Oxygen Vacancies in Pyrochlore Oxides: Powder Neutron Diffraction Study of Pb2Ir2O6.5and Bi2Ir2O7−y

Abstract The structure of the oxygen deficient pyrochlores Bi 2 Ir 2 O 7− y and Pb 2 Ir 2 O 6.5 were refined by Rietveld analysis of time-of-flight neutron powder diffraction data. The diffraction data for Pb 2 Ir 2 O 6.5 have been used to refine the structure in the cubic space group F [formula] m , a = 10.26450(4) A in which there is oxygen vacancy ordering and where each Pb atom is displaced 0.04 A toward its associated vacancy. For Bi 2 Ir 2 O 7− y = 0.2 the structure was fitted within space group Fd 3 m , a = 10.3256(1) A, in which the oxygen vacancies are randomly distributed over the O′ sites. In both structures the Ir atoms are in a nearly regular octahedral coordination whereas the Bi and Pb cations have a distorted eightfold coordination, compressed scalehedron, geometry. XPS studies indicate there is appreciable surface enrichment of the A cation.