Crystal Structure of Pb2V3O9: Rietveld Refinement and Electron Lone-Pair Localization. The Magnetic Susceptibility of Sr2+-Substituted Phases

Abstract

Powder neutron and X-ray diffraction data from Pb2V3O9 were collected and used for structure refinement. The triclinic lattice parameters from neutron data are a = 7.5935(1) Å, b = 16.3898(3) Å, c = 6.9696(1) Å, α = 91.41(1)°, β = 119.34(1)°, γ = 90.49(1)°, C1̄, and z = 2. The nonstandard unit cell was chosen for comparison with the monoclinic structure of Sr2V3O9 with which it is nearly isostructural. The 6s2 electrons of Pb2+ cause the lowering of the symmetry and distort the cooperative arrangement of the vanadyl bonds existing in the V−O framework of Sr2V3O9. One of the two crystallographically independent Pb2+ is in a distorted, capped, octahedral environment formed by seven oxygen atoms with Pb−O bonds ranging from 1.426(8) Å to 2.830(8) Å. The other Pb2+ is surrounded by nine oxygen atoms that form a distorted, tricapped, trigonal prism with Pb−O ranging from 2.421(11) to 2.920(8). The lone-pair 6s2 electrons were located and are about 0.58 Å from the nucleus. Of the four crystallographically independent vanadium atoms, two are in tetrahedral coordination with a mean oxidation number +5. The other two are in octahedral coordination with oxidation number +4. The octahedra link by corner sharing into an infinite chain parallel to [101]. The tetrahedra share corners with the octahedra to form two-dimensional sheets that are then joined by the Pb ions into a three-dimensional structure. One octahedral V4+ cation shows displacement from its polyhedral center toward an oxygen that is corner-shared in the octahedral chain, giving rise to a short vanadyl bond, 1.67(4) Å, and a long bond of 2.18(4) Å. The main difference between the lead and strontium vanadate framework is in the displacement of the second V4+ ion toward an oxygen that is corner-shared with a tetrahedron. Here the short bond length is 1.72(8) Å while it is 2.21(7) Å to the opposite apex. Thus, the cooperative displacement of the VO bond in the Sr2V3O9 chains is broken by the triclinic transformation, leading to a more disordered V4+ arrangement. This is the main reason for the observed differences in the magnetic susceptibilities of the Pb2+ and Sr2+ phases. An alternating J1 − J2 chain model was satisfactorily fitted for Pb2V3O9 while a regular chain model matched the data for Sr2V3O9.