Similarities and differences in the structural and electronic properties of ruthenium and iridium pyrochlores A2M2O7-y (M = Ru, Ir)

Electronic band structures of the ruthenium and iridium pyrochlores A(2)M(2)O(6)O(1-y)' (M = Ru, Ir) were examined using the extended Huckel tight-binding method. The metal versus semiconductor behaviors of these pyrochlores are explained in terms of the Mott-Hubbard localization concept. The M-O, A-O, and A-O' distances and M-O-M angles of A(2)M(2)O(6)O(1-y)' (M = Ru, Ir) were analyzed in terms of the A cation ionic radius. The M-O distance and the M-O-M angle are strongly influenced by the overlap repulsion between the filled shells of the A cation and O2- anions, The O' vacancies of the pyrochlores A(2)M(2)O(6)O(1-y)' with polarizable A cations such as Tl+, Pb2+, or Bi3+ occur most likely to reduce the overlap repulsion associated with the 6s orbitals of the A cations. (C) 1998 Academic Press.