Crystal structures and cation ordering of Sr2AlSbO6 and Sr2CoSbO6

The two double perovskite oxides Sr2AlSbO6 and Sr2CoSbO6 were prepared and their structures studied with the X-ray powder diffraction method. At room temperature the crystal structure of Sr2AlSbO6 is cubic (Fm (3) over barm), with a = 5.6058(1) angstrom. It was found that depending on the preparation conditions, the Al3+ gild Sb5+ cations can be either entirely or partially ordered. In the case of the partially ordered Sr2AlSbO6 sample, the extension of cation ordering was estimated from the hkl-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 Sr2AlSbO6 is stable down to 79 K. The room-temperature crystal structure of Sr2CoSbO6, is trigonal (space group R (3) over bar) with a = 5.6058(1) angstrom and c = 13.6758(3) angstrom. At 470 K, however, the material undergoes a continuous phase transition and its structure is converted to cubic (space group Fm (3) over barm). The studied Sr2CoSbO6 sample was partially ordered, but unlike Sr2AlSbO6, no indication of the formation of anti-phase domains was observed. (C) 2008 Elsevier Inc. All rights reserved.