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 Sr 2 CoTeO 6 synthesised by the freeze-drying method. The room temperature monoclinic P2 1 /n symmetry (a 5 5.6437(2) A ˚, b 5 5.6096(2) A ˚, c 5 7.9271(2) A ˚, b 5 90.059(2)u) is maintained down to 4 K with structural transitions to I2/m then Fm3 ¯m at 373 K (100 uC) and 773 K (500 uC). Below T N 5 18 K, the Type I antiferromagnetic structure is observed with magnetic moments of magnitude 2.25(3) m B rotated 58u 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 21 , 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 21 K 21 , close to the theoretical value for the S9 5 K state of the Co 2+ 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.
The ordered double perovskite Sr 2 MnTeO 6 was prepared by the freeze-drying method. The nuclear and magnetic structures were determined by both X-ray and neutron (D2B and D1B) powder diffraction. A distorted, double perovskite structure type described by the monoclinic P12 1 /n1 space group is observed at room temperature and down to 4 K. The room temperature EPR spectrum shows an isotropic signal centered at a g value of 1.998 indicating that manganese ions are in the +2 oxidation state. At low temperature the EPR signal broadens as the exchange and dipolar interactions between Mn 2+ cations increase. Below 25 K no signal is observed, indicating that the sample is magnetically ordered. The dc magnetic susceptibility shows the existence of antiferromagnetic interactions with an ordering temperature
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