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.

The effect of radiation-induced disordering in a nuclear reactor (fast neutrons fluence Φ = 5 · 10 19 cm 2 , Tirr = 340 K) on resistivity ρ, superconducting transition temperature TC and upper critical field HC 2 of polycrystalline MgCNi3 samples was investigated. It was found that TC decreases under irradiation from 6.5 to 2.9 K and completely recovers after annealing at 600 • C. Temperature dependences ρ(T ) are characteristic of compounds with strong electron-phonon interaction. The dHC 2 /dT behaviour testifies to a considerable decrease in density of electronic state at Fermi level N (EF ) in the course of disordering.Radiation-induced disordering caused by irradiation with high-energy particles is a unique method of investigating the properties of superconducting and normal states of ordered crystals [1,2]. Even in broad-band metals, such as intermetallic compounds with A15 structure, long-range ordering loss leads to considerable rearrangement of the electronic spectrum, resulting in disappearance of individual features of the electronic structure. Disordering causes decrease in densities at Fermi level N (E F ) and respective noticeable drop of T C in compounds with high initial N (E F ) (Nb 3 Sn or V 3 Si), and considerable (from 1.5 to 7 K) rise of T C in compounds with low N (E F ) and T C due to growth of N (E F ) (Mo 3 Si and Mo 3 Ge) [3,4,5]. In type HTSC compounds, disordering leads to more significant changes in properties: fast and complete T C degradation is accompanied with N (E F ) decrease and metal-insulator transition [2]. Thus investigation of response of a system to radiation-induced disordering serves as a kind of a test to reveal the characteristic features of its electron states. It was shown in recent papers [6,7] that T C drop from 38 to 5 K observed at MgB 2 under radiation-induced disordering is connected mainly with considerable drop of N (E F ), similar to Nb 3 Sn or V 3 Si compounds. In our investigation, we concentrated on the effect of disordering on the properties of superconducting compound MgCNi 3 (T C ∼ 8 K) with perovskite cubic structure of type SrTiO 3 , unconventional for intermetallides [8]. Our interest in this system was explained by the fact that its ground state is close to ferromagnetic due to the presence of a narrow peak in N (E) located 45 meV below the Fermi level [9]. This allowed us to regard it as a candidate for an unconventional (possibly triplet) superconductivity, similar to Sr 2 RuO 4 compound. It is known that in Sr 2 RuO 4 , as distinct from conventional superconducting compounds (intermetallides), T C undergoes anomalously strong suppression even under a slight disorder [10]. In MgCNi 3 , maximum T C is achieved at excess of carbon content only (nominal composition MgC 1.5 Ni 3 ), even though, according to neutron diffraction study, the actual composition is closer to Mg 0.96 CNi 3 , and excess carbon occupies the region between sample grain boundaries [11].In the sample preparation, fine powders Mg, C and Ni with purity better than 99.5% were used...

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