The stereochemical effect of 6s2 lone-pair electrons:: The crystal structure of a new lead bismuth oxyphosphate Pb4BiO4PO4

Pb4BiO4PO4 was prepared by heating a stoichiometric mixture of(NH4)(2)HPO4, PbO, and Bi2O3 at 700 degrees C for 4 days with intermediate grinding. The material melts congruently at 805 degrees C; single crystals were obtained by slow cooling of a melt. The compound is triclinic, P (1) over bar a = 6.215(1) Angstrom, b = 7.440(2) Angstrom, c = 10.498(2) Angstrom, alpha = 100.19(1)degrees, beta = 103.73(1)degrees, gamma = 90.05(1)degrees, Z = 2. The structure refinement converged to R = 0.0485. Bi is bonded to 4 O at distances varying from 2.16(3) Angstrom to 2.53(3) Angstrom and two bonds at 3.10(3) Angstrom and 3.15(3) Angstrom. Five atoms form an almost perfect pentagon parallel to (001) with an apical Bi-O bond nearly perpendicular to the pentagonal plane. Presumably the lone pair points to the empty apex to complete a pentagonal bipyramid. Pb(1) is bonded to seven oxygen atoms. Five of these bonds are less than 2.9 Angstrom, while two are 3.12(3) Angstrom and 3.16(3) Angstrom. Four oxygen atoms constitute the equatorial plane of an octahedron with Pb(1) slightly out of that plane. The apex oxygen atoms are tilted in the same direction away from the normal to the equatorial plane. One triangular face is capped by an oxygen atom while another triangular face is open and the lone pair electrons cap this face. Pb(2), Pb(3), and Pb(4) have similar coordination polyhedra. The average of the four P-O bonds is 1.53(5) Angstrom. The Pb-O framework is related to that of tetragonal PbO and the structure is related to that of Pb5SO8. The valence bond sums for two of the Pb atoms are 2.63 and 2.32, while the other two Pb atoms have the more expected values, 2.14 and 2.06. The valence bond sum far Bi is 2.14, much less than the expected trivalent oxidation state. Van der Waals contacts exist between the two Pb with the increased oxidation states and Bi but no contacts exist between the other two Pb and Bi. Where van der Waals contacts are present the Pb 6s(2) energy levels overlap the empty Bi 6p energy levels so that electron transfer into the Ri 6p levels from Pb occurs. The electrons are not itinerant because of the large atomic separations and the material remains insulating. (C) 1999 Academic Press.