Structural transformations in the Na4+xVO(PO4)2 vanadylphosphates

The crystal structures of new sodium vanadylphosphate, Na4.35VO(PO4)2 (a = 15.4450(11)Å, b = 14.7690(10)Å, c = 6.9870(5) A, Z = 8, S.G. Ibam), and new (y-) modification of Na4VO(PO4)2(a = 15.4455(15) A, b = 14.8433(10)Å, c = 7.0081(5)Å, Z = 8, S.G. Pbc21) have been investigated by X-ray single-crystal diffraction. Both structures contain isolated infinite chains of the corner-sharing VO6 octahedra. The octahedra within the chains are additionally linked to each other by the tetrahedral PO4 groups. Sodium atoms are situated in the positions between the chains. Depending on the conditions of synthesis, the number of sodium atoms in the unit cell of the Na4+xVO(PO4)2 compounds may vary resulting in a change of the oxidation state of vanadium atoms and a change of their coordination environment. In Na4.35VO(PO4)2 vanadium atoms have almost regular octahedral coordination with six close V-O separations and all chains in the structure are equivalent. The crystal structure of γ-Na4VO(PO4)2 contains two non-equivalent chain types: the first one is similar to that found in Na4.35VO(PO4)2 whereas the second one contains VO6 octahedra with the short vanadyl bonds. The charge redistribution was supposed in the new γ-modification of Na4VO(PO4)2 where the V4+δ and V4-δ cations orderly occupy octahedral positions in different chains. The origin of this phenomena is discussed.