) MeOH. The open and closed circles indicates XRD peak from the trigonal and monoclinic phase, respectively.

Figure 3

 3 Figure 3 Normalized TDS spectra of the trigonal forms collected from EtOH suspension using Cs2[Mo6Cl14]•H2O as precursor. Arrows indicate characteristic desorption peaks.

Figure 4

 4 Figure 4 FT-IR spectra of different precursors and recrystallized samples in the (a) OH stretching (OH-s) mode and (b) OH bending (OH-b) mode regions. The spectra were obtained by using (1) monoclinic phase crystallized in MeOH, trigonal form crystallized in (2) EtOH or (3) 1-PrOH, and (4) trigonal-form precursor crystallized in acetone solvent. The spectra (5)-(7) indicate the sample (1)-(3) heated in vacuum condition through the TDS measurements, respectively. Arrows indicate characteristic peaks associated with the presence of water.

Figure 5

 5 Figure 5 Time evolution of Cs2[Mo6Cl14] concentration in the supernatant of the suspensions with different precursor/solvent combinations. The open and closed symbols are for combinations with Cs2[Mo6Cl14] and Cs2[Mo6Cl14]•H2O precursors, respectively.

Figure 6

 6 Figure 6 (a) PL spectra of the trigonal-phase precursor and recrystallized trigonal-form Cs2[Mo6Cl14] and monoclinic-form Cs2[Mo6Cl14]•H2O, and (b) TRPL spectra of the trigonal-phase precursor and the samples recrystallized with MeOH, EtOH or 1-PrOH.

Figure 7

 7 Figure 7 Energy band structure of (a) trigonal Cs2[Mo6Cl14] and (b) monoclinic Cs2[Mo6Cl14]•H2O. Difference between E-k dispersion of these compounds can be found in the energy range indicated by a red arrow, and the water related electronic states exist at the energy ranges indicated by two green arrows.

Figure 8

 8 Figure 8 Contour map of electron density around the inserted H2O molecule in Cs2[Mo6Cl14]•H2O.

Table 1

 1 Refined lattice parameters (a and c) and unit cell volume (V) of the trigonal-phase samples prepared with different combination of precursor and solvent.

	Precursor	Solvent	a / nm	Lattice parameters c / nm	V / nm 3
	Cs2[Mo6Cl14]	EtOH	0.9815(2)	1.4161(3)	1.1814(4)
	Cs2[Mo6Cl14]•H2O	EtOH	0.9792(3)	1.4213(3)	1.1801(5)
	Cs2[Mo6Cl14]	1-PrOH	0.9783(3)	1.4183(4)	1.1756(6)
	Cs2[Mo6Cl14]•H2O	1-PrOH	0.9783(3)	1.4218(3)	1.1785(6)
	Precursor purified with acetone	0.9820(2)	1.4200(4)	1.1860(4)

Table 2

 2 Luminescence lifetime (τPL) of different precursors and recrystallized samples.

	Solvent	Obtained phase	τPL / μs
	Acetone	Trigonal + Monoclinic	195
	MeOH	Monoclinic	184
	EtOH	Trigonal	316
	1-PrOH	Trigonal	310

Table 3

 3 Results of population analyses for trigonal Cs2[Mo6Cl14], and monoclinic Cs2[Mo6Cl14]•H2O.Please do not adjust marginsPlease do not adjust marginsFor Table Contents EntryAn octahedral molybdenum cluster compound, Cs2[Mo6Cl14], was purified by recrystallization with hydrophilic alcohol solvents for enhancing its luminescence efficiency.

	Molecule	Form	Mulliken	Hirshfeld
	[Mo6Cl14]	Trigonal Monoclinic	-1.94e -1.78e	-0.44e -0.38e
	H2O	Trigonal Monoclinic	N/A -0.11e	N/A -0.08e

Acknowledgements

This work was carried out as part of the France-Japan international collaboration framework (UMI 3629-LINK Center). The authors thank people involved in LINK and related activities, particularly Drs. David Lechevalier and Mari Kono of Saint-Gobain KK (Tokyo, Japan) for their efforts on international communication and project management, Noée Dumait, Serge Paofai of ISCR for their contributions to contributions to the synthesis of metal clusters precursors. N.S.

ARTICLE

This journal is © The Royal Society of Chemistry 20xx

CrystEngComm | 9

Please do not adjust margins Please do not adjust margins


Norio Saito

Yoshiki Wada

Pierric Lemoine

Stéphane Cordier

Fabien Grasset

Takeo Ohsawa

Noriko Saito

Jeffrey S Cross

Naoki Ohashi
email: ohashi.naoki@nims.go.jp

Norio Saito

Noriko Saito

Theoretical and experimental determination of the crystal structures of cesium-molybdenum chloride

Introduction

Molybdenum(II) halide cluster complexes, [Mo6X

i 8X a 6]
2-(X = halogen, i = inner, and a = apical), 1) consist of an octahedral Mo6 cluster bonded to 14 X ligands. Eight X i ligands are coordinated to the Mo6-cluster on the inner face capping positions, while the remaining six X a ligands lie in terminal apical positions. A range of [Mo6X i 8X a 6]

2-complexes exhibiting different chemical properties can be synthesized by substituting X i and X a with different halogen ions. [2][3][4] As the formal charge of the [Mo6X i 8X a 6]

2-complexes is -2e, this species can be crystallized with counter cations to give Am[Mo6X i 8X a 6] compounds (A = counter cation, m = 2 for monovalent, and m = 1 for divalent). 5) Owing to their large size,

[Mo6X i 8X a 6]
2-complexes can be crystallized not only with elemental inorganic cations (e.g., Cs + ) but also with organic molecular cations. For example, Saito et al. 6) explored the formation of a number of Am[Mo6X i 8X a 6] crystals employing various functional organic molecules as the A-cation. In addition, the incorporation of anionic [Mo6X i 8X a 6]

2-cluster units in organic polymer matrices has been widely developed to produce nanocomposites combining the photophysical properties of clusters with the mechanical and optical properties of polymers. 7) A number of studies have reported the superconductivity of the so-called Chevrel compounds, i.e., metal chalcogenide cluster complexes with the formula Mo6Q i 2Q i-a 6/2Q a-i 6/2 (Q = chalcogen), 8) and as such, investigations focusing on the electric transport properties of Chevrel compounds, such as Pb[Mo6S8], have been conducted. 9) In contrast, compounds based on [Mo6X i 8X a 6]

2-complexes, characterized by their wide energy band gaps, have recently been studied from the viewpoint of optoelectronic applications, such as phosphors, [10][11][12] photocatalysts, [13][14][15] and optical oxygen sensors. 16) In particular,

[Mo6X i 8X a 6]
2-complexes, exhibiting broadband red luminescence under UV or blue light, 10) have received increasing attention, since phosphors with efficient broadband visible emission under blue light excitation are important for the improvement of solid-state lighting efficiency. 17,18) Hence, we were interested in studying the electronic structures of

[Mo6X i 8X a 6]
2-complexes and [Mo6X i 8X a 6]

2--based compounds. Indeed, we recently reported the presence of additional overlapping emission at low temperatures, and the emission process for [Mo6Br14] 2--based complexes. 19) A particularly interesting feature of the luminescence properties of [Mo6X i 8X a 6]

2--based compounds is a large Stokes shift. 10) Such optical properties depend on the combination of X i and X a , 20,21) and so the effects of halogen substitution on the electronic structure of these compounds and their excited states were investigated. 22,23) Based on these investigations into the electronic structure of the [Mo6X 2-, 23,24) [W6X i 8X a 6]

2-, 25) and

[Re6Q i 8X a 6]
4-(Q i = S or Se) 26,27) have been reported. However, these studies utilized molecular orbital calculations for ionized complexes. It is therefore evident that further theoretical investigations into the crystalline form of [Mo6X i the lattice parameters to be a0 = 0.977 nm and c0 = 1.422 nm, 5) while the other quoted a0 = 0.983(5) nm and c0 = 1.431(3) nm. 28) These values differ significantly, and even if the P31c space group was suggested, 28) the lattice symmetry of the trigonal Cs2[Mo6Cl14] was not successfully confirmed. 5,28) Although it appears that the symmetry of both Cs2[Mo6Br14] and Cs2[Mo6I14] belongs to the 31 P c -group, 29) it is not clear whether the trigonal Cs2[Mo6Cl14] possesses 31 P c or P31c symmetry (see Fig. 1). We therefore focused on determination of the crystalline and electronic structures of trigonal Cs2[Mo6Cl14] by density functional theory (DFT) calculations. In particular, plane wave-based DFT simulation codes for the periodic lattice system were used to study the electronic structure of trigonal Cs2[Mo6Cl14].

In addition to theoretical simulations, we experimentally examined the crystal structure of trigonal Cs2[Mo6Cl14] using both X-ray powder diffraction (XRD) and neutron powder diffraction (NPD). Interestingly, we showed that molecules in an experimental environment (e.g., in water) could be inserted into the unoccupied trigonal site of the Cs2[Mo6Cl14] lattice without altering the trigonal lattice. In addition, using powder diffraction results, geometry optimization from DFT calculations, and optical studies employing an fs-pulsed laser for second harmonic generation (SHG), we propose an explanation into the crystal structure ambiguity of the trigonal Cs2[Mo6Cl14]. We herein report our experimental and theoretical investigations into the crystal structure of Cs2[Mo6Cl14].

Materials and methods

Sample preparation

The synthesis of Cs2[Mo6Cl14] was performed according to a method derived from that reported by Healy et al. 28) Initially, (H3O)2[Mo6Cl14]•7H2O was prepared as single crystals using a method reported by Koknat et al. Structural parameters of the Cs2[Mo6Cl14] lattice were analyzed by Rietveld profile refinement using the FullProf and WinPlotr software packages. 31,32) According to the literature, 33) the coherent neutron scattering lengths (bc) adopted for the refinement were as follows: Cs, 5.42 fm; Mo, 6.715 fm; and Cl, 9.577 fm.

As the non-centrosymmetric lattice structure exhibited SHG behavior, 34) photoluminescence studies using high excitation conditions were performed to examine if Cs2[Mo6Cl14] also exhibited SHG behavior. These studies were performed at room temperature using an optical parametric amplifier (Spectra Physics TOPAS Prime) excited by a titanium sapphire regenerative (Ti-Sap) amplifier (Spectra Physics Solstice) as the excitation light source, with 100 fs pulse width, 1 kHz repetition rate, 1.0 mJ/(pulse•cm 2 )

excitation density, and wavelengths of 1000 and 1200 nm. Sample luminescence was monitored using a spectrometer (Jovan-Ybon Triax550) equipped with a charge-coupled device (Andor Technology DU420A).

Computational methods

As two possible structural models have been proposed for the trigonal Cs2[Mo6Cl14] (see Previously reported crystal structure parameters 5,28,[START_REF]Pearson's Crystal Data[END_REF] were used for construction of the model with P31c symmetry, while the model with 31 P c symmetry was constructed according to the crystal structure of Cs2[Mo6I14], which exhibits 31 P c symmetry. 29) These two models were used for the DFT structural relaxation studies.

The CASTEP code, [START_REF] Clark | [END_REF] based on the plane wave pseudo-potential method, 37) was used for DFT calculations. In this study, norm-conserved pseudo-potentials (NCPPs) 38) were generated using the OPIUM code. 39) Generalized gradient approximation (GGA) 40) was selected as the exchange-correlation functional for total energy calculations. In particular, the GGA functional optimized for solid-state compounds, i.e., the PBEsol functional, 41) was used for the majority of calculations. Energy minimizations were performed using Pulay's density mixing scheme. 42) In some cases, the all-bands/EDFT method 43) was adopted to confirm the convergence of calculations using Pulay's scheme. Crystal structures were relaxed using the quasi-Newton method with the Broyden-Fletcher-Goldfarb-Shanno Hessian update scheme. 44) Polarization of electron spin was considered for all DFT calculations. The plane wave cut-off energy was set at 830 eV, while the Monkhorst-Pack grid 45,46) (4×4×3 mesh) was used for Brillouin zone sampling, and a 96×96×144 mesh was adopted for the fast Fourier transformation. The convergence tolerances were set to 5×10 - 5 nm for atomic displacement, 5×10 -6 eV/atom for total energy, 0.1 eV/nm for maximum interatomic force, and 0.02 GPa for pressure. The effective charges of all atoms and ions were calculated using the Mulliken population method. 47,48) Structural optimization for the models assuming 31 P c and P31c space groups was performed by two different means. Firstly, the fully relaxed crystal structure was obtained to determine the unstrained structure with assumed lattice symmetry. Secondly, relaxation of atomic positions with fixed lattice parameters was carried out by fixing the lattice parameters to experimentally obtained values. In addition, structural optimization without any symmetry constraints (P1 group) was performed to examine the possibility of symmetry lowering during the structural relaxation. On calculation of the P1-models, we used the models possessing 31 P c or P31c symmetry as initial structures, and all lattice parameters and atomic positions were relaxed to determine the fully relaxed structure. As we considered the model with P1 symmetry, a conventional unit cell (Z=2) was used for all calculations, although this required relatively large computational resources.

Simulation of XRD patterns using crystal structure parameters determined by DFT calculations was performed using the Mercury code (Cambridge Crystallographic Data Centre) to examine the agreement between the calculated and observed XRD patterns. In addition, following structural optimization by DFT, lattice symmetry was examined using the ADDSYM subroutine in PLATON, 49,50) which enabled us to observe symmetry elements in the relaxed structures.

Results and discussion

Sample characterization and purification

The obtained powder XRD patterns shown in Fig. 2 were identified by assuming that one of the two models shown in Fig. 1 was the major phase. Considering this assumption, some weak diffraction peaks could not be assigned to the trigonal Cs2[Mo6Cl14], as indicated by the arrows in Fig. 2. The intensities of these minor peaks decreased upon heating, while the relative intensities of peaks corresponding to the major phase remained relatively constant.

This suggests that the unidentified XRD peaks are assignable to additional phase(s) that decomposed upon heating. The majority of the discussion will therefore focus only on the XRD peaks assigned to the trigonal Cs2[Mo6Cl14]. As shown in Table I, the lattice parameters of the trigonal Cs2[Mo6Cl14] were altered by thermal treatment. The a-axis clearly shrank upon heat treatment, while the c-axis expanded, resulting in a decrease in unit cell volume upon heating. Interestingly, the lattice parameters of our as-prepared sample were close to those reported by Healy et al., 28) while the parameters corresponding to the sample following heat treatment were comparable to those reported by Potel et al. 5) The latter phase was prepared starting from CsCl and MoCl2 in a silica container using solid-state techniques at high temperatures, thus preventing the insertion of water in the lattice. Indeed, the higher unit cell volume reported in the literature 28) and in the as-prepared sample prior to heating may be attributed to the insertion of water molecules as described below.

FT-IR measurements indicate that changes in lattice parameters correlated with the desorption of H2O upon heating, as desorption occurred by heating under dry N2 gas. As shown in Fig. 3, the modes at 1100-1300 cm -1 and 1600 cm -1 correspond to H2O bending, 51) while the broad band at ~3500 cm -1 correlates with O-H stretching. 51) The presence of such bands indicates that the as-prepared sample contained water molecules. In some complex compounds, such as Cs2[Mo6Cl14]•3H2O, the water molecules are stabilized, forming a hydrogen bond to the Cl -ion at the apical site (X a ). 52) Thus, the insertion of water molecules at interstitial positions could explain the relatively large unit cell volume of the as-prepared sample. In addition, a further vibrational mode found at 1700 cm -1 was assigned to the asymmetric bending of (H3O) + . 53) Assuming that this charged species was incorporated into the Cs2[Mo6Cl14] lattice, the substitution of (H3O) + for a Cs + ion is possible. This is a reasonable consideration, as the size (0.113 nm ionic radius) 54) and positive charge of (H3O) + are comparable with those of the Cs + ion (0.169 nm ionic radius). 54) Furthermore, the desorption of (H3O) + by heat treatment could account for the changes in lattice parameters upon heating. As the lattice parameters following 1 h of treatment in pure N2 gas were similar to those after 3 h, it appears that the rate of H2O/(H3O) + desorption from the lattice was rapid.

Based on these heat treatment results, we concluded that the as-prepared sample contained a number of impurities, and, hence, the sample powder heated in dry N2 gas at 200 °C for 3 h to achieve water desorption represents the intrinsic nature of the trigonal Cs2[Mo6Cl14].

DFT calculations

The electronic structure calculations and structural optimizations were well converged for both assumed crystal structures (see Fig. 1). Indeed, the calculation initiated using the P31c space group model converged to the corresponding structure even when calculations were performed in the absence of symmetrical constraints (i.e., the P1 space group). Furthermore, the calculation initiated from the 31 P c symmetry model converged into the corresponding structure without breaking any symmetrical elements, regardless of the symmetry constraint, indicating that the results of structural relaxation strongly depend on the initial structure prior to relaxation.

The structural parameters of the fully relaxed lattice are summarized in Table II. The calculations using fixed lattice parameters were also well converged. As previously mentioned, the optimization of atomic positions with fixed lattice parameters, i.e., a0 = 0.9789 nm and c0 = 1.4214 nm, was considered to enable direct comparison of the structure determined by DFT optimization to the experimental results shown in Table I. The results of calculations with fixed lattice parameters are also shown in Table II SI (see supplementary data). The structural parameters will be discussed in further detail below.

The calculated total energy (enthalpy) of the optimized lattice possessing P31c symmetry was 6 × 10 -3 eV smaller than that of the optimized lattice with 31 P c symmetry, when both lattice parameters and fractional atomic positions were fully relaxed. In addition, the total energy for the 31 P c symmetry model was 2 × 10 -3 eV smaller than that of the P31c symmetry model with fixed lattice parameters. These differences in total energy between the two models were very small. As the optimized lattice parameters for the two models differed, the unit cell volume (V0) of the optimized 31 P c structure was slightly smaller than that of the P31c structure. This could therefore account for the lower total energy of the fully relaxed (unstressed) P31c model. When considering the total energy using fixed lattice models, the slightly lower total energy of the 31 P c symmetry possibly leads to a more stable structure compared to P31c symmetry. This occurs as P31c symmetry requires a higher compressive stress to maintain the observed lattice volume. However, such small energetic differences between the different symmetries are insufficient for determination of the Cs2[Mo6Cl14] lattice space group from the comparison of total energies alone. showing that the total energies of the two structural models were comparable. In addition, the DOS values for up-spin electrons were identical to those for down-spin electrons, and so the calculated results indicate that no magnetic behavior should be expected from the trigonal Cs2[Mo6Cl14].

As the electronic states are highly localized, the band dispersion of electronic orbitals (E-k dispersion) was flat, as shown in Fig. 4(b). It was therefore not possible to determine if the band gap was direct or indirect, as the exact k-vector at the valence band maximum (VBM) and the conduction band maximum (CBM) could not be determined. The energy band gap deduced from the E-k plot was 2.46-2.48 eV, which is slightly smaller than the experimentally determined optical band gap. 12) As GGA-based DFT calculations give relatively small band gap values due to the self-interaction of electrons, 55,56) a small band gap is possible when employing conventional DFT codes.

Figure 5 shows the projected-DOS (PDOS) of Cs2[Mo6Cl14] exhibiting 31 P c symmetry, which is comparable to the calculated PDOS for the P31c Cs2[Mo6Cl14]. As shown in Fig. 5, the VBM consists of Mo 4d orbitals, and exhibits a rather wide energy dispersion in the valence band. In contrast, the Cl 3p orbitals are separated into two groups, with Cl a being distributed in the top-to-middle range of the valence band, and Cl i being located in the middle-to-bottom range. Furthermore, the electronic state of the Cs valence electrons was observed at 1-2 eV from the VBM.

The effective charge of each ion calculated by Mulliken analysis is shown in Table III.

Variations in lattice symmetry and lattice parameters caused no significant changes in the effective charge of each ion. Hence, the following descriptions are common for all structural models considered in this study. The most notable result is that the effective charges at the Cl a and Cl i sites differ from one another. The effective charge at the Cl a sites was more negative than that at the Cl i sites, indicating that the Cl a sites exhibit a greater ionic tendency.

In addition, the previously mentioned differences in PDOS between the two sites along with the differences in effective charge are consistent with experimental observations. Indeed, core-level photoemission spectra of the X-anions in Am[Mo6X i 8X a 6] complexes contained two peaks with an intensity ratio of approximately 3:4, 57,58) indicating that the chemical state of the six X a ions differed from that of the eight X i ions. Hence, the calculated electronic structure and experimental observations are in good agreement. Furthermore, from the charge analysis, the effective charge of the Cs ions was comparable to its formal charge. This suggests that the [Mo6Cl14] 2-electronic state is highly localized, and the bonds between the complex and the Cs ions are purely ionic.

Crystallographic comparison between optimized crystal structures

Analysis using the ADDSYM subroutine suggested that the space group of the optimized crystal lattice after full relaxation differed from that of the lattice optimized using fixed lattice parameters. The fully relaxed lattice [Table II(a), left columns] belongs to the P31c space group, while the structure optimized using fixed lattice parameters [Table II(a), right columns] belongs to the 31 P c space group. Both calculations were performed by applying constraints without inversion symmetry, allowing Cs2[Mo6Cl14] to maintain its non-centrosymmetry upon increasing the unit cell volume. In contrast, rearrangement of the atoms from a non-centrosymmetric to a centrosymmetric space group occurred when the lattice parameters were fixed to the experimental values. Indeed, XRD simulations employing the structural parameters optimized using the lattice with fixed P31c symmetry produced similar patterns to those obtained using the lattice with fixed 31 P c symmetry, and to the observed pattern for the sample following heat treatment in dry N2 (Fig. S1 in the online supplementary data). As the removal of H2O (H3O + ) and/or -OH was demonstrated from FT-IR measurements, we expected that following heat treatment, the samples would be free from solvent insertion. Therefore, it is likely that the observed XRD intensity profile is closer to the intrinsic form of Cs2[Mo6Cl14], while the structural parameters calculated by DFT better represent the real Cs2[Mo6Cl14] lattice. As suggested from ADDSYM calculations and comparison of the simulated XRD patterns, the structural parameters calculated using both P31c and 31 P c symmetry were comparable, and so the former was considered to be the centrosymmetric structure. Therefore, the DFT calculations suggest that 31 P c symmetry is the most suitable space group for the Cs2[Mo6Cl14] lattice.

High harmonic generation in luminescence

Figure 6 shows the luminescence spectra of the prepared Cs2[Mo6Cl14] upon excitation with a UV pulsed laser source (λ = 410 nm) and with an infrared (IR) pulsed laser (λ = 1000 and 1200 nm) with high power density. Furthermore, the spectrum obtained from the excitation of quartz (α-SiO2) with an IR laser beam (λ = 1000 nm) is given as a reference. A broad emission in the visible-red region centered at λ = 700 nm was observed in all Cs2[Mo6Cl14] luminescence spectra, which can be attributed to a spontaneous luminescence peak. 10,12) This emission was also observed upon excitation using a pulsed IR beam with high excitation density. We expect that this red luminescence resulted from multi-photon absorption.

An additional emission corresponding to third harmonic generation (THG) was also observed in the spectrum obtained from excitation with IR beams (Fig. 6). However, signals corresponding to the SHG were not observed in the Cs2[Mo6Cl14] spectra despite the excitation intensity being sufficient to excite strong SHG emission from α-SiO2. This indicates that the Cs complex prepared herein does exhibit inversion symmetry. Considering these observations together with the DFT calculation results mentioned above, we concluded that the crystal symmetry of Cs2[Mo6Cl14] is most likely be the centrosymmetric 31 P c space group.

Refinement of structural parameters with NPD

Assuming that Cs2[Mo6Cl14] belongs to the 31 P c space group, the Rietveld refinement for the NPD profile was performed (the observed and peak-fitting profiles are shown in Fig. S2, supplementary data). The refinement was well-converged and the resulting reliability factors (R-factors) were Rwp = 2.19%, Re = 0.22%, Rb = 2.70%, Rf = 2.30%, and χ 2 = 103, suggesting that the assumption of 31 P c symmetry was consistent with the observed NPD pattern. The structural parameters refined from the combined NPD and XRD profiles are also shown in Table IV, indicating a consistent atomic arrangement to that determined by DFT calculations, as summarized in Table II. Hence, we concluded that the refined structural parameters accurately represent the intrinsic crystal structure of the Cs2[Mo6Cl14] complex.

Conclusions

Use of a range of analytical techniques revealed that the lattice parameters of Cs2[Mo6Cl14] changed upon the adsorption/desorption of water-related fragments and, hence, the sample-to-sample deference in the reported lattice parameters could be attributed to residual impurities, such as H2O, OH -, and H3O + . Desorption of these impurities was therefore carried out prior to further investigations. XRD and NPD studies showed consistent atomic arrangement to that determined from the electron density in the lattice.

These crystal structures, with theoretically optimized atomic coordinates, were comparable despite imposing the centrosymmetric or non-centrosymmetric constraints. Moreover, the optical measurements monitoring high harmonic generation indicated the presence of inversion symmetry in the Cs2[Mo6Cl14] lattice. Hence, we could conclude that the structural model with 31 P c symmetry was better suited to the obtained results than that with P31c symmetry.

Upon comparison of crystalline phase stability based on the DFT studies, the total energy of the Cs2[Mo6Cl14] lattice with 31 P c symmetry was calculated to be slightly higher than that with P31c symmetry following structural relaxation. However, the lattice parameters deduced for the model with P31c symmetry were not consistent with the observed values.

Furthermore, the results of total energy calculations with the model containing fixed lattice parameters indicated that the model with 31 P c symmetry was more stable, due to an increase in internal energy through compressive stress. As the difference in total energy between the two symmetry models was small (i.e., several meV), it is possible that a meta-stable polymorph of the Cs2[Mo6Cl14] lattice can exist. However, considering the obtained experimental results, we concluded that the total energy calculated using the fixed lattice parameter conditions represents the actual Cs2[Mo6Cl14] cluster compound, while the Cs2[Mo6Cl14] crystallized with 31 P c symmetry is the stable phase under ambient pressure.

Furthermore, impurity analysis for such metal-cluster-complex compounds must be carefully performed to determine the real nature of these compounds. Indeed, the lattice parameters exhibited significant changes upon the desorption of water-related molecules.

Consideration of the desorption behavior is therefore important not only for scientific investigations but also in terms of engineering applications. For instance, for LED phosphor applications, thermal stability is necessary to achieve high power operations.

Finally, as DFT has been shown to effectively reproduce the stable atomic arrangement of the metal cluster compounds, we plan to focus on the understanding and control of the luminescence properties of the compounds reported herein. given as a reference.

Figure Captions

2 -

 2 -based compounds are required to reach a deeper understanding of such compounds, in particular in relation to their crystalline and electronic structures. We herein chose to focus on Cs2[Mo6Cl14], which is a representative [compound with a relatively simple chemical composition. Two previous reports have been published focusing on the crystal structure of the trigonal Cs2[Mo6Cl14]. One study reported

Fig. 1 )

 1 Fig. 1), DFT calculations were performed to compare the total energies of each model. The model shown in Fig. 1(a) represents the P31c space group exhibiting lack of inversion symmetry, while that in Fig. 1(b) represents the 31P c space group, which exhibits an inversion center. Previously reported crystal structure parameters5,28,[START_REF]Pearson's Crystal Data[END_REF] were used for

  . Note that the obtained structures calculated without symmetry constraints (P1 symmetry) were identical to those obtained with structural constraints. It should also be noted that the Mo-Mo and Mo-Cl distances in the [Mo6Cl14] 2-complex determined by DFT structural optimization remained constants with variation in the lattice parameters, although such changes resulted in variation of the Cs-Cl distance along the c-axis. The interatomic distances of the optimized structure are shown in Table

Figure 4 (

 4 Figure 4(a) shows that the total electron density of states (DOS) in the fully relaxed trigonal Cs2[Mo6Cl14] lattice possesses 31 P c symmetry. Results of DOS calculations for both the lattice with P31c symmetry and the model with fixed lattice parameters were comparable to those shown in Fig. 4(a). These results are consistent with previous results

Fig. 1 .

 1 Fig. 1. (Color online) Trigonal Cs2[Mo6Cl14] unit cell. (a) P31c space group, and (b) 31 P c space group.

Fig. 2 .

 2 Fig. 2. (Color online) Powder XRD patterns of Cs2[Mo6Cl14] before and after thermal treatment in air and under dry N2. Arrows denote diffraction peaks originating from the secondary phase.

Fig. 3 .

 3 Fig. 3. (Color online) FT-IR spectra of Cs2[Mo6Cl14] powders both before and after thermal treatment in air and under dry N2.

Fig. 4 .

 4 Fig. 4. (Color online) (a) Density of states and (b) band dispersion in Cs2[Mo6Cl14] with 31 P c symmetry.

Fig. 5 .

 5 Fig. 5. (Color online) Projected density of state of Cs2[Mo6Cl14] crystallized in 31 P c symmetry.

Fig. 6 .

 6 Fig. 6. (Color online) Luminescence spectra of Cs2[Mo6Cl14] under strong excitation with an IR laser beam (λ = 1000 and 1200 nm) and under weaker excitation with a UV laser beam (λ = 410 nm). The spectrum of α-SiO2 under IR laser irradiation (λ = 1000 nm) is given as a reference.

Fig. 1 .

 1 Fig. 1. (Color online) Trigonal Cs2[Mo6Cl14] unit cell. (a) P31c space group, and (b) 31 P c space group.

Fig. 2 .

 2 Fig. 2. (Color online) Powder XRD patterns of Cs2[Mo6Cl14] before and after thermal treatment in air and under dry N2. Arrows denote diffraction peaks originating from the secondary phase.

Fig. 3 .

 3 Fig. 3. (Color online) FT-IR spectra of Cs2[Mo6Cl14] powders both before and after thermal treatment in air and under dry N2.

Fig. 4 .

 4 Fig. 4. (Color online) (a) Density of states and (b) band dispersion in Cs2[Mo6Cl14] with 31 P c symmetry.

Fig. 5 .

 5 Fig. 5. (Color online) Projected density of state of Cs2[Mo6Cl14] crystallized in 31 P c symmetry.

Fig. 6 .

 6 Fig. 6. (Color online) Luminescence spectra of Cs2[Mo6Cl14] under strong excitation with an IR laser beam (λ = 1000 and 1200 nm) and under weaker excitation with a UV laser beam (λ = 410 nm). The spectrum of α-SiO2 under IR laser irradiation (λ = 1000 nm) is

Table I .

 I Refined lattice parameters for Cs2[Mo6Cl14] both before and after thermal treatment.

	Table II. Summary of the theoretically optimized Cs2[Mo6Cl14] structure: (a) P31c (b)
	symmetry and (b) Lattice constants Fully relaxed 31 P c symmetry crystal structure following structural Fixed lattice parameters
	Annealing conditions Temperature (°C) Atmosphere Period (h) a0 / nm 0.980 optimization by DFT calculations. c0 / nm 1.475	Lattice parameters a0 / nm c0 / nm 0.979 1.421	V0 / nm 3
	N/A (As prepared) (a) V0 / nm 3	1.227		0.9833(1) 1.4185(3) 1.1878(2) 1.180
	150 Lattice constants Fully relaxed air 1.0 Fractional atomic positions	0.9803(1) 1.4193(3) 1.1811(2) Fixed lattice parameters
	200 a0 / nm Atom Wyck. dry-N2 0.980 x	1.0 y	0.9789(1) 1.4210(1) 1.1793(2) 0.979 z x y z
	200 c0 / nm Mo1	12i	dry-N2 1.499 0.666	3.0 0.177 0.177 0.666 0.177 0.9789(1) 1.4214(2) 1.1796(2) 1.421 0.175
	References V0 / nm 3 Cl1 4f		1.246 0.667	1.180 0.333 0.046 0.667 0.333	0.038
	Potel et al. 5) Fractional atomic positions Cl2 12i 0.957	0.977 0.330 0.180 0.957 0.330 1.422	1.176 0.177
	Healy et al. 28) Atom Wyck. x Cl3 12i 0.654	0.983(5) z -0.039 0.090 0.653 -0.042 0.085 1.431(3) 1.198 y x y z
	Mo1 Cs1	6c 2c		0.509 0.667	0.180 0.214 0.510 0.178 0.333 0.75 0.667 0.333	0.210 0.75
	Mo2 Cs2	6c 2b		0.515 0	0.340 0.357 0.513 0.337 0 0 0 0	0.361 0
	Cl1	6c		0.031	0.332 0.126 0.039 0.342	0.120
	Cl2	6c		0.312	0.355 0.444 0.308 0.351	0.451
	Cl3	6c		0.368	0.048 0.354 0.371 0.045	0.358
	Cl4	6c		0.379	0.342 0.217 0.375 0.334	0.213
	Cl5	2b		0.333	0.667 0.987 0.333 0.667	0.997
	Cl6	2b		0.333	0.667 0.584 0.333 0.667	0.574
	Cs1	2b		0.333	0.667 0.313 0.333 0.667	0.294
	Cs2	2a		0	0	0.029 0	0	0.031

Table III .

 III List of Mulliken populations in the calculated structure of Cs2[Mo6Cl14]: atomic populations in (a) P31c symmetry and (b) 31 P c symmetry.

	(a)
	Atom s	p	d	Total Charge (e)
	Mo1 2.41 6.37 5.01 13.79 0.21
	Mo2 2.41 6.39 5.01 13.81 0.19
	Cl1	1.94 5.46 0	7.40 -0.40
	Cl2	1.93 5.47 0	7.40 -0.40
	Cl3	1.93 5.17 0	7.10 -0.10
	Cl4	1.93 5.16 0	7.09 -0.09
	Cl5	1.94 5.12 0	7.06 -0.06
	Cl6	1.93 5.13 0	7.06 -0.06
	Cs1	2.05 5.94 0	7.99 1.01
	Cs2	2.08 5.99 0	8.07 0.93
	(b)
	Atom s	p	d	Total Charge (e)
	Mo1 2.41 6.38 5.01 13.80 0.20
	Cl1	1.93 5.12 0	7.05 -0.05
	Cl2	1.93 5.17 0	7.10 -0.10
	Cl3	1.94 5.46 0	7.40 -0.40
	Cs1	2.05 5.95 0	8.00 1.00
	Cs2	2.08 5.99 0	8.07 0.93

Table IV .

 IV Crystallographic parameters of the trigonal Cs2[Mo6Cl14] with 31

							P c
	symmetry refined by combined neutron and X-ray powder diffraction.
	Lattice constants
	a0 / nm		0.9805(1)
	c0 / nm		1.4170(2)
	V0 / nm 3		1.1796(2)
	Fractional atomic positions
	Atom Wyck. x	y	z	Occ. Uiso
	Mo1	12i	0.663(1) 0.177(1)	0.175(1) 1.0	0.0190(3)
	Cl1	4f	2/3	1/3	0.039(1) 1.0	0.0203(3)
	Cl2	12i	0.956(1) 0.330(1)	0.178(1) 1.0	0.0190(3)
	Cl3	12i	0.655(1) -0.035(1) 0.085(1) 1.0	0.0368(3)
	Cs1	2c	2/3	1/3	0.75	1.0	0.0317(5)
	Cs2	4e	0	0	0.028(2) 0.5	0.0317(5)

Acknowledgments

The authors thank those involved in UMI-LINK and its related activities, particularly Dr. Mari Kono and Dr. Benjamin Dierre, of Saint-Gobain KK (Tokyo, Japan), and N. Dumait, Serge Paofai, and Dr. T. Roisnel of ISCR for their contributions to the management of UMI-LINK. N.S. would also like to thank Professor Junzo Tanaka of Tokyo Tech. for support and encouragement. P.L. and S.C. are indebted to the Institut Laue-Langevin (Grenoble, France) for the provision of research facilities. Parts of this study have been financially supported by ANR (CLUSTOP-11-BS08-013-01) and Fondation Langlois.