THE MONOPHOSPHATE PB2V2VO(PO4)(4) - A TUNNEL STRUCTURE WITH THE MIXED-VALENCE V(III)-V(IV)

A new vanadium monophosphate, Pb2V2VO(PO4)4, characterized by the mixed valence V(III)-V(IV), has been synthesized. It crystallizes in the space group C2/c with a = 17.747(2) Å, b = 18.051(2) Å, c = 9.344(1) Å, and β = 117.03(1)°. This structure can be described from the [V2P4O16]∞ framework built up from corner-sharing PO4 tetrahedra and VO6 octahedra. This host lattice consists of cross-linked [VPO8]∞ chains forming [VP2O10]∞ layers parallel to (001); the whole [V2P4O16]∞ framework results from the stacking along [104] of identical [VP2O10]∞ layers. This three-dimensional framework delimits tunnels running along the [001] and [102] where Pb2+ ions are located; it forms also large tunnels running along [110] and [11̄0] which are obstructed by vanadyl ions. The latter form VO5 pyramids with the surrounding oxygens; each VO5 pyramid shares one edge with one PO4 tetrahedron, forming VPO7 units. Bond valence calculations show that V(IV) and V(III) species are distributed in an ordered way, in VO5 pyramids and VO6 octahedra, respectively, whereas lead is in the divalent state. The Pb(II) cations, which exhibit two kinds of coordination, 7 and 8, show a stereoactivity of their 6s2 lone pair. © 1994 Academic Press, Inc.