A new perovskite, CaCo(2+)3V(4+)4O12, has been synthesized at high-pressure and high-temperature (HP-HT) conditions. The properties of this perovskite were examined by a range of techniques. CaCo3V4O12 was found to adopt a double-perovskite cubic lattice [a = 7.3428(6) Å] with Im3 symmetry. We have established that this new perovskite is stable at ambient conditions, and its oxidation and/or decomposition at ambient pressure begins above 500 °C. It undergoes an abrupt antiferromagnetic transition around 98 K. Electrical resistivity data suggest semimetallic conductivity in the temperature range of 1.6-370 K. We have established that the Co(2+) ions in CaCo3V4O12 are in the high-spin state with a sizable orbital moment, even though their square-planar oxygen coordination could be more suitable for the low-spin state, which is prone to Jahn-Teller distortion. Electrical resistivity curves also exhibit a distinct steplike feature around 100 K. CaCo3V4O12 is a first example of perovskite in which the sites A' are fully occupied by Co(2+) ions, and hence its synthesis opens the door to a new class of double perovskites, ACo3B4O12, that may be derived by chemical substitution of the A sublattice by lanthanides, sodium, strontium, and bismuth and by other elements and/or of the B sublattice by some other transition metals.

We investigated the structural, vibrational, magnetic, and electronic properties of the recently synthesized CaCoVO double perovskite with the high-spin (HS) Co ions in a square-planar oxygen coordination at extreme conditions of high pressures and low temperatures. The single-crystal X-ray diffraction and Raman spectroscopy studies up to 60 GPa showed a conservation of its cubic crystal structure but indicated a crossover near 30 GPa. Above 30 GPa, we observed both an abnormally high "compressibility" of the Co-O bonds in the square-planar oxygen coordination and a huge anisotropic displacement of HS-Co ions in the direction perpendicular to the oxygen planes. Although this effect is reminiscent of a continuous HS → LS transformation of the Co ions, it did not result in the anticipated shrinkage of the cell volume because of a certain "stiffing" of the bonds of the Ca and V cations. We verified that the oxidation states of all the cations did not change across this crossover, and hence, no charge-transfer effects were involved. Consequently, we proposed that CaCoVO could undergo a phase transition at which the large HS-Co ions were pushed out of the oxygen planes because of lattice compression. The antiferromagnetic transition in CaCoVO at 100 K was investigated by neutron powder diffraction at ambient pressure. We established that the magnetic moments of the Co ions were aligned along one of the cubic axes, and the magnetic structure had a 2-fold periodicity along this axis, compared to the crystallographic one.

The new title perovskite is synthesized from mixtures of Ca(VO3)2, Co(VO3)2, and Co (14-18 GPa, 900-1300°C, 0.5-8 h). The product is characterized by powder and single crystal XRD, SEM, magnetic susceptibility, and electrical resistivity measurements. CaCo3V4O12 crystallizes in the cubic space group Im3 with Z = 2 and is the first double perovskite with Co 2+ ions at the A' sites. The compound orders antiferromagnetically around 100 K and exhibits semimetallic conductivity in the temperature range 1.6-370 K. -(OVSYANNIKOV*, S. V.; ZAINULIN, Y. G.; KADYROVA, N. I.; TYUTYUNNIK, A. P.; SEMENOVA, A. S.; KASINATHAN, D.; TSIRLIN, A. A.; MIYAJIMA, N.; KARKIN, A. E.; Inorg. Chem. 52 (2013) 20, 11703-11710, http://dx.doi.org/10.1021/ic400649h ; Inst. Solid State Chem., Ural Branch Russ.

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