Neutron and high-resolution synchrotron x-ray diffraction measurements have indicated a tetragonal zirconia phase with both an axial ratio of unity and internal shear deformation: The oxygen displacements from ideal fluorite positions of 0.088 and 0.078 Å are observed for 65 and 70 mol &percent;CeO2–ZrO2samples, respectively, although bothc/avalues are 1.000±0.001. Thec/aratio decreases with increasing ofCeO2content and decreases discontinuously to unity around a 60 mol &percent;CeO2composition, while the oxygen displacement decreases continuously up to about 90 mol &percent;CeO2.©1998 American Institute of Physics.