Structural phase transitions and magnetic and spectroscopic properties of the double perovskites Sr2Co1−xMgxTeO6 (x = 0.1, 0.2 and 0.5)

The structural and magnetic properties of a series of ordered double perovskites with the formula Sr 2 Co 1− x Mg x TeO 6 ( x = 0.1, 0.2 and 0.5) are investigated by X-ray diffraction, low temperature neutron diffraction, electron paramagnetic resonance and magnetic susceptibility. The progressive substitution of the paramagnetic Co 2+ high spin ion by the diamagnetic Mg 2+ , of about the same size, induces changes in the room temperature crystal structure, from a distorted P 2 1 / n phase for the undoped Sr 2 CoTeO 6 oxide to the I 4/ m of the end member (Sr 2 MgTeO 6 ). These perovskites experience structural transitions on heating, the temperature at which the transitions occur being smaller as x increases. The novel approach of mode-crystallography is used for the analysis. All oxides show antiferromagnetic exchange interactions between Co 2+ ions but the long range antiferromagnetic order is not achieved for the phase with x = 0.5. The low temperature neutron diffraction data have been evaluated using a full symmetry analysis. Results are consistent with an unquenched orbital contribution of a high spin Co 2+ ion. Structural and magnetic properties of a series of double perovskites are investigated by X-ray and Neutron diffraction, EPR and magnetic susceptibility. The structural and phase-transitions analysis are done using symmetry mode-analysis approach.