Crystal structures and cation ordering of Sr 2AlSbO 6 and Sr 2CoSbO 6

Abstract

The two double perovskite oxides Sr 2AlSbO 6 and Sr 2CoSbO 6 were prepared and their structures studied with the X-ray powder diffraction method. At room temperature the crystal structure of Sr 2AlSbO 6 is cubic ( Fm 3 ¯ m ) , with a = 5.6058 ( 1 ) A ˚ . It was found that depending on the preparation conditions, the Al 3 + and Sb 5 + cations can be either entirely or partially ordered. In the case of the partially ordered Sr 2AlSbO 6 sample, the extension of cation ordering was estimated from the h k l -dependent broadening of the diffraction peaks and the results were interpreted as evidence of the formation of anti-phase domains in the material. Low-temperature Raman spectroscopic measurements demonstrated that the cubic phase of Sr 2AlSbO 6 is stable down to 79 K. The room-temperature crystal structure of Sr 2CoSbO 6 is trigonal (space group R 3 ¯ ) with a = 5.6058 ( 1 ) A ˚ and c = 13.6758 ( 3 ) A ˚ . At 470 K, however, the material undergoes a continuous phase transition and its structure is converted to cubic (space group Fm 3 ¯ m ) . The studied Sr 2CoSbO 6 sample was partially ordered, but unlike Sr 2AlSbO 6, no indication of the formation of anti-phase domains was observed.