We report on the synthesis of large single crystals of a new FeSe layer superconductor Cs(0.8)(FeSe(0.98))(2). X-ray powder diffraction, neutron powder diffraction and magnetization measurements have been used to compare the crystal structure and the magnetic properties of Cs(0.8)(FeSe(0.98))(2) with those of the recently discovered potassium intercalated system K(x)Fe(2)Se(2). The new compound, Cs(0.8)(FeSe(0.98))(2), shows a slightly lower superconducting transition temperature (T(c) = 27.4 K) in comparison to 29.5 in (K(0.8)(FeSe(0.98))(2)). The volume of the crystal unit cell increases by replacing K by Cs-the c parameter grows from 14.1353(13) to 15.2846(11) Å. For the alkali metal intercalated layered compounds known so far, (K(0.8)Fe(2)Se(2) and Cs(0.8)(FeSe(0.98))(2)), the T(c) dependence on the anion height (distance between Fe layers and Se layers) was found to be analogous to those reported for As-containing Fe superconductors and Fe(Se(1 - x)Ch(x)), where Ch = Te, S.

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The new perovskite PbVO 3 was synthesized under high-temperature and high-pressure conditions. Its crystal structure (a ) 3.80005(6) Å, c ) 4.6703(1) Å, Z ) 1, S.G. P4mm) contains isolated layers of corner-shared VO 5 pyramids, which are formed instead of octahedra due to a strong tetragonal distortion (c/a ) 1.23). The lead atom is shifted out of the center of the unit cell toward one of two [VO 2 ]-layers due to the influence of the lone pair. This new perovskite exhibits a semiconductor-like F(T) dependence down to 2 K. This behavior can be qualitatively explained by taking into account strong electron correlations in electronic structure calculations.

We report on a comparative study of the crystal structure and the magnetic properties of FeSe1−x (x = 0.0 − 0.15) superconducting samples by neutron powder diffraction and magnetization measurements. The samples were synthesized by two different methods: a "low-temperature" one using powders as a starting material at T ≃ 700 o C and a "high-temperature" method using solid pieces of Fe and Se at T ≃ 1075 o C. The effect of a starting (nominal) stoichiometry on the phase purity of the obtained samples, the superconducting transition temperature Tc, as well as the chemical stability of FeSe1−x at ambient conditions were investigated. It was found that in the Fe-Se system a stable phase exhibiting superconductivity at Tc ≃ 8K exists in a narrow range of selenium concentration (FeSe0.974±0.005).