Structural changes of ZrO2-CeO2 solid solutions around the monoclinic-tetragonal phase boundary

High-resolution neutron-diffraction experiments have been performed to study the structural changes of ZrO2 induced by CeO2 doping. The crystal structures of monoclinic [P21/c, Z=4] and tetragonal [P42/nmc, Z=2] phases for ZrO2-X mol % CeO2 (X=0, 2, 5, 8, 10, 12, and 15) have been refined by the Rietveld analysis of the diffraction patterns measured at 298 K. The unit-cell parameters change anisotropically with an increase of CeO2 content X. In the monoclinic phase am and cm increase considerably while bm shows a rather small increase with increasing X where the suffix m denotes the monoclinic phase. The angle βm decreases with increasing X. The thermal ellipsoids also show large anisotropies, which correlate strongly with the directions of the movements of oxygen ions during the phase transition. The unit-cell parameters and the atomic positions discontinuously change during the monoclinic-tetragonal phase change. The crystal structure of monoclinic phase approaches that of tetragonal phase with an increase of X: (1) the length of am approaches that of bm with increasing X, (2) the angle βm decreases with X, and (3) all positional parameters of the monoclinic phase approach those of the tetragonal structure.