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

Single crystals of Ba2K2Te2O9 (dibarium dipotassium nonaoxidoditellurate), (I), Ba2K2Te2O9 (dibarium potassium sodium nonaoxidoditellurate), (II), and Ba2CaTeO6 (dibarium calcium hexaoxidotellurate), (III), were obtained from KNO3/KI or KNO3/NaNO3 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(0)([12co]) (B)2([6o]) (B0O9)-O-[6o]. They crystallize in the 6H-BaTiO3 structure family in space-group type P63/mmc, with the A, A0, B and B0 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-0([6o]) B '' O-[6o](6) double perovskite structure in space-group type Fm(3)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.