Crystal structure and magnetism in CoSb2O6 and CoTa2O6

CoSb2O6 was investigated by neutron diffraction and magnetic susceptibility techniques. The structure found was tetragonal, of the trirutile type, with space group P42mnm, a = 4.6495(2)Å, c = 9.2763(6)Å. Low-temperature neutron data gave no evidence of structural change down to 20 K. Susceptibility data showed a deviation from Curie-Weiss behavior below 100 K and a maximum at 35 K associated with low-dimensional ordering. A Curie-Weiss law fit of the data above 100 K gave μeff = 4.62(1) μB and θ = −32.4(3) K. The ordered magnetic structure as determined by neutron diffraction was indexed on a supercell (2a, a, 2c) and is similar to that found previously for FeTa2O6. Measured magnetic moments below Tc gave an extrapolated zero-temperature moment of 3.3(2) μB for Co2+ and critical parameters β = 0.12(2) and Tc = 12.64(5) K, consistent with the two-dimensional Ising model. The crystal structure of CoTa2O6 was found to be the same as that for CoSb2O6 with a = 4.7358(3)Å and c = 9.1708(7)Å and with no structural change down to 10 K. At 4.2 K magnetic reflections were observed that could be indexed with a propagation vector k0 = (14, 14, 14). A two-cone axis helical spin structure is proposed which can account for the observed intensities. Some of the differences in magnetic properties of the two compounds can be understood in terms of bond distances within the CoO layers.