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 con ditions yielded in a pH range between 6.5 and 7.6 and at synthesis times lo nger than 10 days a new blue Na3VO(PO4)(HPO4) phase. It crystallizes in spa ce 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 co mposed of VOPO4 layers, with additional HPO4 tetrahedra between the layers. A symmetry supergroup/subgroup examination of all compounds containing VOX O4 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 N a+ cations occupy every second possible position between the vanadium and t he phosphate polyhedra of the VOPO4 layers. This is in contrast to the clas s of MVOPO4 . nH(2)O (M = Na, K, Rb, Ca, Sr, Pb, Ba) (n = 3-4) compounds, w here only every fourth possible position is occupied. (C) 1999Academic Pres s.