The bulk structures and surface compositions of these O defect pyrochlore oxides were studied in order to determine the relationship between anion vacancies, electrical conductivity, cationic mobility, and surface segregation. The bulk structures were refined by means of Rietveld analysis, combined with powder neutron and X-ray synchrotron diffraction data. It was found that the oxides were isostructural (F¯43m, a = 1.03501 [Ti] = 1.06051nm [Sn]). The Ti (or Sn) and W atoms were randomly distributed on the 16e sites, and O-vacancy ordering occurred on the pyrochlore type-8b sites. In both cases, the Pb cation was surrounded by seven O atoms in a compressed scalenohedral arrangement in which the eighth vertex was occupied by the Pb 6s lone-pair electrons. In the Ti-containing pyroclore, the Pb atoms were displaced by 0.00328nm, along the [111] direction, towards the associated vacancy. In the larger Sn-containing pyroclore, the Pb cation was not displaced away from the ideal 16d site position. This difference was due to the Pb-O bonding requirements. The surface compositions of both oxides were identical to the bulk compositions. It was concluded that electrical conductivity was a critical factor which governed surface enrichment by potentially mobile A-type cations.

Ismunandar, B.J.Kennedy, B.A.Hunter: Journal of Solid State Chemistry, 1997, 130[1], 81-9