Internal distortion in ceria-doped hafnia solid solutions: High-resolution x-ray diffraction and Raman scattering

High-resolution synchrotron x-ray powder diffraction and Raman scattering have indicated the existence of tetragonal ceria-doped hafnia with both an axial ratio cf/af of unity and internal shear defomation. The values of cf/af ratio in HfO₂--X-mol%-CeO₂ samples $(X=80,$ 85, and 90) are estimated to be 1±0.00004 from the estimated standard deviation of the peak positions and to be 1±0.002 from the full width at half maximum of the 400f reflection peaks. However, the Raman spectra of these samples clearly demonstrate the tetragonal phase. The intensity of the Raman band around 280 cm^-1, which is one of the characteristic bands of the tetragonal phase, continuously decreased and approached zero between X=90 and 95, which reflects the tetragonal-cubic phase boundary is located within this compositional range. The lattice distortion does not induce the cubic-tetragonal phase transition but an internal distortion in oxygen sublattice, oxygen displacement along the cf axis from the ideal fluorite position, does.