2005 Spring Meeting

Hydroxyapatite, HVA, Pb10(VO4)6(OH)2 was prepared by wet precipitation method and analysed by X-ray absorption scpectroscopy (XANES, EXAFS), Raman scattering and X-ray difraction (XDR). The VO4 tetrahedra were found to be strongly distorted , with three different values of vanadium oxygen distances (1.69, 1.72 snd 1.75 A). The XRD pattern was compatible with space group P63/m (176) and cell parametres a=10.1938 A and c=7.4499 A. The spectroscopic data were used in a Rietveld refinement of the XRD pattern; a crystallographic structure of the compound is proposed, in agreement with these data. The electronic structures of clusters representing the proposed HVA structure were investigated, employing the first-principles self-consistent discret variational method and local-density theory. Mulliken atomic-orbital populations, densities of states and charge-density maps were obained to study the Pb-O interaction and evaluate the influence of the lone-pair electrons of lead at sixfold positions in determining the location of the OH ions and the chemical reactivity associated with Pb sites. The electronic structure of vanadinite, Pb10(VO4)6Cl2, was also investigated and used as a structural prototype. The utility of mixed vanadinite/lead hydroxyvanadinite nanoparticles as low temperature catalysts is discussed.