High-resolution X-ray and neutron scattering studies of powder samples of pure and Zr-doped (5at%) material revealed that mesoscopic compositional inhomogeneities existed, in the doped compound, which were unlikely to be revealed by conventional diffraction methods. Even if structural refinement of the doped compound gave quite good reliability factors within a single-phase model, a more careful analysis of the data proved the existence of 2 phases. The main phase (90%) had compositions and structures which were very close to those of the pure oxide. The phase which was richer in Zr exhibited site selectivity. An abnormally high value (4.7Å2 at 294K) of the thermal displacement parameter of the site, as affected by Zr substitution, was explained in terms of a split-atom model which involved an off-site displacement of 0.38Å. Additional O atoms occupied the vacancies in the structure, and provided charge balance for the Zr dopant.

G.Baldinozzi, J.F.Bérar, M.Gautier-Soyer, G.Calvarin: Journal of Physics - Condensed Matter, 1997, 9[45], 9731-44