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

Abstract

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 P2₁/n for both compounds with a = 5.4569(2) Å, b = 5.5904(2) Å, c = 7.7399(2) Å, β = 90.239(2)° and a = 5.6417(2) Å, b = 5.6063(2) Å, c= 7.9234(2) Å, β = 90.117(4)° 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 φ = 4.8° for Sr₂CoTeO₆ and φ = 15.1° for Ca₂CoTeO_6. The low temperature magnetic structures were determined by NPD, selected among the possible magnetic solutions compatible with the P2₁/n space group, according with the group theory representation. The propagation vectors for the Ca and Sr compounds are k = 0 and k = (½, ½, 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) μ_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) μ_B.