Crystal structure of Pb5Bi18P4O42: A fluorite-related superstructure

Single crystals of a new oxyphosphate of bismuth and lead, Pb5Bi18P4O42, we re studied using short-wavelength synchrotron X-ray radiation. The compound is monoclinic, I2/m, a=11.885(2) Angstrom, b=11.540(2) Angstrom, c=15.636( 3) Angstrom, beta=90.23(3)degrees, Z=2. The crystal is twinned by pseudo-me rohedry. The structure refinement converged to R=0.0366. There are five cry stallographically independent Bi atoms. The oxygen coordination polyhedron around Bi(1) is a slightly distorted cube with Bi-O bonds varying from 2.38 1(11) to 2.662(10) Angstrom. The other four Bi atoms are in a severely dist orted oxygen environment with bond lengths varying from 2.102(5) to 3.16(2) Angstrom. 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 distance s vary from 2.13(2) to 3.38(2) Angstrom. Of 20 crystallographically indepen dent oxygen ions 11 are disordered. The structure consists of two Bi-O laye rs parallel to (101). They are interconnected perpendicular to [010] by lay ers consisting of Pb and PO4. The two crystallographically independent PO4 moieties are disordered. This compound displays a delta-Bi2O3 fluorite supe rstructure. The locations of the nonbonded lone-pair electrons were calcula ted and vary from 0 to 1.1 Angstrom. These displacements from the cation nu cleus are indicative of the extent of s-p hybridization, where zero displac ement represents the spherical nonbonded 6s(2) orbital. The extent of hybri dization is a function of the distortion of the oxygen coordination polyhed ron around the cation. (C) 2000 Academic Press.