Crystal Structure of Pb$_5$Bi$_{18}$P$_4$O$_{42}$: A Fluorite-Related Superstructure
Résumé
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 , b=90.23(3)°, Z=2. The crystal is twinned by pseudo-merohedry. The structurere refinement converged to R=0.0366. There are five crystallographically independent Bi atoms. The oxygen coordinationpolyhedron around Bi(1) is a slightly distorted cube with Bi+Obonds varying from 2.381(11) to 2.662(10) Å . The other four Bi atoms are in a severely distorted oxygen environment with bondlengths 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 6s$^2$ orbital. The extent of hybridization is a function of the distortion of the oxygen coordination polyhedron around the cation.