Structural studies of the fast oxygen ion conductor BICOVOX.15 by single-crystal neutron diffraction at room temperature

The room temperature crystal structure of the fast oxygen ion conductor Bi- 4(V0.85Co0.15)(2)O11-delta (BICOVOX.15) was refined within the average tetr agonal unit cell from single-crystal neutron diffraction data, The unit cel l dimensions are a(T) = 3.929(2) Angstrom and c(T) = 15.476(9) Angstrom in space group I4/mmm with Z = 1. The conventional R-w factor is 0.067. The st ructure consists of intergrowth, along the c-axis, of alternating (Bi2O2)(2 +) sheets and (VO(3.5)square(0.5))(2-) oxygen-deficient perovskite-like lay ers. The compound, which exhibits a high ionic conductivity at moderate tem peratures, undergoes a phase transition around 780 K between the disordered high-temperature gamma-phase and the lower temperature gamma'-phase, showi ng a modulated superstructure. Oxygen vacancies are localized in the perovs kite-like layers, oxygen stoichiometry is derived, and oxygen diffusion pat hs are proposed for the gamma'-phase at room temperature, (C) 1998 Academic Press.