Crystal Structure of Pb5Bi18P4O42: A Fluorite-Related Superstructure

Abstract

Single crystals of a new oxyphosphate of bismuth and lead, Pb5Bi18P4O42, were studied using short-wavelength synchrotron X-ray radiation. The compound is monoclinic, I2/m, a=11.885(2) Å, b=11.540(2) Å, c=15.636(3) Å, β=90.23(3)°, Z=2. The crystal is twinned by pseudo-merohedry. The structure refinement converged to R=0.0366. There are five crystallographically independent Bi atoms. The oxygen coordination polyhedron around Bi(1) is a slightly distorted cube with Bi–O bonds varying from 2.381(11) to 2.662(10) Å. The other four Bi atoms are in a severely distorted oxygen environment with bond lengths varying from 2.102(5) to 3.16(2) Å. A greater variation of the oxygen coordination polyhedra exists around the three crystallographically independent Pb atoms changing from a somewhat elongated cube to a monocapped pentagonal bipyramid; bond distances vary from 2.13(2) to 3.38(2) Å. Of 20 crystallographically independent oxygen ions 11 are disordered. The structure consists of two Bi–O layers parallel to (101). They are interconnected perpendicular to [010] by layers consisting of Pb and PO4. The two crystallographically independent PO4 moieties are disordered. This compound displays a δ-Bi2O3 fluorite superstructure. The locations of the nonbonded lone-pair electrons were calculated and vary from 0 to 1.1 Å. These displacements from the cation nucleus are indicative of the extent of s–p hybridization, where zero displacement represents the spherical nonbonded 6s2 orbital. The extent of hybridization is a function of the distortion of the oxygen coordination polyhedron around the cation.