Crystal structure of Pb2V3O9: Rietveld refinement and electron lone-pair localization. The magnetic susceptibility of Sr2+-substituted phases

Powder neutron and X-ray diffraction data from Pb2V3O9 were collected and u sed for structure refinement. The triclinic lattice parameters from neutron data are a = 7.5935(1) Angstrom, b = 16.3898(3) Angstrom, c = 6.9696(1) An gstrom, alpha = 91.41(1)degrees, beta = 119.34(1)degrees, gamma = 90.49(1)d egrees, C1 and z = 2. The nonstandard unit cell was chosen for comparison w ith the monoclinic structure of Sr2V3O9 With which it is nearly isostructur al. The 6s(2) electrons of Pb2+ cause the lowering of the symmetry and dist ort the cooperative arrangement of the vanadyl bonds existing in the V-O fr amework of Sr2V3O9 One of the two crystallographically independent Pb2+ is in a distorted, capped, octahedral, environment formed by seven oxygen atom s with Pb-O bonds ranging from 1.426(8) Angstrom to 2.830(8) Angstrom. The other Pb2+ is surrounded by nine oxygen atoms that form a distorted, tricap ped, trigonal prism with Pb-O ranging from 2.421(11) to 2.920(8). The lone- pair 6s(2) electrons were located and are about 0.58 Angstrom from the nucl eus. Of the four crystallographically independent vanadium atoms, two are i n tetrahedral coordination with a mean oxidation number +5. The other two a re 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 th en 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 vanad yl bond, 1.67(4) Angstrom, and a long bond of 2.18(4) Angstrom, The main di fference between the lead and strontium vanadate framework is in the displa cement of the second V4+ ion toward an oxygen that is corner-shared with a tetrahedron. sere the short bond length is 1.72(8) Angstrom while it is 2.2 1(7) Angstrom to the opposite apex. Thus, the cooperative displacement of t he V=O 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 J(1) - J(2) chain model was satisfactorily fitt ed for Pb2V3O9 while a regular chain model matched the data for Sr2V3O9.