Crystal structures of the triple perovskites Ba(2)K(2)Te(2)O(9) and Ba(2)KNaTe(2)O(9), and redetermination of the double perovskite Ba(2)CaTeO(6)

Single crystals of Ba(2)K(2)Te(2)O(9) (dibarium dipotassium nona­oxidoditellurate), (I), Ba(2)KNaTe(2)O(9) (dibarium potassium sodium nona­oxidoditellurate), (II), and Ba(2)CaTeO(6) (dibarium calcium hexa­oxidotellurate), (III), were obtained from KNO(3)/KI or KNO(3)/NaNO(3) flux syntheses in platinum crucibles for (I) and (II), or porcelain crucibles for (III). (I) and (II) are isotypic and are members of triple perovskites with general formula A (2) ([12co]) A′([12co]) B (2) ([6o]) B′([6o])O(9). They crystallize in the 6H-BaTiO(3) structure family in space-group type P6(3)/mmc, with the A, A′, B and B′ sites being occupied by K, Ba, Te and a second Ba in (I), and in (II) by mixed-occupied (Ba/K), Ba, Te and Na sites, respectively. (III) adopts the A (2) ([12co]) B′ ([6o]) B′′ ([6o])O(6) double perovskite structure in space-group type Fm [Image: see text] m, with Ba, Ca and Te located on the A, B′ and B′′ sites, respectively. The current refinement of (III) is based on single-crystal X-ray data. It confirms the previous refinement from X-ray powder diffraction data [Fu et al. (2008). J. Solid State Chem. 181, 2523–2529], but with higher precision.