Na3V4+O(PO4)(HPO4), a new phase in the Na/V/P/H2O system under hydrothermal conditions at 473 K

A new examination of the Na/V/P/H2O systems at 473 K under hydrothermal conditions yielded in a pH range between 6.5 and 7.6 and at synthesis times longer than 10 days a new blue Na3VO(PO4)(HPO4) phase. It crystallizes in space group P2(1)/m with a = 9.6402(7) Angstrom, b = 6.4075(5) Angstrom, c = 6.2637(6) Angstrom, and beta = 104.86(7)degrees. The crystal structure is composed of VOPO4 layers, with additional HPO4 tetrahedra between the layers. A symmetry supergroup/subgroup examination of all compounds containing VOXO4 layers shows that all space groups of these compounds can be derived by reducing the symmetry elements of the aristotypic space group I4/mmm. The Na+ cations occupy every second possible position between the vanadium and the phosphate polyhedra of the VOPO4 layers. This is in contrast to the class of MVOPO4 . nH(2)O (M = Na, K, Rb, Ca, Sr, Pb, Ba) (n = 3-4) compounds, where only every fourth possible position is occupied. (C) 1999Academic Press.