Crystal structures of the triple perovskites Ba2K2Te2O9 and Ba2KNaTe2O9, and redetermination of the double perovskite Ba2CaTeO6

The triple perovskites Ba2K2Te2O9 and Ba2KNaTe2O9 crystallize in the 6H-BaTiO3-type of structure, whereas Ba2CaTeO6 represents a double perovskite (cubic elpasolite structure type).


Chemical context
During a recent project on the structure determination of barium oxotellurates(VI), different preparation methods were applied for single-crystal growth of the phases Ba[H 4 TeO 6 ], Ba[H 2 TeO 5 ], Ba[Te 2 O 6 (OH) 2 ] and Ba[TeO 4 ] . Owing to the different water content that defines the thermal stability range of the respective phase, relatively mild temperatures < 600 K had to be adjusted for the three hydrous phases using either a diffusion method in aqueous solutions (room temperature) or hydrothermal methods (ca 470 K), whereas for the anhydrous phase higher temperatures could be employed. However, Ba[TeO 4 ] decomposes into Ba[TeO 3 ] with release of oxygen at temperatures above 1000 K, which prevents prolonged heating near this temperature. Although very small crystals of Ba[TeO 4 ] with a rather poor quality could eventually be grown by heating Ba[H 4 TeO 6 ] at 873 K for some days , alternative crystal-growth methods were tested with the intention of obtaining larger crystals with better quality. With the upper stability range of the target phase Ba[TeO 4 ] in mind, KNO 3 /KI or KNO 3 / NaNO 3 mixtures were used for crystal-growth experiments. Such salt mixtures have low eutectic melting points, e.g. 498 K for a 50:50 mol% mixture of NaNO 3 /KNO 3 (Berg & Kerridge, 2004). At least for the latter eutectic mixture, crystal-growth experiments from the melt have already been applied successfully for another barium phase, viz. Ba 2 As 2 O 7 (Weil,

Structural commentary
The three title compounds belong to the vast family of perovskites (Tilley, 2016). The ideal cubic A [12co] B [6o] O 3 perovskite structure comprises of corner-sharing [BO 6 ] octahedra. In the centre of the resulting 3 1 [BO 6/2 ] network, the Asite cation occupies a 12-coordinate cuboctahedral site. The 2H hexagonal perovskite structure contains chains of facesharing [BO 6 ] octahedra that are separated by chains of A-site cations. In an alternative description, perovskite structures can be derived from closed-packed arrangements of the anions with different stacking sequences (Lufaso & zur Loye, 2005a;Stö ger et al., 2010). For example, in the cubic perovskite an ABC stacking and in the hexagonal 2H perovskite an AB stacking is observed. More complex structures that are realized in double perovskites or triple perovskites can include both cubic (c) and hexagonal stacking sequences (h) and consequently structure motifs of corner-sharing and facesharing [BO 6 ] octahedra like in the triple perovskites discussed below.
Ba 2 K 2 Te 2 O 9 (I) and Ba 2 KNaTe 2 O 9 (II) are isotypic and members of the triple perovskite family with general formula They crystallize in the 6H-BaTiO 3 structure type in space-group type P6 3 /mmc with Z = 2. In (I) the A, A 0 , B and B 0 sites are occupied by K1, Ba1, Te1 and Ba2, and in (II) by mixed-occupied (Ba/K)1, Ba1, Te1 and Na2, respectively. The 6H-BaTiO 3 structure type is sometimes also referred to as the BaFeO 2+x structure type with possible values for Z = 2, 3 or 6, dependent on the overall formula sum of the compound. The stacking sequence for this structure type is (cch) 2 (Tilley, 2016). About 240 entries of this structure family are compiled in the recent version of the Inorganic Crystal Structure Database (ICSD, 2018), with hexagonal BaTiO 3 being the first phase that has been structurally determined (Burbank & Evans, 1948). Only four Te-containing phases have been reported so far to adopt this structure type, viz. Ba 3 Fe 2 TeO 9 (Harari et al., 1972), K 3 LaTe 2 O 9 (Zhang et al., 2015), Ba 3 Cr 1.94 Te 1.06 O 9 (Li et al., 2016) and the high-pressure phase Ba 2 NiTeO 6 (Z = 3; Aoba et al., 2016). A review of this structure type and of perovskites in general was given recently by Tilley (2016). In both structures (I) and (II), Ba1 is situated on Wyckoff position 2b (site symmetry 6m2), the K1 site in (I) and the mixed-occpied (Ba/K)1 site (occupancy ratio 1:1) in (II) on 4f (3m.), Ba2 in (I) and Na2 in (II) on 2a (3m.), and in both structures Te1 4f (3m.), O1 on 6h (mm2) and O2 on 12k (.m.), respectively. Hence the smaller Te VI atoms occupy the face-sharing octahedral B site while the larger barium (Ba2 in (I)) or sodium cations (Na2 in (II)) occupy the corner-sharing octahedral B 0 site. The inner angles of the two face-sharing [TeO 6 ] octahedra in (I) and (II) ( Table 1) are more similar than those in isotypic triple perovskites (Lufaso & zur Loye, 2005a), with center shifts of 0.076 Å in (I) and of 0.191 Å in (II). Representative for both (I) and (II), the crystal structure of Ba 2 K 2 Te 2 O 9 is given in Fig. 1. It should be noted that the A (= K1) position in (I) has only nine coordination partners, while in (II) twelve oxygen atoms surround the corresponding site that is statistically occupied by Ba 2+ and K + (= (Ba/K)1).
The current refinement of Ba 2 CaTeO 6 (III) is based on single crystal X-ray data and confirms the previous structure determination from X-ray powder diffraction data, but with higher precision (reliability factors for the previous determination: R wp = 0.159, R p = 0.112; Fu et al., 2008 present at the B 0 and B 00 sites, double perovskites are functional oxide materials with interesting electronic and magnetic properties (Vasala & Karppinen, 2015). In the crystal structure of (III), Ba, Ca and Te are located on the A, B 0 and B 00 sites, respectively. The Wyckoff positions and site symmetries of the four sites present in the structure of (III) are: Ba on 8c (43m), Ca on 4a (m3m), Te on 4b (m3m), and O on 24e (4m.m). Since Ba 2 CaTeO 6 represents the highest possible symmetry of a double perovskite structure (cubic elpasolite-type in space group type Fm3m), tilting of the B 0 O 6 or B 00 O 6 octahedra (Howard et al., 2003), like in the monoclinic structure of Sr 2 CaTeO 6 (Prior et al., 2005), is not observed. The ordering of the CaO 6 and TeO 6 octahedra in a checkerboard arrangement in (III) is displayed in Fig. 2.
With the exception of the Na-O bond length, all other bond lengths (Table 1) are characteristic for their respective coordination polyhedra and in good agreement with mean values compiled recently for alkali and alkaline earth cations bonded to oxygen: K-O = 2.955 Å for coordination number (CN) 9, 3.095 Å for CN 12; Ca-O = 2.668 Å for CN 12; Ba-O = 2.689 Å for CN 6, 2.965 Å for CN 12 (Gagné & Hawthorne, 2016). The same is valid for the mean value of octahedrally coordinated Te VI with a mean Te-O bond length of 1.923 Å (Gagné & Hawthorne, 2018). As noted above, the Na-O bond length deviates from the mean value. At 2.3037 (16) Å it is considerably shorter than the mean of 2.441 Å for CN 6 (Gagné & Hawthorne, 2016). Such a compression has also been reported for other 6H-BaTiO 3 -type structures containing sodium. For example, the Na-O distance in K 3 NaOs 2 O 9 has nearly the same value [2.313 (6) Å ; Mogare et al., 2012] but is reported to be significantly shorter in Ba 3 NaRuIrO 9 [2.058 (9) Å ; Lufaso & zur Loye, 2005b]. Ba[H 4 TeO 6 ] was prepared according to a literature protocol (Engelbrecht & Sladky, 1965) and its purity checked by X-ray powder diffraction. One gram of dried Ba[H 4 TeO 6 ] was mixed with five grams of a KNO 3 /KI mixture (stoichiometric ratio 2:1) for (I) or a KNO 3 /NaNO 3 mixture (stoichiometric ratio 1:1) for (II). The mixtures were placed in platinum crucibles and heated within six h to 773 K, held at that temperature for four days and cooled to room temperature within 12 h. The solidified melts were leached out with water and the remaining solid filtered off, washed with water and ethanol. Colourless single crystals with a hexagonal form for both (I) and (II) were selected from the reaction products. In one case a porcelain crucible was used to reproduce the formation of (I). In this batch, very few colourless crystals of Ba 2 CaTeO 6 (III) had formed as a minor by-product. The porcelain crucible is an adventitious source of calcium that is present in feldspars such as oligoclase used for manufacturing.

Figure 2
Projection of the crystal structure of Ba 2 CaTeO 6 (III) along

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. For refinements of (I) and (II) the coordinates of isotypic Ba 3 LaRuO 9 (Doi et al., 2002) were used as starting parameters. In the structure of (II), the M1 position with site symmetry 3m. of Wyckoff site 4f is statistically occupied by K + and Ba 2+ cations. For refinement of (III), the starting parameters were taken from the previous sructure determination based on X-ray powder diffraction data (Fu et al., 2008). The type of element on the metal positions was checked by free refinement of the respective site-occupation factors, which confirmed Ca and Ba, respectively.

Funding information
The X-ray centre of TU Wien is acknowledged for financial support and for providing access to the single-crystal diffractometer. Computer programs: APEX2 and SAINT (Bruker, 2015), SHELXL2017 (Sheldrick, 2015), ATOMS for Windows (Dowty, 2006) and publCIF (Westrip, 2010

Crystal data
Ba 2 K 2 Te 2 O 9 M r = 752.08 Hexagonal, P6 3 /mmc a = 6.047 (3)  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 3.87 e Å −3 Δρ min = −1.68 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq