New triple molybdate and tungstate Na5Rb7Sc2(XO4)9 (X = Mo, W) Chimica Techno Acta ARTICLE published by Ural Federal University 2021, vol. 8(4), № 20218412 eISSN 2411-1414; chimicatechnoacta.ru DOI: 10.15826/chimtech.2021.8.4.12 1 of 8 New triple molybdate and tungstate Na5Rb7Sc2(XO4)9 (X = Mo, W) Tatyana S. Spiridonova a* , Aleksandra A. Savina ab , Evgeniy V. Kovtunets a , Elena G. Khaikina a a: Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Sciences, 670047 Sakh’yanova st., 6, Ulan-Ude, Russia b: Skolkovo Institute of Science and Technology, 121205 Bolshoy blvd., 30, Moscow, Russia * Corresponding author: spiridonova-25@mail.ru This article belongs to the regular issue. © 2021, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract 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 crys- tal structures were refined with the Rietveld method using the pow- der X-ray diffraction data. The thermal expansion of Na5Rb7Sc2(WO4)9 was studied by high-temperature powder X-ray dif- fraction; it was shown that this triple tungstate belongs to high thermal expansion materials. Keywords sodium rubidium scandium triple molybdate triple tungstate synthesis crystal structure thermal expansion Received: 03.12.2021 Revised: 10.12.2021 Accepted: 14.12.2021 Available online: 16.12.2021 1. 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 chemis- try, crystal chemistry and materials science. The great- est attention is paid to the synthesis, study of the struc- ture and properties of complex oxides, among which binary Mo (VI) and W (VI) compounds of various com- positions occupy a significant place. In the last two dec- ades, triple molybdates have been actively studied, and in recent years, triple tungstates have also attracted much attention as interesting research objects. Con- stant interest in such compounds is maintained due to their wide range of functional properties, such as cata- lytic, luminescent, laser, nonlinear optical, ferroelec- tric, ion-conducting, and others. Thus, numerous publi- cations are devoted to triple molybdates and tungstates with the BaNd2(MoO4)4-type structure, which are repre- sented 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 com- pounds as photo- and IR-phosphors, materials for UV radiation dosimeters, laser materials are shown in [1–10]. 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: Tb3+/Yb3+ can be used as a spectral converter [12]. Triple molybdates Na25Cs8R5(MoO4)24 (R = In, Sc or Fe) [13, 14] and Na10Cs4Со5(MoO4)12 [15], built on the basis of alluaudite (Na,Ca)Mn(Fe,Mg)2(PO4)3, M1–xA1–xR1+x(MoO4)3 [16, 17] with the NASICON-type structure, K5ScHf(MoO4)6 [18] with the K5InHf(MoO4)6-type structure, and AgRb2In(MoO4)3 [19], Ag3Rb9Sc2(WO4)9 [20], Ag5Rb7Sc2(XO4)9 (X = Mo, W) [21] that formed their own structural types are considered to be promising solid state electrolytes. In this work, the family of triple molybdates and tung- states 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. http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2021.8.4.12 https://orcid.org/0000-0001-7498-5103 https://orcid.org/0000-0002-7108-8535 https://orcid.org/0000-0003-1301-1983 https://orcid.org/0000-0003-2482-9297 http://creativecommons.org/licenses/by/4.0/ Chimica Techno Acta 2021, vol. 8(4), № 20218412 ARTICLE 2 of 8 2. Experimental 2.1. Preparation of materials Commercially available chemically pure MoO3, WO3 (ReaKhim, Ltd, Russia), AgNO3 (KhimKo, Ltd, Russia) and high purity Sc2O3 (SibMetallTorg, Ltd, Russia), Rb2CO3 (Sig- ma-Aldrich, China) were used as starting materials for pre- paring molybdates and tungstates. Rb2ХO4 (Х = Mo, W) was prepared by high temperature annealing of a stoichiometric mixture of Rb2CO3 and ХO3 (723–823 K, 80 h). Sc2(ХO4)3 (Х = Mo, W) was obtained from Sc2O3 and ХO3 (Х = Mo, 773–1023 K, 80 h; Х = W, 773–1123 K, 80 h). Anhydrous Na2XO4 (X = Mo, W) used in this work were obtained by cal- cining 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 agree- ment with the literature data [23]. 2.2. 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 monochroma- tor). High temperature X-ray measurements of Na5Rb7Sc2(WO4)9 were performed with the same instru- ment using an Anton Paar HTK 16 high temperature cham- ber 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 determina- tion 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). 3. Results and discussion 3.1. Synthesis and characterization of Na5Rb7Sc2(ХO4)9 (Х = Mo, W) Polycrystalline Na5Rb7Sc2(ХO4)9 (Х = Mo, W) were synthe- sized by annealing the stoichiometric mixtures of Na2ХO4, Rb2ХO4 and Sc2(ХO4)3 at 773–823 K for 80 h (Х = Mo), 100 h (Х = W). 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). 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 tung- states 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 corre- sponding scheme differs from that for ternary tungstate only in shorter synthesis times. Both Mo- and W-based Na5Rb7Sc2(ХO4)9 melt incongru- ently at 857 K (X = Mo) and 889 K (X = W) (Fig. 1). Reflec- tions of both NaSc(MoO4)2 and a phase with an alluoudite- type 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 tung- states RbSc(WO4)2 and NaSc(WO4)2 and an alluaudite-like phase. The amount of the latter phase was dominant. Fig. 1 The DSC curves for polycrystalline Na5Rb7Sc2(ХO4)9 (Х = W, Mo) Chimica Techno Acta 2021, vol. 8(4), № 20218412 ARTICLE 3 of 8 The PXRD patterns of prepared single-phase com- pounds 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 tung- state 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. Table 1 Unit cell parameters for Na5Rb7Sc2(ХO4)9 (Х = Mo, W) Х a, Å c, Å V, Å3 Mo 10.1264(1) 35.6570(7) 3172.80 W 10.1899(2) 35.6096(9) 3202.12 Table 2 The PXRD data for Na5Rb7Sc2(WO4)9 h k l 2exp,° I/I0 dexp, Å  = 2exp – 2calc,° h k l 2exp,° I/I0 dexp, Å  = 2exp – 2calc,° 1 0 1 10.322 1 8.563 –0.003 2 1 13 42.680 3 2.1167 –0.002 0 1 2 11.183 2 7.906 –0.002 0 4 5 42.879 2 2.1074 +0.000 1 0 4 14.117 1L 6.268 +0.003 3 0 12 43.273 2 2.0891 +0.000 0 0 6 14.911 1L 5.936 +0.004 0 1 17 44.418 1L 2.0379 –0.005 0 1 5 15.974 1L 5.544 +0.004 4 0 7 44.739 2 2.0240 –0.001 1 1 0 17.390 20 5.095 +0.001 1 2 14 44.781 2 2.0222 –0.009 1 1 3 18.938 42 4.682 +0.001 3 2 1 44.813 2 2.0208 –0.011 1 0 7 20.131 1 4.407 +0.000 1 3 10 44.914 1 2.0165 –0.013 0 2 1 20.264 1 4.379 –0.001 2 3 2 45.033 1 2.0114 –0.003 2 0 2 20.724 2 4.283 –0.001 0 2 16 45.615 1L 1.9871 +0.000 0 0 9 22.454 13 3.9563 –0.002 0 0 18 45.831 7 1.9783 –0.001 1 1 6 22.987 100 3.8658 +0.000 3 2 4 45.930 1 1.9742 +0.002 2 0 5 23.694 1 3.7520 +0.007 3 1 11 46.475 2 1.9523 +0.000 0 2 7 26.722 2 3.3333 +0.001 2 3 5 46.604 1 1.9472 –0.004 2 1 1 26.824 15 3.3209 +0.000 2 2 12 46.970 2 1.9329 +0.000 1 0 10 26.973 1L 3.3029 +0.005 4 1 0 47.157 3 1.9257 –0.001 1 2 2 27.178 3 3.2784 +0.000 4 1 3 47.813 5 1.9008 –0.002 1 1 9 28.541 59 3.1249 –0.001 2 0 17 48.045 1L 1.8921 –0.004 0 1 11 29.345 2 3.0411 +0.018 3 2 7 48.348 2 1.8810 –0.001 1 2 5 29.546 5 3.0208 +0.002 4 0 10 48.505 1L 1.8753 –0.004 0 0 12 30.094 1 2.9671 –0.004 2 1 16 49.167 1L 1.8516 +0.007 0 3 0 30.362 54 2.9415 –0.001 3 0 15 49.280 1L 1.8476 +0.005 3 0 3 31.304 1 2.8551 –0.001 1 1 18 49.379 5 1.8441 –0.002 2 1 7 32.067 2 2.7889 –0.006 2 3 8 49.422 1 1.8426 –0.008 1 2 8 33.548 3 2.6690 –0.002 4 1 6 49.737 8 1.8317 –0.001 3 0 6 33.989 5 2.6354 –0.002 1 3 13 49.930 1 1.8250 –0.002 2 0 11 34.335 1L 2.6096 –0.006 0 4 11 49.993 2 1.8229 –0.006 1 1 12 34.964 15 2.5641 –0.002 1 2 17 51.478 2 1.7737 –0.005 2 2 0 35.200 5 2.5475 +0.000 3 1 14 51.749 1L 1.7651 +0.045 2 2 3 36.031 1 2.4906 –0.002 0 5 1 51.820 1L 1.7628 +0.001 0 1 14 36.738 1L 2.4443 +0.004 3 2 10 51.916 1 1.7598 –0.006 1 3 1 36.779 2 2.4417 –0.002 2 2 15 52.659 4 1.7367 –0.002 2 1 10 36.898 1L 2.4340 –0.004 4 1 9 52.831 15 1.7314 –0.002 3 1 2 37.057 1L 2.4240 –0.012 0 2 19 53.025 1 1.7256 +0.018 3 0 9 38.091 6 2.3605 –0.002 4 0 13 53.279 1 1.7179 –0.009 2 2 6 38.423 8 2.3409 –0.001 2 3 11 53.336 1 1.7162 –0.009 3 1 5 38.875 2 2.3147 +0.001 3 3 0 53.946 5 1.6983 –0.002 1 3 7 40.884 1 2.2055 –0.001 3 3 3 54.538 2 1.6812 +0.001 4 0 1 40.950 1 2.2021 +0.002 0 5 7 55.016 1L 1.6677 +0.011 0 4 2 41.201 1 2.1892 –0.004 2 4 1 55.077 2 1.6660 +0.005 1 0 16 41.799 1L 2.1593 +0.025 4 2 2 55.268 1L 1.6607 +0.010 1 1 15 41.952 9 2.1518 –0.002 1 3 16 55.766 1L 1.6471 +0.017 2 2 9 42.156 19 2.1418 –0.002 3 0 18 55.969 13 1.6416 –0.001 Cu Kα1 radiation ( = 1.54056 Å) Chimica Techno Acta 2021, vol. 8(4), № 20218412 ARTICLE 4 of 8 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 refine- ment of the Na5Rb7Sc2(ХO4)9 (Х = Mo, W) structures by the Rietveld method. The refinement was carried out by gradual- ly 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 atom- ic displacement parameters, cation occupancies and main se- lected interatomic distances are presented in Tables 4–7. The crystal structures of Na5Rb7Sc2(MoO4)9 and Na5Rb7Sc2(WO4)9 were deposited in the Cambridge Crys- tallographic Data Centre with Cambridge Structural Data- base (CSD) № 2124713 and № 2124691, respectively [28]. Table 3 Main structure parameters for Na5Rb7Sc2(ХO4)9 (Х = Mo, W) after the Rietveld refinement Compound Na5Rb7Sc2(MoO4)9 Na5Rb7Sc2(WO4)9 Sp. gr. R32 R32 a, Å 10.13752(9) 10.19247(9) c, Å 35.6615(4) 35.6191(4) V, Å3 3173.91(7) 3204.59(7) Z 3 3 2θ-interval, º 8–100 8–100 Rwp, % 4.15 4.56 Rp, % 3.20 3.42 Rexp, % 2.04 1.81 χ2 2.04 2.51 RB, % 1.64 2.11 Fig. 2 Observed, calculated and difference diffractograms of Na5Rb7Sc2(MoO4)9 Fig. 3 Observed, calculated and difference diffractograms of Na5Rb7Sc2(WO4)9 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 three- fold axes; Rb3, Mo2 (W2), and Na3 are settled at twofold axes, and Mo1 (W1) and oxygen atoms are in general posi- tions. 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). Similar "lanterns" [M2(TO4)9] (M is an octahedrally co- ordinated cation, TO4 is a tetrahedral oxoanion), and hex- agonal layers formed by them also characterize the struc- tures of previously studied Ag5Rb7Sc2(MoO4)9, Ag5Rb7Sc2(WO4)9 and Na5Cs7Yb2(MoO4)9 [21]. The relation- ship between structure of the considered family M′5M″7R2(XO4)9 (X = Mo, W) and many rhombohedral tri- ple molybdates and tungstates with а = 9–10 Å and large c-periods (more than 20 Å) was discussed in [21]. 3.3. 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: aV = V/3 = (2a+с)/3. Thermal expansion anisotropy is quantitatively defined as |a – с|. Chimica Techno Acta 2021, vol. 8(4), № 20218412 ARTICLE 5 of 8 a b c Fig. 4 The crystal structure of Na5Rb7Sc2(ХO4)9 (Х = Mo, W): a general view (a); [Sc2(XO4)9] cluster (b); layers of [Sc2(XO4)9] clusters (c) Table 4 Fractional atomic coordinates and isotropic displacement parameters (Å2) for Na5Rb7Sc2(MoO4)9 Atom x y z Biso Occ. Rb1 0 0 0.2332(1) 1.9 (1) 1 Rb2 0 0 0.1074(1) 2.0(1) 1 Rb3 0 0.3687(3) 0 4.6(2) 1 Na1 0 0 0 1.9(6) 1 Na2 0 0 0.3376(8) 2.1(3) 0.5 Na3 0.4030(9) 0.4030(9) 0.5 2.1(3) 1 Sc2 0 0 0.4253(3) 2.1(2) 1 Mo1 0.3429(2) 0.3265(3) 0.39176(4) 1.30(9) 1 Mo2 0.7331(2) 0.7331(2) 0.5 0.9(1) 1 O1 0.458(1) 0.269(1) 0.3746(4) 3.2(4) 1 O2 0.362(1) 0.480(1) 0.3662(3) 1.4(4) 1 O3 0.406(1) 0.394(1) 0.4344(3) 0.4(3) 1 O4 0.147(1) 0.180(1) 0.3938(3) 1.8(2) 1 O5 0.566(1) 0.719(1) 0.4937(4) 1.6(4) 1 O6 0.822(1) 0.839(2) 0.5391(3) 1.8(2) 1 Table 5 Fractional atomic coordinates and isotropic displacement parameters (Å2) for Na5Rb7Sc2(WO4)9 Atom x y z Biso Occ. Rb1 0 0 0.2331(2) 1.1(2) 1 Rb2 0 0 0.1083(2) 1.7(2) 1 Rb3 0 0.3671(5) 0 3.1(2) 1 Na1 0 0 0 2(1) 1 Na2 0 0 0.329(2) 3(1) 0.5 Na3 0.418(1) 0.418(1) 0.5 4.3(6) 1 Sc2 0 0 0.4241(3) 0.5(3) 1 W1 0.3410(1) 0.3251(1) 0.39174(3) 0.8(1) 1 W2 0.7329 (1) 0.7329(1) 0.5 0.9(1) 1 O1 0.4660(7) 0.2580(8) 0.3785(3) 1.0(2) 1 O2 0.376(1) 0.4839(7) 0.3633(2) 1.0(2) 1 O3 0.4104(8) 0.3974(9) 0.4371(2) 1.0(2) 1 O4 0.1487(9) 0.1765(9) 0.3976(4) 1.0(2) 1 O5 0.5570(8) 0.7196(7) 0.4896(2) 1.0(2) 1 O6 0.823(1) 0.840(1) 0.5412(2) 1.0(2) 1 Chimica Techno Acta 2021, vol. 8(4), № 20218412 ARTICLE 6 of 8 Table 6 Main bond lengths (Å) in Na5Rb7Sc2(MoO4)9 Mo1-tetrahedron Mo2-tetrahedron Sc-octahedron Mo1–O1 1.660(9) Mo2–O5 1.64(1) (× 2) Sc–O4 2.02(1) (× 3) –O2 1.73(1) –O6 1.71(1) (× 2) –O6 2.14(1) (× 3) –O3 1.66(1) 1.67 2.08 –O4 1.79(1) 1.71 Na1-octahedron Na2-trigonal prism Na3-octahedron Na1–O1 2.39(1) (× 6) Na2–O2 2.45(2) ( 3) Na3–O3 2.34(1) (× 2) –O4 2.62(2) (× 3) –O5 2.192(9) (× 2) 2.53 –O5 2.78(1) (× 2) 2.44 Rb1-polyhedron Rb2-polyhedron Rb3-polyhedron Rb1–O5 3.04(1) (× 3) Rb2–O5 3.18(1) (× 3) Rb3–O1 3.05(1) (× 2) –O3 3.20(1) (× 3) –O3 3.00(1) (× 3) –O2 3.00(1) (× 2) –O2 3.16(1) (× 3) –O1 3.01(1) (× 3) –O2 3.11(1) (× 2) 3.13 3.06 –O4 3.18(1) (× 2) 3.08 Table 7 Main bond lengths (Å) of Na5Rb7Sc2(WO4)9 W1-tetrahedron W2-tetrahedron Sc-octahedron W1–O1 1.785(5) W2–O5 1.768(6) (× 2) Sc–O4 1.92(1) (× 3) –O2 1.788(7) –O6 1.786(8) (× 2) –O6 2.12(1) (× 3) –O3 1.771(8) 1.777 2.02 –O4 1.792(7) 1.784 Na1-octahedron Na2-trigonal prism Na3-octahedron Na1–O1 2.406(9) (× 6) Na2–O2 2.30(2) ( 3) Na3–O3 2.247(8) (× 2) –O4 2.96(5) (× 3) –O5 2.76(1) (× 2) 2.63 –O5 2.69(1) (× 2) 2.57 Rb1-polyhedron Rb2-polyhedron Rb3-polyhedron Rb1–O5 2.870(8) (× 3) Rb2–O5 3.202(9) (× 3) Rb3–O1 3.043(9) (× 2) –O3 3.212(8) (× 3) –O3 2.997(7) (× 3) –O2 2.874(9) (× 2) –O2 3.27(1) (× 3) –O1 2.88(1) (× 3) –O2 3.209(9) (× 2) 3.12 3.03 –O4 3.32(1) (× 2) 3.11 The reflections in the X-ray diffraction patterns of Na5Rb7Sc2(WO4)9 regularly shift with increasing tempera- ture (Fig. 5) due to an increase in the unit cell parameters (Fig. 6). Fig. 5 Fragments of Na5Rb7Sc2(WO4)9 diffractograms from 303 K to 823 K The parameter a changes with temperature almost lin- early; the temperature variation of the parameter c is de- scribed by a polynomial of the second degree (Table 8). Table 8 also presents the coefficients of thermal linear expansion and thermal expansion anisotropy. The ob- tained results allowed classifying Na5Rb7Sc2(WO4)9 as be- longing to high thermal expansion materials. 4. Conclusions Two new compounds Na5Rb7Sc2(ХO4)9 (Х = Mo, W) were obtained by a solid-phase synthesis, supplementing the previously discovered family of isostructural triple molyb- dates and tungstates of the composition M'7M''5R2(XO4)9. The thermal stability of obtained compounds was studied and the thermal expansion of Na5Rb7Sc2(WO4)9 was exam- ined by the high-temperature XRD diffraction method; it was shown that this compound belongs to highly expand- ing substances. The crystal structure of Na5Rb7Sc2(ХO4)9 (Х = Mo, W) was refined by the Rietveld method using the PXRD data. Chimica Techno Acta 2021, vol. 8(4), № 20218412 ARTICLE 7 of 8 Table 8 Fitting polynomials for temperature dependent LTECs and average LTECs for Na5Rb7Sc2(WO4)9 in the temperature range 303–823 K Composition Polynomials for a(T) and c(T), Å LTEC10–6, K–1 αa αс αaV |αa–αc| Na5Rb7Sc2(WO4)9 а = 0.0003Т + 10.18 с = 1×10–6Т2 + 0.0002Т + 35.612 27.8(3) 22.9(2) 26.2(3) 4.9 The obtained compounds crystallize in the chiral sp. gr. R32 and together with their formula and structural ana- logues Ag5Rb7Sc2(ХO4)9 (Х = Mo, W), Na5Cs7Ln2(MoO4)9 (Ln = Tm, Yb, Lu) belong to the series of rhombohedral triple molybdates and tungstates with а = 9–10 Å and long c-parameter, more than 20 Å; many of those have noticea- ble ionic conductivity at elevated temperatures [19–21]. 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 proper- ties 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 con- sidered structural type to reveal the influence of the na- ture of one-, three- and hexavalent elements on the value of thermal expansion coefficients and anisotropy in these phases. 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