BETA-NBPO5 AND BETA-TAPO5 - BRONZOIDS, 2ND MEMBERS OF THE MONOPHOSPHATE TUNGSTEN BRONZE SERIES (PO2)4(WO3)2M

The oxides beta -NbPO//5 and beta -TaPO//5 have been studied by X-ray diffraction and high resolution electron microscopy. They exhibit different supercells based on an orthorhombic subcell with the parameters a//0 equals 11. 27 Angstrom, b//0 equals 5. 28 Angstrom, c//0 equals 6. 62 Angstrom. It is shown that their framework corresponds to the member m equals 2 of the series of monophosphate tungsten bronzes (PO//2)//4(WO//3)//2//m with pentagonal tunnels (MPTB//p. The structure can thus be described as built up from ReO//3-type slabs which are two octahedra wide and connected through phosphate planes. The stability of these bronzoeids is discussed with respect to that of the MPTB//p compounds. The relationships between the structures of the alpha and beta forms of NbPO//5 and TaPO//5 are studied.