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 synthesized and grown as single crystals. The ALPHA form is isostructural with the Ba and Sr homologues previously described by Kang et al. The BETAform exhibits an original tridimensional (3D) framework (a=9.715(1), b=13.820(1), and c=10.999(1) ?). The latter crystallizes in the space group Pnma and consists of tritetrahedral P2VO10 units and tripolyhedral units V2PO14 (2VO6 octahedra+1PO4 tetrahedron) forming chains running along b. VPO8 chains in which one PO4 tetrahedron alternates with one VO6 octahedron, running along a and forming [V2P2O14] layers are also observed. This 3D framework forms large tunnels running along b where the Pb2+ cations are located. The infrared study supports the structure description and gives a tentative correlation between framework units and vibrational behaviour.