Factors determining the effect of Co(II) in the ordered double perovskite structure: Sr₂CoTeO₆

Double perovskites are of interest due to their diverse properties that are of potential use in technological applications. In order to determine the effect of Co(II) in the double perovskite structure in the absence of other paramagnetic species, mixed valency or mis-site disorder, we have studied the ordered perovskite Sr₂CoTeO₆ synthesised by the freeze-drying method. The room temperature monoclinic P2₁/n symmetry (a = 5.6437(2) Å, b = 5.6096(2) Å, c = 7.9271(2) Å, β = 90.059(2)°) is maintained down to 4 K with structural transitions to 12/m then Fm3̄m at 373 K (100 °C) and 773 K (500 °C). Below T _N = 18 K, the Type I antiferromagnetic structure is observed with magnetic moments of magnitude 2.25(3) μ_B rotated 58° out of the ab plane. AC magnetic susceptibility and specific heat data show maxima at 19 K, associated with long-range antiferromagnetic order. EPR and diffuse reflectance spectroscopies confirm that Co is in the +2 oxidation state, in highly regular octahedral coordination and is highly ionic. A small magnetic irreversibility, with H_c and M_r of 36 Oe and 0.5 emu mol, is observed at low temperature but this is not due to conventional spin glass or cluster glass behaviour. Calculations from specific heat give a magnetic entropy of 4.2 J mol^-1 K^-1, close to the theoretical value for the S′ = 1/2 state of the Co²⁺ cation at low temperature. Distortions of the structure, demonstrated to be primarily rotations of highly regular octahedra, change the geometry of the magnetic exchange pathways but are insufficient to explain the variation in ordering temperatures and magnetic structure types observed, with orbital energies within the exchange pathways having a significant influence on the properties of these and similar technologically important materials.