Two new members of a series of monoclinic sodium phosphate tungsten bronzes NaxP4O8(WO3)2m: NaxP4W8O32 (m = 4) and NaxP4W12O44 (m = 6)

Abstract

Monoclinic, P21/a. NaxP4W803~2: a = Introduction. The investigation of the P-W-O and A-P-W-O systems allowed us to synthesize three series of tunnel structures which are all built up from ReOa-type slabs connected by PO4 tetrahedra. The first family, the P4Os(WO3)2, n orthorhombic phosphate tungsten bronzes (Giroult, Goreaud, Labb6 & Raveau, 1981b), is characterized by empty pentagonal tunnels, the connection between two successive 'WO 3' slabs being made by 'planes' of single PO 4 tetrahedra. In the second family, AxP4Oa(WOa)2m (Giroult, Goreaud, Labb6 & Raveau, 1980, 1981a), called monoclinic pyrophosphate tungsten bronzes, the 'WO 3' slabs are connected through planes of P207 groups; the members of this series, which exhibit wide distorted hexagonal tunnels, are stabilized by big ions such as K +, Rb + or T1 +. Recently, the structural study of the bronze KxP2W4Ol6 (Giroult, Goreaud, Labb6 & Raveau, 1982) led us to predict the existence of a new series of monoclinic phosphate tungsten bronzes with the same formulation AxP4Oa(WO3)Em. The framework of the latter bronzes also contains distorted hexagonal tunnels, but with a smaller size than those of the second family, because the 'WO 3' slabs are not connected through pyrophosphate groups but through single PO 4 tetra- hedra. Only one member, m = 4, could be obtained in this series, and only in the case of potassium; however, the structure of MO4Oll described by Kihlborg (1963) corresponds to the member m = 6 of this series in which MoO4 tetrahedra replace the PO 4 tetrahedra, the tunnels being empty. Thus, it appeared that this structure could be stabilized by ions smaller than potassium. The present work deals with the structure of two sodium phosphate bronzes belonging to this series, NaxPaWsO32 and NaxP4W12044.