Preparation, crystal and magnetic structures of two new double perovskites:: Ca2CoTeO6 and Sr2CoTeO6

Ca2CoTeO6 and Sr2CoTeO6 double perovskites have been prepared as polycrystalline powders by solid state reaction, in air. These materials have been studied by X-ray, neutron powder diffraction ( NPD) and magnetic measurements. At room temperature, the crystal structure is monoclinic, space group P2(1)/n for both compounds with a 5 5.4569( 2) Angstrom, b = 5 5.5904( 2) Angstrom, c = 7.7399( 2) Angstrom, b = 90.239( 2)degrees and a = 5.6417( 2) Angstrom, b = 5.6063( 2) Angstrom, c = 7.9234( 2) Angstrom, beta = 90.117(4)degrees for Ca2CoTeO6 and Sr2CoTeO6 respectively. The perovskite lattices contain an almost completely ordered array of CoO6 and TeO6 octahedra. The monoclinic distortion is larger in Ca2CoTeO6 than in Sr2CoTeO6, which is associated with the tilting of the CoO6 and TeO6 octahedra, displaying tilting angles phi = 4.8degrees for Sr2CoTeO6 and phi = 15.1degrees for Ca2CoTeO6. The low temperature magnetic structures were determined by NPD, selected among the possible magnetic solutions compatible with the P2(1)/n space group, according with the group theory representation. The propagation vectors for the Ca and Sr compounds are k = 0 and k =( 1/2, 1/2, 0), respectively. A canted antiferromagnetic structure is observed for Ca2CoTeO6 below T-N = 10 K, which remains stable down to 2.2 K, with an ordered magnetic moment of 2.37( 3) mu(B) for Co2+ cations. For Sr2CoTeO6, an antiferromagnetic ordering is displayed below T-N = 15 K, that remains stable down to 2.9 K, with an ordered magnetic moment for Co2+ of 2.15( 4) mu(B).