Surface segregation and oxygen vacancy ordering in defect pyrochlores

The bulk structure and surface composition of the two oxygen defect pyrochlore oxides Pb-2(M1.5W0.5)O-6.5 (M = Ti, Sn) have been studied in order to determine the relationship between anion vacancies, electrical conductivity, cationic mobility, and surface segregation in pyrochlore oxides. The bulk structures were refined by Rietveld analysis combining powder neutron and X-ray synchrotron diffraction data. The two oxides are isostructural, space group F (4) over bar 3m (No. 216), with a = 10.3501(1) Angstrom for M = Ti and a = 10.6051(2) Angstrom for M = Sn, where the M and W atoms are randomly distributed in the 16e site and oxygen-vacancy ordering occurs on the pyrochlore type 8b sites, In both cases the Pb cation is surrounded by seven oxygen atoms in a compressed scalenohedral arrangement where the eighth vertex is occupied by the Pb 6s lone pair electrons. In Pb-2(Ti1.5W0.5)O-6.5, the Ph atoms are displaced by 0.032(8) Angstrom along the [111] direction toward the associated vacancy, while in the larger Pb-2(Sn1.5W0.5)O-6.5 the Pb cation is not displaced away from the ideal 16d site position. This difference is a consequence of the Pb-O bonding requirements. The surface composition of both oxides is identical to that of the bulk composition. By comparison with other studies these results demonstrate that electrical conductivity is a critical factor in determining surface enrichment by the potentially mobile A-type cations in pyrochlore oxides. (C) 1997 Academic Press.