Structural and Vibrational Properties of the Ordered Y₂CaGe₄O₁₂ Germanate: A Periodic Ab Initio Study

DFT calculations with six LDA, GGA, and hybrid functionals have been performed using the CRYSTAL09 code to describe the crystal structure and vibrational spectra of Y₂CaGe₄O₁₂ cyclotetragermanate, a new optical host. Two space groups <i>P</i>4/<i>nbm</i> and <i>Cmme</i> have been considered. The former corresponds to a mixed (0.5 Ca + 0.5 Y) distribution at the octahedral sites found from the results of Rietveld refinement of room temperature powder XRD pattern; the latter refers to the model of crystallographically nonequivalent calcium and yttrium atomic setting in distorted oxygen octahedrons. The most accurate geometry description has been obtained with the WC1LYP and PBE (<i>n</i> = 6) hybrid functionals, while the B3LYP calculation provides the best agreement between the recorded infrared and Raman spectra and their computed counterparts. Assignments of most of the observed bands to vibrational modes are given. The comparison between calculated and experimental frequencies shows a general good agreement for the spectra below 600 cm^–1. The relationship between selected infrared bands and Raman lines, internal vibrations of the [Ge₄O₁₂] unit, and external modes is briefly discussed

New Antiferromagnetic Perovskite CaCo₃V₄O₁₂ Prepared at High-Pressure and High-Temperature Conditions

A new perovskite, CaCo²⁺₃V⁴⁺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 [<i>a</i> = 7.3428(6) Å] with <i>Im</i>3̅ 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 Co²⁺ 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 Co²⁺ 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

Structural and Magnetic Transitions in CaCo₃V₄O₁₂ Perovskite at Extreme Conditions

We investigated the structural, vibrational, magnetic, and electronic properties of the recently synthesized CaCo₃V₄O₁₂ double perovskite with the high-spin (HS) Co²⁺ 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-Co²⁺ ions in the direction perpendicular to the oxygen planes. Although this effect is reminiscent of a continuous HS → LS transformation of the Co²⁺ 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 CaCo₃V₄O₁₂ could undergo a phase transition at which the large HS-Co²⁺ ions were pushed out of the oxygen planes because of lattice compression. The antiferromagnetic transition in CaCo₃V₄O₁₂ at 100 K was investigated by neutron powder diffraction at ambient pressure. We established that the magnetic moments of the Co²⁺ 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