Dimorphism of the Vanadium(V) Monophosphate PbVO_2PO_4: #alpha#-Layered and #beta#-Tunnel Structures

摘要

Two forms of V(V) monophosphate #alpha#- and #beta#-PbVO_2 PO_4 have been synthesized and grown as single crystals. The #alpha# form is isostructure with the Ba and Sr homologues previously described by Kang et al. The #beta# from exhibits an original tridimensional (3D) framework (a=9.715(1) b=13.820(1), and c=10.999(1) A). The latter crystallizes in the space group Pnma and consists of tritetrahedral P_2VO_(10) units and tripolyhedral units V_2PO_(14) (2VO_6) octahedra+1PO_4 tetrahedron) forming chains running along b. VPO_8 chains in which one PO_4 tetrahedron alternates with one VO_6 octahedron, running along a and forming [V_2P_2O_(14)]_infinity layers are also observed. This 3D framework forms large tunnels running along b where the Pb~(2+) cations are located. The infrared study supports the structure description and gives a tentative correlation between framework units and vibrational behaviour.