Dimorphism of the vanadium(V) monophosphate PbVO2PO4: alpha-layered and beta-tunnel structures

Two forms of V(V) monophosphate alpha- and beta- PbVO2PO4 have been synthes ized and grown as single crystals. The a form is isostructural with the Pa and Sr homologues previously described by Kang et al, The beta form exhibit s an original tridimensional (3D) framework (a = 9.715(1), b = 13.820(1), a nd c = 10.999(1) Angstrom). The latter crystallizes in the space group Pnma and consists of tritetrahedral P2VO10 units and tripolyhedral units V2PO14 (2VO(6) octahedra + 1PO(4) tetrahedron) forming chains running along b. VP O8 chains in which one PO4 tetrahedron alternates with one VO6 octahedron, running along a and forming [V2P2O14](infinity) layers are also observed, T his 3D framework forms large tunnels running along b where the Pb2+ cations are located. The infrared study supports the structure description and giv es a tentative correlation between framework units and vibrational behaviou r. (C) 2000 Academic Press.