Crystal Structure, Polymorphism, and Properties of the New Vanadyl Phosphate Na4VO(PO4)2

The new vanadyl phosphate Na4VO(PO4)2 was synthesized and investigated by X-ray powder and single-crystal diffraction, high-temperature X-ray diffraction, electron diffraction, high-resolution electron microscopy, thermal analysis, magnetic susceptibility, and conductivity measurements. The compound undergoes a reversible phase transition at about 200 C. The crystal structure of low-temperature (beta)-Na4VO (PO4)2 was solved using X-ray single-crystal data. This phase has an orthorhombic unit cell with lattice parameters a = 16.0068(12) A, b = 14.5129(8) A, c = 7.0231(5) A, S.G. Pbca, and Z = 8. The crystal structure of -Na4VO(PO4)2 is built by isolated chains formed by corner-shared V4+O6 octahedra linked additionally via corners by two PO4 tetrahedra. All chains in the structure are equivalent. Na cations are located between the chains in an ordered manner. High -temperature (alpha)-Na4VO(PO4)2 also has an orthorhombic cell with lattice parameters a = 15.595 (1) A, b = 14.651(2) A, c = 7.0262(6) A, S.G. Ibam, and Z = 8. Electron diffraction study revealed an existence of various structural transformations occurring in situ in the transmission electron microscope. In both (alpha)- and -(beta)modifications, the susceptibility follows a Curie-Weiss law with a very small Curie-Weiss temperature, indicating a very weak magnetic exchange among the V4+ ions.