New triple molybdate and tungstate Na5Rb7Sc2(XO4)9 (X = Mo, W)

New compounds of the composition Na5Rb7Sc2(XO4)9 (X = Mo, W) were obtained via the ceramic technology. The sequences of chemical transformations occurring during the formation of these compounds were established, and their primary characterization was performed. Both Na5Rb7Sc2(XO4)9 (X = Mo, W) were found to melt incongruently at 857 K (X = Mo) and 889 K (X = W). They are isostructural to Ag5Rb7Sc2(XO4)9 (X = Mo, W), Na5Cs7Ln2(MoO4)9 (Ln = Tm, Yb, Lu) and crystallize in the trigonal crystal system (sp. gr. R32). The crystal structures were refined with the Rietveld method using the powder X-ray diffraction data. The thermal expansion of Na5Rb7Sc2(WO4)9 was studied by high-temperature powder X-ray diffraction; it was shown that this triple tungstate belongs to high thermal expansion materials. Keywords


Introduction
The search for new functional inorganic materials based on the development of ideas about the relationships between their structure and properties is one of the high-priority directions of modern solid state chemistry, crystal chemistry and materials science. The greatest attention is paid to the synthesis, study of the structure and properties of complex oxides, among which binary Mo (VI) and W (VI) compounds of various compositions occupy a significant place. In the last two decades, triple molybdates have been actively studied, and in recent years, triple tungstates have also attracted much attention as interesting research objects. Constant interest in such compounds is maintained due to their wide range of functional properties, such as catalytic, luminescent, laser, nonlinear optical, ferroelectric, ion-conducting, and others. Thus, numerous publications are devoted to triple molybdates and tungstates with the BaNd2(MoO4)4-type structure, which are represented by two families of compounds: LiMR2(MoO4)4 (M = K, Tl, Rb; R = Bi, Ln) and Li3Ba2Ln3(ХO4)8 (Х = Mo, W). The prospects for possible application of these compounds as photo-and IR-phosphors, materials for UV radiation dosimeters, laser materials are shown in [1][2][3][4][5][6][7][8][9][10]. The latter is also facilitated by the fact that the maximum anisotropy of thermal expansion in several representatives of this family is lower than that in other successfully used laser crystals [11]. The molybdate phosphor NaCaLa(MoO4)3: Tb 3+ /Yb 3+ can be used as a spectral converter [12].
In this work, the family of triple molybdates and tungstates represented by formula M′5M″7R2(XO4)9 [21,22] is extended by two new compounds, Na5Rb7Sc2(XO4)9 (X = Mo, W). The primary characterization of these phases was carried out. Their crystal structure was refined by the Rietveld method from powder X-ray diffraction data. In addition, the thermal expansion of Na5Rb7Sc2(WO4)9 was studied. Na2XO4 (X = Mo, W) used in this work were obtained by calcining the corresponding crystalline hydrates at 823-873 K. The phase purity of the prepared samples was confirmed by powder X-ray diffraction (PXRD). The PXRD patterns of Na2ХO4, Rb2ХO4, Sc2(ХO4)3 (Х = Mo, W) were in good agreement with the literature data [23].

Instrumental characterization methods
The processes that occur during the solid-state syntheses were monitored with PXRD using a D8 ADVANCE Bruker diffractometer (VANTEC detector, Cu Kα radiation, λ = 1.5418 Å, reflection geometry, secondary monochromator). High temperature X-ray measurements of Na5Rb7Sc2(WO4)9 were performed with the same instrument using an Anton Paar HTK 16 high temperature chamber in the temperature range of 303-823 K. The heating rate was 20 K min -1 . Prior to measurements, the sample was kept at a specified temperature for 25 min.
The unit cell parameters of Na5Rb7Sc2(XO4)9 (X = Mo, W) were refined by the least-squares method using ICDD program package for preparing experimental standards. The Smith-Snyder F30 criterion was used as a validation criterion for X-ray patterns indexing [24]. The crystal structures refinement of Na5Rb7Sc2(XO4)9 (X = Mo, W) at room temperature and the unit cell parameters determination in high-temperature studies were carried out by the Rietveld method [25] using the TOPAS 4.2 software [26].
The thermal measurements were carried out using an STA 449 F1 Jupiter NETZSCH thermoanalyser (Pt crucible, heating rate of 10 K min -1 in a flow of argon). The final powder products are of white color, insoluble in water and common organic solvents, soluble in HCl (Na5Rb7Sc2(MoO4)9 at room temperature, Na5Rb7Sc2(WO4)9 -at heating).

Results and discussion
According to the results of PXRD data, the sequence of chemical transformations in the course of Na5Rb7Sc2(WO4)9 formation from a stoichiometric mixture of simple tungstates can be illustrated by the following scheme: Scheme 1 The sequence of chemical transformations in the course of Na5Rb7Sc2(WO4)9 formation In the Mo-containing system the formation of Na5Rb7Sc2(MoO4)9 started at the stage when NaRb3(MoO4)2 and RbSc(MoO4)2 appeared. The corresponding scheme differs from that for ternary tungstate only in shorter synthesis times.
Both Mo-and W-based Na5Rb7Sc2(ХO4)9 melt incongruently at 857 K (X = Mo) and 889 K (X = W) ( Fig. 1). Reflections of both NaSc(MoO4)2 and a phase with an alluouditetype structure together with the initial phase were found in the PXRD pattern of the cooled Na5Rb7Sc2(MoO4)9 melt. The cooled melt of Na5Rb7Sc2(WO4)9 contains the double tungstates RbSc(WO4)2 and NaSc(WO4)2 and an alluaudite-like phase. The amount of the latter phase was dominant. The PXRD patterns of prepared single-phase compounds Na5Rb7Sc2(ХO4)9 (Х = Mo, W) are similar and show that these complex oxides are isostructural to trigonal Na5Cs7Yb2(MoO4)9, Ag5Rb7Sc2(ХO4)9 (Х = Mo, W) (sp. gr. R32, Z = 3) [21,22]. This allows satisfactorily indexing the PXRD patterns of Na5Rb7Sc2(ХO4)9 (Х = Mo, W) (in the case of molybdate F(30) = 141.6 (0.0056; 38), for tungstate F(30) = 287.2 (0.0028; 37)). The obtained crystallo-graphic characteristics are shown in Table 1, the results of indexing of Na5Rb7Sc2(WO4)9 are shown in Table 2 as an example. 3.2. Rietveld refinement of Na5Rb7Sc2(ХO4)9 (Х = Mo, W) structure The positional atomic parameters for the Ag5Rb7Sc2(MoO4)9 structure [21] were taken as a starting model for the refinement of the Na5Rb7Sc2(ХO4)9 (Х = Mo, W) structures by the Rietveld method. The refinement was carried out by gradually adding the refined parameters with the simultaneous graphical simulation of the background. The Pearson VII Function was used to describe the shape of peaks. Isotropic displacement parameters (Biso) for all atoms in Na5Rb7Sc2(MoO4)9 were refined separately, while for the O atoms in Na5Rb7Sc2(WO4)9 they were taken as equal. The refinement procedure included corrections for the sample preferred orientation and broadening of peaks due to anisotropy within the model of spherical harmonics [27]. The refinement results for Na5Rb7Sc2(ХO4)9 (Х = Mo, W) are shown in Table 3. Experimental, theoretical and difference PXRD patterns for Na5Rb7Sc2(ХO4)9 (Х = Mo, W) are shown in Fig. 2 and 3. The fractional atomic coordinates, isotropic atomic displacement parameters, cation occupancies and main selected interatomic distances are presented in Tables 4-7.
The crystal structures of Na5Rb7Sc2(MoO4)9 and Na5Rb7Sc2(WO4)9 were deposited in the Cambridge Crystallographic Data Centre with Cambridge Structural Database (CSD) № 2124713 and № 2124691, respectively [28]. In the structures of Na5Rb7Sc2(ХO4)9 (Х = Mo, W), Na1 and Na2 atoms are located in threefold special positions with the point symmetry 32; Sc, Rb1, and Rb2 sit at threefold axes; Rb3, Mo2 (W2), and Na3 are settled at twofold axes, and Mo1 (W1) and oxygen atoms are in general positions. Both Mo and W atoms have tetrahedral coordination, while Sc, Na1 and Na3 possess octahedral coordination. It is worth noting that, unlike the octahedron surrounding Na1, the octahedron around Na3 is distorted. The half-occupied Na2 site has a trigonal-prismatic environment. Rb1 and Rb2 atoms have 9-fold environments, while Rb3 exhibits CN = 8. The general view of the structure is illustrated in Fig. 4a.
The characteristic details of the title compounds are so-called 'lanterns' [Sc2(XO4)9] (X = Mo, W) composed by two ScO6 octahedra sharing corners with six terminal and three bridging XO4 tetrahedra (Fig. 4b). Together with the Rb1, Rb2 and Na3 cations they form two-tiered hexagonal layers parallel to (001) plane, which resemble the motif of the K3Na(SO4)2 glaserite structure [29]. The layers are folded with a displacement along the b axis and are connected by Na3, Na1 and Rb3 cations (Fig. 4c).

Thermal expansion of Na5Rb7Sc2(WO4)9
The thermal expansion of Na5Rb7Sc2(WO4)9 was studied by high-temperature X-ray diffraction. The thermal expansion of this compound, which crystallizes in a trigonal symmetry, is defined by two linear thermal expansion coefficients (LTECs) measured along (с) and across (a) the threefold axis. The average LTEC can be calculated as follows:    The reflections in the X-ray diffraction patterns of Na5Rb7Sc2(WO4)9 regularly shift with increasing temperature (Fig. 5) due to an increase in the unit cell parameters (Fig. 6). The parameter a changes with temperature almost linearly; the temperature variation of the parameter c is described by a polynomial of the second degree (Table 8). Table 8 also presents the coefficients of thermal linear expansion and thermal expansion anisotropy. The obtained results allowed classifying Na5Rb7Sc2(WO4)9 as belonging to high thermal expansion materials.
The thermal stability of obtained compounds was studied and the thermal expansion of Na5Rb7Sc2(WO4)9 was examined by the high-temperature XRD diffraction method; it was shown that this compound belongs to highly expanding substances. The crystal structure of Na5Rb7Sc2(ХO4)9 (Х = Mo, W) was refined by the Rietveld method using the PXRD data. The obtained compounds crystallize in the chiral sp. gr.
For two representatives of the M'7M''5R2(XO4)9 family, namely, Ag5Rb7Sc2(ХO4)9 (Х = Mo, W), we confirmed this experimentally earlier [21]. This stimulates our research to find new representatives of this group of phases, as well as to continue the study of the ion-conducting properties of already obtained compounds -(Na5Rb7Sc2(ХO4)9 (Х = Mo, W) and Na5Cs7Ln2(MoO4)9 (Ln = Tm, Yb, Lu). In addition, it seems expedient to carry out a further study of thermophysical properties for representatives of the considered structural type to reveal the influence of the nature of one-, three-and hexavalent elements on the value of thermal expansion coefficients and anisotropy in these phases. Fig. 6 Temperature dependences of the a and c unit cell parameters for Na5Rb7Sc2(WO4)9