Ľubomír Smrčok

Armel Le Bail
email: armel.le_bail@univ-lemans.fr

Miroslav Boca
email: miroslav.boca@savba.sk

Aydar Rakhmatullin
email: aydar.rakhmatullin@cnrs-orleans.fr

Polymorphism of K 2 ZrF 6

Keywords: di-potassium zirconium fluoride, polymorphism, synchrotron powder diffraction, 19 F MAS NMR

Heating of the K 2 ZrF 6 room temperature monoclinic form-I reveals a complex polymorphism investigated by synchrotron powder temperature-dependent diffraction. The first thermal event at 240 °C corresponds to the quasi-simultaneous apparition of four different phases (noted II to V) of which only three could be indexed and two completely characterized structurally.

Form-II (structure unrefined) is very probably hexagonal (a = 6.4473(1) Å, c = 3.8606(1) Å at 264 °C, Z = 1). Form-III is cubic (space group Fm m, a = 9.0804(1) Å at 264 °C, Z = 4), related to 3 K 3 ZrF 7 . Form-IV remains unindexed, presenting broad diffraction peaks. Form-V is present as a weak impurity at this stage, increasing in proportion after the second thermal event at 288 °C where a new form-VI is built. Form-V is tetragonal (space group P4/nmm, a = 6.3281(1) Å, c = 9.3671(2) Å at 295 °C, Z = 2) and form-VI is orthorhombic (space group Cmcm, a = 6.40840(5) Å, b = 20.7697(2) Å, c = 19.9888(2) Å, at 295 °C, Z = 16). At 317 °C, form-VI transforms into form-V. At 449 °C, a different orthorhombic form-VII is disclosed (space group Pnma, a = 10.3084(1) Å, Page 1 of 33 ACS Paragon Plus Environment Crystal Growth & Design 2 b = 6.0291(1) Å, c = 23.8023(2) Å, at 495 °C, Z = 8), coexisting with the cubic form-III. On cooling, form VII of which an approach of the crystal structure is provided, returns to form V at 411 °C which looks stable down to 139 °C giving form I back. Isolated ZrF 6 octahedra are present in forms-III, -V, -VI, -VII. Form-VI contains also dimeric entities Zr 2 F 12 built from a ZrF 7 monocapped trigonal prism sharing a corner with a ZrF 6 octahedron. Form-VII structure is related to form-V by quadrupling the tetragonal a parameter, the reversible VVII transition is very probably topotactic. The coexistence of several phases at each temperature above the first transition at 240 °C was observed. Peak multi-splitting is observed at many temperatures that suggests the occurrence of coexisting micro-phases. The parallel existence of several phases was observed also by 19 F MAS NMR spectroscopy measured from 250 o C → 410 o C → RT.

INTRODUCTION

In the concept of so-called "Generation IV" Molten Salts Nuclear Reactors (MSR) fission power should be produced in a circulating molten salt fuel mixture. Currently, these types of reactors are considered to be fuelled with uranium or thorium fluorides dissolved in a mixture of molten fluorides, with Na/K and Zr fluorides being a primary option 1 . The compounds belonging to the KF-ZrF 4 system have several potential uses: i) as solvent media in a reactor core or as solvent for pyrochemical reprocessing of actinides and lanthanides produced in nuclear reactions; ii) as moderators; iii) as cooling media of the reactor core, iv) as high temperature transport media providing transport of heat to hydrogen production facilities employed for the Hydrogen Economy Concept [START_REF] Forsberg | Hydrogen, nuclear energy, and the advanced high-temperature reactor[END_REF] . In all of the above applications the compatibility of the molten salt with reactor construction materials is necessary to avoid chemical attack by a corrosive liquid. Considering the working regime of a MSR it is evident that reliable reference data on the dynamics of phase transitions in zirconium fluorides are essential for design of a cooling unit. However, the only available phase diagram of KF-ZrF 4 system was reported in 1961 [START_REF] Novoselova | The KF-ZrF4 system[END_REF] and although it has brought initial information, some results are seriously limited by the use of low resolution laboratory powder diffraction. None of the high temperature phases was the subject of a crystal structure refinement by the Rietveld method. Among the compounds belonging to the KF-ZrF 4 system it is K 2 ZrF 6 which seems to be, mainly for the economic reasons, the most promising candidate for future application. For its solid-solid phase transformations the following scheme was suggested For the region above 585 °C a series of phase transformations resulting in the co-existence of solid and liquid phases was suggested, but the region has not yet been explored in details. At room temperature, the crystal structure of K 2 ZrF 6 is monoclinic, C2/c [START_REF] Hoppe | Die Kristallstruktur von K2ZrF6[END_REF][START_REF] Gerasimenko | Synthesis and complex investigation of potassium ammonium hexafluorozirconates: I. Synthesis and X-ray diffraction study of K2-x(NH4)xZrF6 (0 < x < 2) crystals[END_REF] . The structure of β-K 2 ZrF 6 is not reliably known in spite of several suggestions [START_REF] Bode | Über Strukturen von Hexafluorozirkonaten und Hexafluorohafnaten[END_REF][START_REF] Kavun | Ion Mobility and Electrophysical Properties of Potassium Hexafluorozirconate K2ZrF6[END_REF] . Phase transitions in the temperature range of ~240 to 245 °C were independently confirmed only by NMR [START_REF] Kavun | Ion Mobility and Electrophysical Properties of Potassium Hexafluorozirconate K2ZrF6[END_REF] . Recently, some more detailed analysis of phase transformation of K 2 ZrF 6 was reported in the region up to 330 o C 8, 9 using XRD, DTA/DSC, Raman spectroscopy and statistical methods PCA and MCR, as well as phase diagram of the system (LiF-NaF-KF) eut -K 2 ZrF 6 [START_REF] Kubikova | Phase analysis and volume properties of the (LiF-NaF-KF)(eut)-K2ZrF6 system[END_REF] . Detailed room temperature analysis by XPS and NMR spectroscopy was provided, as well [START_REF] Boča | X-ray photoelectron spectroscopy as detection tool for coordinated or uncoordinated fluorine atoms demonstrated on fluoride systems NaF, K2TaF7, K3TaF8, K2ZrF6, Na7Zr6F31 and K3ZrF7[END_REF][START_REF] Rakhmatullin | Solid state NMR and XPS of ternary fluorido-zirconates of various coordination modes[END_REF] .

The very poor knowledge of the K 2 ZrF 6 thermal behaviour regarding to its potential industrial and economic importance incited us to re-examine the problem by synchrotron temperaturedependent diffraction experiments. Our results led to the observation of seven K 2 ZrF 6 phases instead of four, requiring to change the above Greek notation by using forms-I to VII.

Correspondences with the polymorphism of K 2 HfF 6 [START_REF] Saalfeld | The polymorphism of dipotassium hexafluorohafnate[END_REF] , for which structure models were proposed (but no Rietveld or single crystal structure refinement) for six different phases, are discussed.

EXPERIMENTAL SECTION

Preparation of K

DSC-TGA Analysis:

Thermogravimetric experiments were carried out on a thermoanalyzer TA instrument DT-Q600 under argon at a heating rate of 5 °C min -1 from room temperature up to 500 °C and back, allowing to characterize a strong hysteresis (Figure 1). This DSC shows mainly four endothermic effect on heating (240, 288, 317 and 445 °C) and two exothermic effects on cooling (411 and 139 °C). If one does not consider the 317 °C peak, the heating seems to be compatible with the previous model 3 of αβ transitions given in the introduction, but not with any reversibility (also compare with 9 ). Avance III HD NMR spectrometer operating at 17.6 T using the previously described diode laser heated system 9 . In addition, to enhance resolution at room temperature a probe with 1.3 mm diameter rotors spinning at a frequency of 50 kHz was used. The parameters were: pulse width 0.95 μs (π/2), relaxation delay 300 s (6T 1 ), and 4 scans. The 19 F chemical shift was referenced to CFCl 3 . The DMFIT program was applied to simulate the spectra and fit the peaks [START_REF] Massiot | Modelling one-and two-dimensional solid-state NMR spectra[END_REF] .

Powder X-ray Diffraction (PXRD): Data were recorded using beamline I11 at the Diamond Light Source, Didcot, UK. Samples were mounted in a sapphire capillary, in dry box under argon atmosphere, that was closed by glue on both sides for measurements in transmission geometry.

Two main experiments were made, using the Mythen detector configuration on sample 1 and the multianalyzing crystal-detectors (MACs) on sample 2, both operating at wavelength close to 0.827 Å. The beamline set-up and characteristics are described in the literature [START_REF] Thompson | Beamline I11 at Diamond: a new instrument for high resolution powder diffraction[END_REF][START_REF] Thompson | Fast X-ray powder diffraction on I11 at Diamond[END_REF][START_REF] Parker | High-throughput powder diffraction on beamline I11 at Diamond[END_REF] . The diffraction data were collected in order to obtain significant patterns in the temperature range 20-500 °C.

Data interpretations were sometimes facilitated by the previous results about K 2 HfF 6 in 1983 by Saalfeld and Guse [START_REF] Saalfeld | The polymorphism of dipotassium hexafluorohafnate[END_REF] . From their GUINIER-powder diagrams (high-temperature chamber), they indexed a tetragonal phase (190-250 °C, a = 6.29(8) Å, c = 9.27(4) Å at 210 °C, suggested space group P4/mmm) and proposed it to be related to distorted variants of the cubic elpasolite [START_REF] Sabelli | Structure refinement of elpasolite from Cetine Mine, Tuscany, Italy[END_REF] .

KNaAlF 6 having similar cell parameters (a ~ 6.5 Å, c ~ 9.5 Å for (NH 4 ) 3 MF 6 , M = In, Sc) described by Bode and Voss [START_REF] Bode | Strukturen der Hexafluorometallate(III)[END_REF] . Saalfeld and Guse 13 identified a cubic K 2 HfF 6 phase (210-380 °C, a = 9.01 (6) Å at 240 °C, space group Fm m, proposing a structure related to elpasolite or to K 2 PtCl 6 ) [START_REF] Schefer | Neutron and X-ray Diffraction Study of the Thermal Motion in K2PtCl6 as a Function of Temperature[END_REF] and 3 sometimes a hexagonal phase (170-220 °C, a = 6.40 (5), c = 3.81(8) at 200 °C, no systematic extinction) which they proposed to be similar to β 1 -K 2 UF 6 and β 1 -K 2 ThF 6 , with the space group P6 2m, that were investigated by Zachariasen [START_REF] Zachariasen | Double Fluorides of Potassium or Sodium with Uranium, Thorium or Lanthanum[END_REF] . Saalfeld and Guse 13 noted that the -K 2 ZrF 6 powder pattern from Novoselova et al. [START_REF] Novoselova | The KF-ZrF4 system[END_REF] was analogous to that of their hexagonal K 2 HfF 6 phase, unfortunately they never confirmed their structure models by any Rietveld refinement. These K 2 HfF 6 hexagonal, cubic and tetragonal three phases were recognized (using search-match identification process by means of the EVA software 24 combined with the ICDD-PDF-4+ powder patterns database) [START_REF]PDF4+ database[END_REF] to occur simultaneously in the case of K 2 ZrF 6 , with similar cell parameters as those of the Hf-based compounds, they were noted phases II, III and V, respectively. A fourth phase noted IV is also present at this stage, all of these phases corresponding to the "β-K 2 ZrF 6 " in the Novoselova et al. notation. [START_REF] Novoselova | The KF-ZrF4 system[END_REF] .

ACS Paragon Plus Environment

Crystal Growth & Design

6

A contour plot of a synchrotron thermo-diffractogram of K 2 ZrF 6 covering the range 185-300 °C is shown in Figure 2, with phase numbering. This figure shows that the 288 °C thermal event corresponds to the complete collapse of the hexagonal phase-II (phase IV having not totally disappeared yet at 295 °C), a strong decrease in the proportion of the cubic phase-III, a strong increase in the proportion of the tetragonal phase-V and the occurrence of a new phase VI for which no similar powder pattern could be found in the K 2 HfF 6 previous study. The 317 °C DSC peak in Figure 1 is interpreted as the transition from form-VI to the tetragonal form-V which is the main phase up to 449 °C corresponding to the transition between the tetragonal form-V to a new phase VII accompanied by the reappearing of more of the cubic phase-III. On cooling, the 411 °C peak of the DSC corresponds to the reverse VIIV transition, with a strong hysteresis. The tetragonal phase-V is then stable on cooling down to 139 °C, where K 2 ZrF 6 returns back to phase-I, monoclinic.

The new phases VI and VII were indexed by using the McMaille 26 software and were found to be both orthorhombic. Phase IV could never be indexed. It is to be noted that its diffraction peaks are quite broader than for phases II (showing the narrowest peaks) and III. Hypotheses that the phase IV peaks could be sattellites of either a commensurate or an incommensurate supercell of either phases II or III found no success, so it cannot be excluded that what we call form-IV is in fact a mixture of 2 phases. Then the ab initio structure determination of phases V, VI and VII were undertaken with success by using the direct space Monte Carlo software ESPOIR [START_REF] Bail | ESPOIR: A Program for Solving Structures by Monte Carlo Analysis of Powder Diffraction Data[END_REF] (but the space group P4/mmm and the model proposed [START_REF] Saalfeld | The polymorphism of dipotassium hexafluorohafnate[END_REF] for the tetragonal K 2 HfF 6 phase finally could not be applied to K 2 ZrF 6 form-V). Rietveld [START_REF] Rietveld | A Profile Refinement Method for Nuclear and Magnetic Structure[END_REF] refinement as well as Le Bail fits [START_REF] Bail | Whole powder pattern decomposition methods and applications: A retrospection[END_REF] were realized by using the FULLPROF 30 software. The cubic structure of K 2 ZrF 6 form-III was better Rietveld-refined at 495 °C when mixed with form-VII, using the K 3 ZrF 7 cubic structure 31 as a starting model.

Attempts to refine the hexagonal structure of form-II using the various proposed models for K 2 HfF 6 were never satisfying due to the presence of phases III, IV (the latter unindexed) and V.

Crystallographic data for the K 2 ZrF 6 forms I to VII are summarized in Table 1.

All the solid-state DFT structures optimizations were done according the procedure described elsewhere 32 .

RESULTS AND DISCUSSION

K 2 ZrF 6 form-I. In the whole range between the room temperature and the DSC peak at 240 °C, form-I (previously noted α-K 2 ZrF 6 ) is found to be monoclinic [START_REF] Hoppe | Die Kristallstruktur von K2ZrF6[END_REF][START_REF] Gerasimenko | Synthesis and complex investigation of potassium ammonium hexafluorozirconates: I. Synthesis and X-ray diffraction study of K2-x(NH4)xZrF6 (0 < x < 2) crystals[END_REF] , however, peak shapes are so distorted and even split (in two or three components) that the Rietveld refinements are always quite bad. There was some controversy in the past about the symmetry: monoclinic or orthorhombic, monoclinic at room temperature and then possibly orthorhombic at higher temperature. For sample 1, the 0k0 reflections are split clearly in two main peaks (Figure 3), suggesting the existence of at least two microphases presenting different cell parameters. The splitting is visible at all temperatures. Comparison of samples 1 and 2 at 200 °C show that sample 2 looks much better than sample 1 but presents itself a small 0k0 splitting (see the 060 below) and is not free of peak shape problems (see the 133 or 422 in Figure 3). The Rietveld refinement for sample 2 at 200 °C led to R P = 13.0 % and R B = 11.5 %, and it was not better than R P = 35.1 % and R B = 24.3 % for sample 1. Our explanation about these differences is that sample 1 was too highly crushed and that sample 2 was not enough crushed (the bad fit for sample 2 is not only due to broad peaks like the 422 or 133, but also to some abnormally intense peaks typical of the presence of single crystals in the powder). Similar problems on sample grain size response was observed also on DTA/DSC experiments 9 . Then, sample 2 was placed 12 h at 470 °C, then cooled freely in order to see if some high temperature phase could be quenched. The result was the form-I presenting broader peaks than for unheated sample 1 and 2, but which produced an excellent fit (Figure 4) by the Rietveld method in spite of some anisotropic broadening which was treated by separating the much narrower h00 and hh0 from the others. This sample contains monoclinic P2 1 /c ZrO 2 as impurity, and also a third phase never encountered before, having peaks at positions very close to those of the cubic form-III, but convincingly indexed in a rhomboedral cell with parameters a = 6.34818(5), c = 15.65071(13) Å, in too small proportion for an accurate structure refinement (R B ~10 % was obtained with the R m space group suggesting the K 3 ZrF 5 O formula, to be confirmed), so it was 3 treated by a Le Bail fit, whereas m-ZrO 2 was treated by using the atomic coordinates from the literature, fixed. The Rietveld refinement from synchrotron data leads to similar results as from the two previous single crystal studies [START_REF] Hoppe | Die Kristallstruktur von K2ZrF6[END_REF][START_REF] Gerasimenko | Synthesis and complex investigation of potassium ammonium hexafluorozirconates: I. Synthesis and X-ray diffraction study of K2-x(NH4)xZrF6 (0 < x < 2) crystals[END_REF] (none presenting anisotropic thermal parameters because of crystals being of low quality, which is compatible with our problems with sample 1 and 2 freshly prepared) and confirms the monoclinic cell (Table 2). Interatomic distances are in Table 3. The structure is built up from infinite chains of ZrF 8 square antiprisms sharing edges, extending along the c-axis (Figure 5). The tenfold coordination of the potassium is harder to define, but can be described as a strongly distorted cube having two adjacent faces capped. The absence of extinction leads to sixteen possible space groups for the hexagonal (or trigonal) form-II. It was proposed [START_REF] Saalfeld | The polymorphism of dipotassium hexafluorohafnate[END_REF] for the equivalent K 2 HfF 6 form (a = 6.40, c = 3.81 Å, "sometimes additional lines occur" in the 170-220 °C range), but not confirmed by any refinement, that it could be isostructural with the β 1 -K 2 MF 6 (M = U, Th) investigated by Zachariasen [START_REF] Zachariasen | Double Fluorides of Potassium or Sodium with Uranium, Thorium or Lanthanum[END_REF] , with the space group P 2m. Zr would have an unusual ninefold coordination according to that model. 6

Unfortunately, no satisfying Rietveld fit could be obtained by following this approach. No more success was obtained by trying other models selected among the A 2 BX 6 known structures with relatively similar cell parameters, including the K 2 GeF 6 structure-type [START_REF] Hoard | Structures of Complex Fluorides. Potassium Hexafluogermanate and Ammonium Hexafluogermanate[END_REF] .

The cubic form-III could be best refined (R B = 4.14 %) by applying the K 3 ZrF 7 model 31 to the 495 °C synchrotron pattern (refined together with form-VII). Crystallographic parameters are gathered in Table S1 and interatomic distances in Table S2. The same model used in the attempts to fit the 264 °C complex pattern could lead to a Rietveld Bragg reliability R B ~14 % while nothing better than R B ~25 % was attained for form-II. It is believed that form-II belongs to the β 1 -K 2 MF 6 (M = U, Th) structure type but that disorder exists and could not be characterized due to the mixture of phases. Indeed, the K 3 ZrF 7 compound presents a strong disorder of the F atom positions so that it is difficult to estimate if all the zirconium polyhedra are ZrF 7 ones or if there is a distribution of three possibilities including also ZrF 6 and ZrF 8 ones. This is not the same for K 2 ZrF 6 form-III since all polyhedra around the zirconium have to be octahedra. If only the F1 atoms are considered, they build perfect ZrF 6 octahedra (Zr-F = 1.977 Å), but there is a non-negligible proportion of F atoms at the F2 position corresponding to longer Zr-F distances (2.178 Å) compatible with larger coordination which could not be compensated by smaller ones in this case, since the average polyhedra is ZrF 6 that time. It is more probable that all polyhedra are ZrF 6 octahedra adopting five possible positions: that corresponding to F1 or one of the four possibilities suggested by F2.

Moreover, there is a supplementary cause for a disorder of the F atoms in K 2 ZrF 6 form-III which is the partial occupancy of the potassium sites (fully occupied in K 3 ZrF 7 ). In order to answer to the question "has this phase really a K 2 ZrF 6 formula or is it in fact K 3 ZrF 7 ?" we fixed the stoichiometry to the latter and the Rietveld R B value increased from 4.14 to 9.87 %. K 2 ZrF 6 form-V. This tetragonal form is present in large temperature ranges on heating (240-449 °C) and on cooling (411-139 °C), however, it is never a unique phase and the diffraction peaks are showing strange shapes/splitting, especially at the higher temperatures (Figure 9). It is to be noted that the a cell parameter decreases when the temperature increases, whereas c increases in a normal thermal expansion behaviour. The best pattern selected for the structure refinement is that of sample 2 at 200 °C on cooling.

The space group P4/mmm proposed (no refinement) for the tetragonal form of K 2 HfF 6 could not fit for K 2 ZrF 6 , the powder pattern showing clearly systematic extinctions leading to the P4/nmm space group. After structure solution, the refinement could be managed with an estimation of the anisotropic thermal parameters. The anisotropic peak broadening was undertaken by a simple approach, separating the narrower reflections (hh0) from the others. The crystallographic parameters and interatomic distances are in Tables S3 andS4, respectively. The Rietveld plot is in Figure 10.

The crystal structure (Figure 11) of form-V is built up from ZrF 6 octahedra, and K(1)F 6 octahedra (as in KF) alternating in corner-sharing chains with the ZrF 6 ones (Figure 12). In these K(1)F 6 octahedra, the potassium atom is strongly off-centre. The second independent potassium atom K( 2) is in a 8+4 coordination very close to a cuboctahedron (the square faces are rectangularly distorted). K 2 ZrF 6 form-VI. This orthorhombic form (space group Cmcm) has a small domain of existence in temperature (288-317 °C) and is mixed with form-V and form-III. The unindexed form-IV continues also to exist but its proportion decreases with the temperature increasing and the peaks become even broader than previously. A relatively fast cooling was made in situ during the synchrotron measurements (from sample 1) from 300 to 54 °C, and the forms VI and V were also S5 andS6, respectively, and the Rietveld plots are in Figure 13 (295 °C) and 14 (54°C). As for form-V, K 2 ZrF 6 form-VI contains one isolated Zr(1)F 6 octahedra but there is also an uncommon dimeric entity formulated Zr 2 F 12 built from a Zr(2)F 7 monocapped trigonal prism sharing a corner with a Zr(3)F 6 octahedron (Figure 15). There are five different potassium atom sites, showing all a different polyhedral shape: K(1)F 8 is in a bicapped trigonal prism; K(2)F 11 is a tricapped cube; K(3)F 7 is a monocapped trigonal prism; K(4)F 6 is an octahedron; K(5)F 8 is a hexagonal-based bipyramid. There is a considerable decrease of the b and c axes lengths between 295 and 54 °C. The atom which had a quite abnormally big thermal factor at 295 °C was F( 9), but it is F(7) at 54 °C. Both fluorine atoms belong to the Zr(3) environment and show abnormally short Zr-F distances which could be released according to some positional disorders presenting differences at the two temperatures. K 2 ZrF 6 form-VII. The pattern selected for structure solution and refinement is at 495 °C. Some ZrO 2 is appearing due to the high reactivity of fluorides and the contact with the sapphire capillary.

Two ZrO 2 forms are present, the monoclinic and the tetragonal. The cubic form-III is present in large quantity so that its structure could be refined simultaneously as noted previously. The form-VII refined crystallographic parameters and selected interatomic distances are in Tables S7 andS8, respectively, the Rietveld plot is in Figure 16. There are considerable differences between the DFT-optimized atomic coordinates and the Rietveld-refined ones and therefore on the interatomic distances (Tables S7 andS8). The indexing and the space group assignment were straightforward from the first 20 peaks at small diffraction angles, and the Le Bail fit allowed to extract intensities that led very directly to the full structural model by the direct space method. However, when the final Rietveld fit is examined in details, many peaks are badly fitted due to splitting (see especially the 301 peak split in three components in the Figure 16). There is no splitting at all for form-III peaks. Even if the ZrF 6 octahedra were recognized at the solution stage, soft restraints had to be used on the Zr-F distances during the refinement, in spite of which some distances are too short in Table S8. We have obviously to consider this structure as an imperfect approach, and even if the DFT optimization is capable to provide more realistic interatomic distances (to a point that we have chosen this time to show the drawing of the optimized crystal structure), using the fixed DFT coordinates does not improve the Rietveld fit, indeed, it is worst. Anyway, this phenomenon of peak splitting was also seen for form-V especially at the vicinity of the reversible VVII transition, and also for some of the form-I various preparations. From the comparison of the crystal structures of forms VII and V (Figure 17), the VVII transition is more probably topotactic, given the cell parameters obvious relations However, such a non-reconstructive transition suggests that a strong disorder may affect both phases since either 4 a V or 4 b V can become c VII , this could explain the quasi impossibility to fit the form-V powder patterns at high temperature, so that what we call "micro-phases" could correspond to different local arrangements at the interface of domains having the main structure.

a VII ~ c V , b VII ~ b V , c VII ~
It is hard to give the polyhedra shapes around of the potassium atoms, if the limit in the K-F maximum distance is fixed at 3.5 Å, the polyhedra shapes may be slightly different in the DFToptimized and Rietveld-refined models.

Based on the XRD results the scheme a scheme of phase transformations of K 2 ZrF 6 is suggested on Figure 18. In order to support the suggested models high temperature [START_REF] Parker | High-throughput powder diffraction on beamline I11 at Diamond[END_REF] F MAS NMR spectroscopy was applied. As was mentioned in our previous work 9 In the present case, we observed a number of fluorine resonances much smaller than the number of non-equivalent fluorines. We explain this discrepancy by the dynamic behavior of fluorine atoms, which consists of a fast hoping of the fluorine on each position around one polyhedron. We think that in the measured spectra of K 2 ZrF 6 we observe a dynamic behavior of fluorine atoms, each signal corresponds to a different polyhedron and the chemical shift is the barycenter of the

CONCLUSIONS

From this study, not everything is explained yet (forms II and IV, more details on hypothetical micro-phases) but a considerable step is made in the knowledge of the K 2 ZrF 6 complex polymorphism that reveals to be seriously different from the old expectations. An incredible diversity of polyhedral shapes is adopted by the zirconium and the potassium atoms, with coordinations from 6 to 7 and 6 to 12, respectively. The "β-K 2 ZrF 6 " phase characterized previously by hyperfine quadrupolar interaction of 181 Ta at Zr sites [START_REF] Martínez | Temperature dependence of the hyperfine quadrupolar interaction in K2ZrF6[END_REF] , or by RAMAN [START_REF] Dracopoulos | Raman spectroscopic studies of molten ZrF4-KF mixtures and of A2ZrF6, A3ZrF7 (A = Li, K or Cs) compounds[END_REF] and NMR 7 before any crystallography study, is a mixture of up to four phases, indeed, two of which being still not completely understood (forms-II and IV). Three new crystal structures types were established ab initio from powder diffraction data for forms-V, VI and VII. Rietveld refinements were concluding for the ubiquitous cubic form-III related to K 3 ZrF 7 suggesting a possible conduction by K + ions as a consequence of 2/3 sites occupied. Strange enough is the fact that the forms VI and VII at high temperature present both a lower symmetry (orthorhombic with Z = 16 and 8, respectively) than the tetragonal form-V (Z = 2) existing also at much lower temperatures. A scheme of phase transformations of K 2 ZrF 6 was suggested. It could be noted that the phase transformation of K 2 ZrF 6 (here all Zr atoms are eight coordinated at RT forming ZrF 8 ) is related with the breaking of infinite chains of bridged zirconia atoms through F atoms followed by the formation of phase with dimeric fragments finished with island like fragment of ZrF 6 .

A re-examination of the polymorphism of K 2 HfF 6 looks desirable, given the high degree of isomorphism of Zr-and Hf-based fluorides. The fact is that no K 2 HfF 6 powder patterns corresponding to our K 2 ZrF 6 complex forms VI and VII (nor IV) was observed, though the forms I, II, III and V were listed [START_REF] Saalfeld | The polymorphism of dipotassium hexafluorohafnate[END_REF] together with some others which we could not disclose. In case of future use of the K 2 ZrF 6 formulation for the "Generation IV" Molten Salts Nuclear Reactors, these results will have to be taken into consideration, especially the strong variation of the volume occupied by the molecular unit in the various solid states when cooling from the molten state. The nice dream of a phase transition from a crystal structure to another, both perfectly ordered, is not fulfilled in the K 2 ZrF 6 case, for reasons remaining to be explained.

ASSOCIATED CONTENT

Supplementary material contain crystallographic parameters and selected bond distances or K 2 ZrF 6 forms III, V, VI and VII phases tables of indexed powder patterns of phases I-VII, full dataset for K 2 ZrF 6 at 265°C and a CIF file for every synchrotron refinement including the Fullprof .pcr and .dat files for Rietveld or Le Bail fits. This material is available free of charge via the internet at http://pubs.acs.org.

CORRESPONDING AUTHOR

*Institute of Inorganic Chemistry, Department of molten systems, Dubravska cesta 9, Bratislava; 845 36, Slovakia. E-mail: miroslav.boca@savba.sk.

For Table of Contents Use Only

Polymorphism of K 2 ZrF 6

Ľubomír Smrčo † , Armel Le Bail, Miroslav Boca*, Aydar Rakhmatullin SYNOPSIS Phase transformations of K 2 ZrF 6 were analysed in situ using synchrotron powder diffraction and high temperature [START_REF] Parker | High-throughput powder diffraction on beamline I11 at Diamond[END_REF] F MAS NMR. Exceptional multiple solid-solid transformation was described.

Figure 1 .

 1 Figure 1. DSC thermal analysis of K 2 ZrF 6 by DSC under argon at a heating rate of 5 °C min -1 from room temperature up to 500 °C and back.

Figure 2 .

 2 Figure 2. Contour plot from the synchrotron thermo-diffractometry on K 2 ZrF 6 sample 1 in the range 185-300 °C, on heating.

Page 8 of 33 ACSFigure 3 .

 333 Figure 3. Parts of the synchrotron diffraction patterns of the two starting K 2 ZrF 6 samples at 200 °C illustrating their differences and suggesting the coexistence of micro-phases with slightly different cell parameters.

Figure 4 .

 4 Figure 4. Refined diffraction pattern from synchrotron data for K 2 ZrF 6 form-I at 22 °C after free cooling from 470 °C. Red dots represent the observed data, the black line represents the calculated ones. Bragg ticks are the peak positions (3 sets from top to bottom: reflections of form-I, new rhombohedral phase related the cubic K 3 ZrF 7 , monoclinic ZrO 2 ) and the blue curve shows the difference between the observed and calculated patterns,  = 0.82563 Å.

Figure 5 .K 2

 52 Figure 5. Projection of the unit cell of K 2 ZrF 6 form-I along the a axis showing the infinite chains of ZrF 8 square antiprisms sharing edges.

Figure 6 .

 6 Figure 6. Comparison of the two samples synchrotron patterns at 265 °C. The tetragonal form-V is more present in the case of sample 1. Some of the main peaks corresponding to form-IV (unindexed) are indicated.

Figure 7 .

 7 Figure 7. Le Bail fit on selected zones of the sample 2 synchrotron pattern at 265 °C using only the dominant forms II and III.

Figure 8 .

 8 Figure 8. The structure of K 2 ZrF 6 form-III projected along the [111] direction, with all possible positions for the F atoms represented, delimiting a kind of isolated sphere around of the Zr atoms due to the various orientations of the ZrF 6 octahedra.

Page 14 of 33 ACSFigure 9 .

 339 Figure 9. Splitting of the (002) reflection (K 2 ZrF 6 form-V) at the higher temperatures, strange peak shapes, a cell parameter decreasing with the increasing temperature as seen from the (110) reflection positions. Where not specified, temperatures were on cooling from 495 °C.

Figure 10 .

 10 Figure 10. Refined diffraction pattern from synchrotron data for K 2 ZrF 6 form-V at 200 °C on cooling. Red dots represent the observed data, the black line represents the calculated ones. Bragg ticks are the peak positions (3 sets from bottom to up: hk0 reflections of form-V, cubic K 2 ZrF 6 form-III, other peaks of form-V) and the blue curve shows the difference between the observed and calculated patterns,  = 0.82695 Å. Traces of m-ZrO 2 were managed by small excluded zones.

Figure 11 .

 11 Figure 11. Unit cell projection of the K 2 ZrF 6 form-V (at 200 °C on cooling) structure along the a axis. Ellipsoids are represented at 50% probability.

Figure 12 .

 12 Figure 12. Alternating corner-sharing trans-chains of ZrF 6 and KF 6 octaedra along the c axis of the K 2 ZrF 6 form-V structure. Ellipsoids are represented at 50% probability.

Page 17 of 33 ACS

 33 that temperature. The refined crystallographic parameters and interatomic distances are in Tables

Figure 13 .

 13 Figure 13. Refined diffraction pattern from synchrotron data for K 2 ZrF 6 form-VI at 295 °C. Red dots represent the observed data, the black line represents the calculated ones. Bragg ticks are the peak positions (up: form-VI, bottom: form V) and the blue curve shows the difference between the observed and calculated patterns,  = 0.82711 Å.

Figure 14 .

 14 Figure 14. Refined diffraction pattern from synchrotron data for K 2 ZrF 6 form-VI at 54 °C after natural cooling from 300 °C. Red dots represent the observed data, the black line represents the calculated ones. Bragg ticks are the peak positions (up: form-VI, bottom : form V) and the blue curve shows the difference between the observed and calculated patterns,  = 0.82711 Å.

Figure 15 .

 15 Figure 15. Unit cell projection of the K 2 ZrF 6 form-VI (at 295 °C) structure along the short a axis showing the isolated ZrF 6 octahedra and the dimeric entities formulated Zr 2 F 12 built from a ZrF 7 monocapped trigonal prism sharing a corner with a ZrF 6 octahedron.

Figure 16 .

 16 Figure 16. Refined diffraction pattern from synchrotron data for K 2 ZrF 6 form-VII at 495 °C. Red dots represent the observed data, the black line represents the calculated ones. Bragg ticks are the peak positions (from up to down: orthorhombic form-VII, cubic form-III, tetragonal ZrO 2 , monoclinic ZrO 2 ) and the blue curve shows the difference between the observed and calculated patterns,  = 0.82695 Å. The splitting of some form-VII peaks is exemplified by the enlarged part showing the tripled 301 peak.

  4 a V and the observation that relatively small translations and rotations of the ZrF 6 octahedra and translations of the K atoms allow to reproduce one crystal structure from the other.

Figure 17 .

 17 Figure 17. K 2 ZrF 6 form-VII (up) DFT-optimized model compared to form-V (down) extended on four times the a V cell parameter. The comparison shows the rotations and translations of ZrF 6 octahedra as well as the translations of the K atoms occurring during the topotactic transition.

Figure 18 .Figure 19 .

 1819 Figure 18. Scheme of phase transformations of K 2 ZrF 6 .

  a desiccator at ambient temperature (1.147 g, 40 %). The studied compound can be prepared by the described procedure using zirconium fluoride (0.01 mol, 1.672 g) instead of zirconium oxide and/or potassium fluoride (0.02 mol, 1.162 g) instead of potassium hydrogen fluoride. It is crucial to use the right concentration of hydrofluoric acid. The use of highly concentrated solution is resulting into formation of solvated product K 2 ZrF 6 •HF[START_REF] Gerasimenko | Dipotassium hexafluoridozirconate(IV) hydrogen fluoride, K2ZrF6•HF[END_REF] , while from pure water solution K 3 ZrF 7 crystallises[START_REF] Hampson | The Structure of Ammonium Heptafluozirconate and Potassium Heptafluozirconate and the Configuration of the Heptafluozirconate Group[END_REF] . Two K 2 ZrF 6 samples were made, noted samples 1 and 2 below, which were used for mainly two different synchrotron experiments (both fraction might differ in grain size).

	Zirconium oxide (0.01 mol, 1.232 g) was dissolved in hydrofluoric acid (10 wt % in H 2 O;
	30 mL) by continuous stirring and heating (80-90 °C) in a platinum dish. Potassium hydrogen
	fluoride (0.02 mol, 1.562 g) was added and the reaction mixture was stirred for ca. one hour. The

2 ZrF 6 : For the preparation of the title compound the following chemicals were used: ZrO 2 (Chiron, 98.5 %), KHF 2 (Lachema, 99%), KF (Sigma-Aldrich, 99 %), ZrF 4 (prepared at the Institute of Chemistry KSC RAS, Apatite, Russia; min. 99.5%; no impurities could be detected by powder XRD), HF (38-40 wt % in H 2 O; Lachema). KF was dried in vacuum at 130 °C for 24 h; other chemicals were used without further purification. The handling of KF and ZrF 4 was done in a glove box under dry argon atmosphere (Messer Tatragas, 99.9990 %) with moisture content below 10 ppm. resulting clear solution was filtered through a folded paper filter into a plastic beaker and left to crystallise at ambient temperature. After two days, the oblong colourless crystals were removed and dried in

Table 1 .

 1 Crystallographic data for the K 2 ZrF 6 forms I to VII (IV excluded)

		I	II	III	V	VI	VII
	crystal system	monoclinic	hexagonal	cubic	tetragonal	orthorhombic	orthorhombic
	space group	C2/c	?	Fm-3m	P4/nmm	Cmcm	Pnma
	temperature (C)	22*	264	495	200**	295 / 54	495
	(K)	295	537	768	473	568 / 327	768
	a (Å)	6.56646(2)	6.4473(1)	9.15641(3)	6.33605(2)	6.40840(5) / 6.37122(8)	10.30844(7)
	b (Å)	11.44440(4)				20.7697(2) / 20.5603(3)	6.02909(3)
	c (Å)	6.94202(2)	3.8606(1)		9.29458(5)	19.9888(2) / 19.6642(3)	23.8017(2)
	β (deg)	90.5668(3)
	V (Å 3 )	521.661(3)	138.975(2)	767.675(5)	373.136(3)	2660.52(4) / 2575.90(6)	1479.29(2)
	Z	4	1	4	2	16	8
	V/Z	130.41	138.97	191.92	186.57	166.28/160.99	184.91
	Wavelength (Å)	0.82563	0.82695	0.82695	0.82695	0.82711	0.82695
	2 range (deg)	7-100		3-49	4-59	3-50 / 3-60	3-49
	refined coordinates	13		3	6	32	32
	soft restraints	none		none	none	none	18 (ZrF 6 )
	R P	3.82		5.72	7.73	3.04 / 3.10	5.72
	R WP	4.94		9.69	10.5	4.65 / 4.45	9.69
	R B	1.99		4.14	5.10	7.63 / 6.03	13.4
	R F	0.96		5.67	7.97	7.85 / 5.25	22.3
	other phases	ZrO 2 ,	III, IV, V	VII, ZrO 2	III, ZrO 2	IV, V / V	III, ZrO 2
		K 3 ZrF 5 O ?

Table 2 .

 2 Crystallographic parameters for K 2 ZrF 6 form-I at 22 °C (C2/c) after cooling from 470 °C. Second line, from Hoppe and Mehlhorn (1976) 4 third line from Gerasimenko et al. (2006) 5

	atom	site	x	y	z	B iso or U iso
	K	8f	0.01939(7)	0.34331(4)	0.00129(7)	1.725(9)
			0.0197(2)	0.3428(1)	0	0.84
			0.01844(5)	0.34287(3)	0.00238(3)	0.02044(6)
	Zr	4e	0	0.04711(1)	1/4	0.167(4)
				0.04761(7)		0.71

Table 3 .

 3 Selected bond distances (angstroms) in K 2 ZrF 6 form-I from synchrotron X-ray diffraction refinement

	bond	length	bond	length	bond	length
	Zr-F1 x 2	2.0201(9)	K-F2	2.6897(11)	K-F2	2.8048(11)
	Zr-F2 x 2	2.0691(10)	K-F1	2.7236(10)	K-F3	2.8256(8)
	Zr-F3 x 2	2.1503(8)	K-F2	2.7240(11)	K-F1	2.8274(10)
	Zr-F3 x 2	2.1946(8)	K-F2	2.7418(12)	K-F3	3.0206(11)
	average	2.1085	K-F1	2.7879(10)	K-F1	3.1272(10)
					average	2.8273

  , spectrum at 250 °C contains two resonances at -16.3 ppm and at -33.7 ppm with ratio 3:1, respectively. It should be noted that the signals have very different values of the chemical shift anisotropy, 28 and 10 ppm, respectively.An increase of the temperature from 250 °C to 290 °C leads to a decrease of the line width (full width at half-maximum, FWHM) for low field peak from 740 to 460 Hz and increase for high field signal from 440 Hz to 640 Hz. In addition, a small shift of around 1 ppm in low field was measured.At 310 °C two new peaks appear at -15.9 and -19.1 ppm. In the range of the temperature 310-340 °C we observe 4 signals. We note that at 340 °C two initially observed peaks almost disappeared.At 360 °C the integral intensity of the resonance at -15.5 ppm reaches maximum and then it starts to decreases at 380 °C until the signal disappears completely at 410 °C. At 410 °C only one peak was observed. NMR data are summarized in Table3.Usually, at room temperature (or at lower temperatures) the high-resolution solid state NMR spectrum should contain the number of lines corresponding to the number of crystallographically non-equivalent atoms depending on the local chemical environment, in other words different nucleus in a molecule resonates at different frequencies. On other hand, the NMR spectra acquired in molten state consist in a single Lorentzian line, characteristic of a rapid exchange at the timescale of NMR between the different atomic configurations around the observed nucleus[START_REF] Bessada | In situ NMR approach of the local structure of molten materials at high temperature[END_REF] . In some cases the measured chemical shift might thus be the average of the individual chemical shifts of the different species even at room temperature or temperatures below melting as was discussed many times for K 2 TaF 7[START_REF] Boca | Differences in XPS and solid state NMR spectral data and thermo-chemical properties of isostructural compounds in the series KTaF6, K2TaF7 and K3TaF8 and KNbF6, K2NbF7 and K3NbF8[END_REF] and refs. therein. Some other examples of the dynamics of fluorine atoms can be Ba 5 AlF 13[START_REF] Martineau | Structure determination of Ba5AlF13 by coupling electron, synchrotron and neutron powder diffraction, solid-state NMR and ab initio calculations[END_REF] , Rb 3 TaOF 6[START_REF] Laptash | Dynamic orientation disorder in rubidium fluorotantalate. Synchronous Ta-O and Ta-F vibrations[END_REF] , or (NH 4 ) 2 ZrF 6[START_REF] Voit | Quantum-chemical study of potassium and ammonium hexafluorozirconates[END_REF] .

Table 3 .

 3 [START_REF] Parker | High-throughput powder diffraction on beamline I11 at Diamond[END_REF] F NMR data obtained from the simulation of the[START_REF] Parker | High-throughput powder diffraction on beamline I11 at Diamond[END_REF] F NMR spectra at 17.6 T and MAS 7 kHz during the heating of the sample from RT up to 410 °C

	Page 25 of 33					Crystal Growth & Design
			Zr 2 F 12	ZrF 6	β-K 2 ZrF 6	phase-III (K 3 ZrF 7 )
			δ iso , ppm	Integral,	δ iso , ppm	Integral,	δ iso , ppm	Integral,	δ iso , ppm	Integral,
	T,	°C	(±0.1 ppm)	% (± 1%)	(±0.1 ppm)	% (± 1%)	(±0.1 ppm)	% (± 1%)	(±0.1 ppm)	% (± 1%)
	410	-	-	-18.2	100	-	-	-	-	phase-VII
	390	-15.3	16.8	-18.3	83.2	-	-	-	-
	380	-15.4	39	-18.4	61	-	-	-	-
											phase-VI
	380	-15.4	52.9	-18.4	47.1	-	-	-	-
	360	-15.5	75	-18.8	25	-	-	-	-
	340	-15.6	75	-18.8	23	-13.3	1	-32.9	1	phase-VI;
	320	-15.8	70	-19	24	-14	4	-33.1	2	β-phase;
	310	-15.9	7	-19.1	30	-14.7	51	-33.2	12	phase-III
	290	-	-	-	-	-15.1	80	-33.2	20	phase-II +
				-		-					Phase-IV =
											β-phase;
	250	-		-		-16.3	77	-33.7	23	phase-III
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ACKNOWLEDGMENTS

This work was supported by the Slovak Research and Development Agency under the contract No. APVV-15-0479. This work was financially supported by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences, grant no. Vega 2/0024/20. Financial support from TGIR-RMN-THC Fr3050 CNRS for conducting the research is gratefully acknowledged.

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individual resonances. Similar behavior was observed also in the case of K 18 Ta 5 Zr 5 F 63 compound [START_REF] Boča | The structure of the metastable K18Ta5Zr5F63 phase[END_REF] .

In the range 250 °C to 290 °C we detected two signals for K 2 ZrF 6 . The value of the chemical shift at -33.7 ppm is slightly different from others, and it is very close to the signal of K 3 ZrF 7 (-34/-37 ppm at RT 9,[START_REF] Rakhmatullin | Solid state NMR and XPS of ternary fluorido-zirconates of various coordination modes[END_REF] ), and we assigned this signal to the phase-III related to K 3 ZrF 7 . The second signal can be assigned to β-K 2 ZrF 6 . At 410 °C NMR spectrum is characterized by one line at -18.2 ppm and it can be attributed to the phase-VII with ZrF 6 environments (Figure 19). The phase-VI consists of two different polyhedrons: ZrF 6 and Zr 2 F 12 . The signal at -15.4 ppm was assigned to dimeric fragment Zr 2 F 12 and signal around -18.4 ppm to the island like ZrF 6 fragment. The phase transition between 390 °C and 410 °C is accompanied not by the appearance of a new signal, but by the disappearance of the signal at -15.3 ppm corresponding to the dimer Zr 2 F 12 and preserving the signal at -18.3 ppm. Based on this observations it could be estimated that β-phase that was for the first time mentioned by [START_REF] Novoselova | The KF-ZrF4 system[END_REF] and lastly discussed by 9 probably consist of structural motives based on bridging of zirconium atoms by fluorine atoms; Zr-F-Zr.

The spectral changes were not reversible upon cooling to RT. A cooling up to 170 °C does not lead to changes in the spectrum, only one line was observed, which just shifted from -18.2 ppm (410 °C) to -20 ppm (170 °C). Then two new small peaks were observed. In order to reach the maximum resolution, a very fast spinning rate of 50 kHz was used after cooling the sample. As in case the heating-cooling cycle RT-250 °C-RT 9 , in addition to the expected signals for the α-phase (I=97.3 %), two small signals at -34.6 ppm (I=1.6 %) and -36.7 ppm (I=0.1 %) were detected. This spectral evolution during the cooling process is in a good agreement with the obtained XRD data.

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Constance Jame
email: constance.jame@hcts.uni-heidelberg.de

Introduction

The concept of heritage in Qatar came into being in the 1970s, following the country's independence in 1971 and the foundation of a nation-state. Based on the European model of modernity, this willing to create a heritage of its own led to the opening of a National Museum in 1975. After his coup d'état in 1995, the former Emir, Sheikh Hamad bin Khalifa Al-Thani, instituted the development of a cultural program in the country to establish Qatar as a major international cultural centre. This perspective was reaffirmed through the creation in 2008 of the Qatar National Vision 2030 project which aims to 'preserve Qatar's national heritage and enhance Arab and Islamic values and identity'. The creation of a museum solely dedicated to Islamic art in 2008 is part of this process, beside the opening of the Mathaf, dedicated to modern and contemporary Arab art in 2010 and the refurbishment of the National Museum in 2018. Two official reasons have been underlined to explain the opening of the Museum of Islamic Art (MIA). First, the economy and the rapid development of Qatar being based on the oil and gas exploitation, which will be in the long term, used up, the country tried to reinvent its economy, through the investments in museums, but also international universities and sport, to become a cultural hub in the region and attract tourists. Secondly, in the growing context of Islamophobia, MIA is used as a bulwark by the government 'to raise the global debate about the nature and principles of Islamic civilization and to alter the perceived images of that civilization'. However, beyond this scope, based on Benedict Anderson's theory of the role of the museum in building national imagination and Roland Barthes' mythology's theory I would like to argue that MIA aims to state the belonging of Qatar to the Islamic world and its heritage through the shaping of a myth. This mythology asserts the transnational identity of the country. This paper is based on my museographic analysis ranging from the architecture of the building and the scenography of the works to the mediation around them. In addition to my evaluations of the museum space, I conducted semi-structured interviews with different actors of the museum to understand their decisions. These data were collected during my doctoral field research in 2019 and 2021.

Museum: shaping the myth of national identity

Drawing from Ferdinand de Saussure's theory on semiology, in Mythologies, Barthes demonstrated that a myth is a language, and as such a mode of signification. For Saussure, the linguistic sign, the word, consists of the double articulation signifier and signified (meaning conveyed by the sign). For Barthes, in the mythological sign (an image or an object), a third term is added to the relationship between the signifier and the signified, the signification. He argued that the connection between both is an act of signification. The signifier has already a signification, but the myth transformed it in an empty form, to create a new signification. This connection becomes over time naturalized and the signification universal, and so the language of myth takes over reality to become the reality. Barthes' perception of myth applied also to museum, the museum being a system of communication delivering a message through the display of objects. As such, the linguist Louise Ravelli developed the concept of 'museum-as-text' in arguing that the museum draws on a variety of semiotic resources, like physical with the practice of design and display, discursive with community involvement and government directives, and written with museum's texts. Drawing from Barthes' myth theory and Ravelli's concept of 'museum-as-text', I argue that the museum is a place where myth can be formed. Indeed, museums express signification through the lens of the presentation of objects. Their ways of being displayed, the selection of the objects, the labels giving sense to them, the scenography designing the display, and the architecture surrounding the objects, every single detail within the museum is meaningful. This signification delivered by the museum changed over time, such as the myth, which is not permanent, because it reflects a moment in society. The myth is created, because the museum displays a story, a historical one or artistic one, that is presented as permanent and the unique reality, in highlighting one layer of meaning of the objects.

Since its inception at the end of the 18 th and beginning of the 19 th centuries in Europe, the museum has been at the heart of the construction of the modern nation-state, beginning with the Louvre and its creation after the French revolution and the birth of the French Republic. The Louvre symbolized the Republic in displaying and giving access to each citizen to the nationalized collections of the former royal family, of the nobles and the Church. The national museum was supposed to gather citizens around a new myth, the national identity of the French Republic. The Louvre symbolized what Benedict Anderson called an 'imagined community'. In his text, he defined this concept of a modern nation as an 'imagined community', because it is a construction to gather people at a moment when 'Enlightenment and Revolution were destroying the legitimacy of the hierarchical dynastic realm' and order. The Louvre was opened to the public in that perspective, when France was searching for a new political structure and foundation able to gather people, after being a monarchy during centuries. In the context of Qatar, the same process can be seen. Indeed, being a new modern nation-state looking also for a common identity, the sudden buildings of these different museums reflect the willing of constructing a community, which I will argue in the following is imagined, because manufactured through the belonging to a mythological past.

A museum built from a connoisseur perspective?

According to the museum discourse through a recent temporary exhibition, A Falcon's Eye: Tribute to Sheikh Saoud Al Thani, the MIA collection, among other Qatar Museum's collection, has been created by the Sheikh Saoud Al Thani. The exhibition highlighted the role of Saoud in the founding of Qatar Museum's collections, as a man of taste, presenting him at the image of the 19 th century connoisseur: '(he) was so influential in the constitution of collections for Qatar Museums and the State of Qatar in general', 'a collector of crucial importance to Qatar Museums', 'he laid the foundations of the major collections', 'his collecting vision', 'a real passion and eyes for arts and culture'.

The MIA collection has been built ex nihilo by Sheikh Saoud between 1995 and 2000s at the request of his cousin, the former Emir, Sheikh Hamad. Saoud was appointed president of the National Council for Cultures, Arts and Heritage to pursue this quest of gathering objects in the purpose of building a collection and then a museum dedicated to Islamic art. So, the museum is not exhibiting Saoud's personal collection, but the State's collection gathered by Saoud, as a representant of the State. Saoud has been chosen by his cousin for this task, because he was already a collector and passionate about art. However, behind the figure of the Sheikh as art amateur, drawn in the exhibition, there is the figure of Guy Bennett, a former Christie's Islamic art expert and dealer, who has been appointed as Qatar Museum's director of collection and acquisition. With Bennett's support, the collection has been built on the western art market mostly at Christie's and Sotheby's. Part of the collection is also from former private collections. Thus, the collection reflects mainly what was on the art market at that moment, and considered by the experts as the most beautiful, valuable objects and most representative of what could define 'Islamic art'.

A first layer of the myth is created through the figure of the father of the collection. The exhibition dedicated to Sheikh Saoud is doubly the construction of a mythology. Before this exhibition, the role of the Sheikh was known, but not particularly highlighted by the museum. There was no mention on the website of the museum and even within the museum's galleries regarding the formation of the collection. This can be explained by the royal disgrace which affected the Sheikh from 2005. He was accused to have amassed a personal collection with the state money when he was supposed to collect for the state. After that, he was put beside until his death. Dedicating an entire temporary exhibition within MIA to the Sheikh as a tribute to his collector's work, also shows a change of perspective by the state itself regarding his person. The state supports or even causes the change of his image. Through this myth of a father, a history is created, asserting that the presence of this Islamic art collection is not the fruit of the state, but the result of one of its citizen's passions presented as 'a modern Renaissance man of the Arab world'. Thus, the collection itself has a proper history, putting in the shadow the influence of the state within the formation of the collection and the museum.

The lack of collection's history has been hidden through the choice of display. Following, Western presentation of Islamic art since Munich 1910 exhibition, Masterpieces of Muhammedan Art, the beauty of the objects is highlighted, through the aesthetization of the collection. Indeed, the objects are presented in a white cube. Labels give very restrictive information on each object: name, materials, period of production, geographical origin. Objects are either isolated in a solo showcase in the centre of the room, or exhibited in group, but with space between each object. An atmosphere of preciosity is created, through the solely lighting of the objects contrasting with the darkness of the room. This choice of display is linked to the character of the collection itself. Indeed, Oliver Watson, former director of MIA, wrote in the museum's collection catalog that the collection is not meant to represent the entire Islamic art production, but to highlight 'masterpieces'. The process of collecting on the art market wasn't as such based on the taste of the collector, but on the value of the object, an object, which could be labeled as 'masterpiece'. According to Hans Belting, the term was used to designate an ideal of art, the highest form of artistic creation. For him, the concept of 'masterpiece' became normalized through the rise of museum, the musealization of objects gave them this status. Eva Troelenberg in her analysis of the 1910 Munich exhibition argued that after this exhibition it became canonical to display Islamic art in an 'unhistorical' and 'unethnographical' approach. As such, objects are identified as art.

This double decontextualization, from the way of collecting and from the display, helped to the construction of the myth. The display allowing the creation of the myth, that this, those objects, is Islamic art.

A transcultural architecture: 'Arab', 'Islamic' and 'Modernism'

The architecture of MIA has been widely analyzed and also described by his creator. I.M.Pei stated knowing nothing about Islamic art and architecture, he had to travel around the Arab world to get inspired for the purpose of constructing the building. For the building skeleton, he draws his inspiration from the fountain of the mosque of Ahmad Ibn Tulun (876-879) in Cairo, which is one of the oldest mosques known today. Also, the architectural vocabulary refers to Islamic art tradition. For example, the left courtyard, surrounding the main building, has a raw of arches, also a typical element of the Islamic architecture, which can be found for example in the Umayyad architecture such at the Cordoba's Mosque-Cathedral or at the Anjar palace in Lebanon. Inside the building, this vocabulary is also present. One of the most visible reference to Islamic art vocabulary is the geometric pattern on the ground in the atrium reminding the mosaics of the Alhambra. Also in the atrium, a great window referred by its form and position to the mihrab, the niche in mosque's wall indicating the direction of the Mecca. For Karen Exell, 'the rhetoric of fusing the traditional and the modern represents the region's approach to modernity, an agenda of retaining cultural identity in balance with aspects of a more secular modernity'. Pei is not copying Islamic architecture but capturing elements of western modern architecture and Islamic architecture to emulate his own architecture. The transcultural vocabulary of the architecture inscribed Pei's building within the Qatari architectural movement, but also widely in the Gulf, to create a contemporary Islamic architecture. Here, Pei's architecture is also a tool to build a myth of the belonging of the country to Islamic artistic heritage. In making references to the 'Islamic' architecture tradition from different regions and periods in the Arab world, Pei states that Islamic architecture is a 'global' heritage, which as such belongs to Qatar and to the world.

Naming the myth

The myth is stated from the origin in the name of the museum itself, Museum of Islamic Art. The name witnesses the aim of anchoring the country within a history, a global culture, such as the global Islamic heritage. The use of the name 'Islamic art' is problematic for museums and in the research. Since almost its creation, the field of Islamic art is struggling with its designation. This debate has been increased since the beginning of the century and after 9/11, especially within western museums leading to the MET's choice to not entitle their gallery as 'Islamic'. In this context, Doha's choice bears a powerful meaning. For Tim Stanley, Islamic art curator at the V&A, 'in the late 20 th century, it (Islamic art) gained new legitimacy as part of the intellectual apparatus of self-consciously Islamic states'. Thus, by choosing this name, despite the debate around it, the Qatari government wants to settle the country as one of the heirs of this Islamic great history and culture. This myth is made not only for Qataris but also for worldwide vision of the country. Coming back to the architecture, the choice of Pei is symbolic of this desire to register the country for international community within the Islamic global history. Indeed, Pei wasn't supposed to be the architect of the museum. Another architect, Rasem Badran, won the competition, and Pei didn't even participate to the competition. The father Emir requested him to be the architect. The choice of Pei is symbolic of the status given to the museum by the Emir himself, and as such within the state of Qatar, because Pei is not only a worldwide famous architect, but he is also the creator of the Louvre's Pyramid. The Louvre, being the biggest and most famous museum in the world, symbol of the creation of the French Republic.

Conclusion

At each level, from the inception of the collection and the choice of the architect to the presentation of the objects, the museum has been staged to shape a mythological past to Qatar. Why is it a myth? Qatar is a very young nation-state looking for its own identity. However, the human presence on his soil is very old, because artifacts from the Ubaid period (c. 6500-3800 BC) have been found on its territory. However, as the National Museum shows Qataris were until the 19 th century Bedouins. As such, their material culture is not identified as Islamic art, but as ethnographic material, which can be questioned. As we already mentioned, Islamic objects are identified as art, underlining the fact that it is beautiful objects with an artistic value. Objects defined as such were produced mainly in the court context. Also, Islamic art, by its old definition which today is debatedis defined as being produced between the 7 th until the 19 th century. MIA follows this established definition in its exhibition. Modern and contemporary artworks can be found in another museum, Mathaf. As such, it seems impossible that MIA exhibits Qatar's cultural heritage. Thus, the collection presents no objects produced on Qatar soil or even in the Gulf. Through different interviews in 2019 and 2021, many members of the museum's staff at different levels (curators, director, and head of exhibitions) confirmed the fact that Qataris did not feel attracted by the museum due in part to the lack of connection with their own history, identity and culture. One of the first reason raised by them was the absence of objects from Qatar on the contrary to the recently refurbished National Museum, which since its opening has been attracting much more Qataris than its neighbor, MIA. The building and separation between the 1975 and then 2018 refurbished National Museum, the 2008 MIA and the 2010 Mathaf witnesses Qatar's strategy in presenting different layers of identity. The National Museum presents material cultures from Qatar's territory, as such defending a national (local) identity based on the principle of 19 th century idea of nation-state and identity. The Mathaf defends the anchor of Qatar within a regional Arab identity. When, MIA, in addressing Islamic art definition as a unified civilization drawing from different periods and geographical areas defends the belonging to a global identity, the Islamic one. Through MIA, Qatar registered itself within an identity beyond the nation defined as nation-state. It challenged the idea of identity based on the 19 th century European model, that an identity is defined by its territory. Qatar defends the belonging to a transnational identity which is shaped through the museum.