Zirconia-ceria solid solutions of tetragonal symmetry (t'-ZrO,) containing 30 to 65 mol% ceria were prepared by annealing c'-ZrO, at 627°C samples sintered at 1660-1760"C, where c'-ZrO, is defined as a cubic or tetragonal phase whose axial ratio c / a (tetragonality) is equal to 1. The lattice parameters and the cube root of the unit cell volumes of the annealed samples increased linearly with the ceria content but, on the other hand, the axial ratio cla (tetragonality) decreased to 1.000 at 70 mol%. During the annealing of each sample, the cell volume decreased because the residual Ce3+ ions were oxidized to the smaller Ce4+. The temperature-time-transformation (TTT) diagram of the 1:l solid solution was investigated for the ~' 4 ' diffusionless phase transition and the c'-(t + c) diffusional reaction.The d -t ' transformation was found to behave as a thermally activated process with an activation energy estimated to be 113 kJ/mol. This value may indicate that c'-t' transformation is controlled by oxygen diffusion accompanied by dimensional changes of the cerium ions.

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Diffusionless tetragonal (t') u cubic (c') phase transformation in 65-mol%-Ce0,-ZrO2 was investigated around the c'-t' equilibrium temperature Ti'" using powder X-ray diffraction, where c' was defined as a tetragonal or cubic phase with an axial ratio of unity. The (400) peak profile of t'-ZrO, broadened at 860" and 900°C. This indicated that the t'-ZrO, transformed partially into c'-ZrO,. The (400) peak profile of c'-ZrO, also broadened at 860" and 9OO0C, which indicated that the c'-ZrO, partially transformed to t'-ZrO,. The finish point of t ' -d transformation T:' " was investigated by annealing the t'-ZrO, containing 30-65 mol% CeO, at various temperatures. The T:'"' line existed in the vicinity of the cubic solubility limit within the t + c two-phase region. This Tf-" location, which should have been located in the vicinity of Ti"", could not be explained by a simple regular solution model. However, it was described successfully by a thermodynamic model based on Landau's phenomenologic theory.

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The structural geometry, vibrations and deformation density Ap for lithium niobate, LiNbO3, and lithium tantalate, LiTaO3, are derived from synchrotron radiation diffraction measurements. Electron density is transferred from the Nb (