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 c(f)/a(f) of unity and internal shear defomation. The values of c(f)/a(f) ratio in HfO2-X-mol %-CeO2 samples (X= 80, 85, and 90) are estimated to be 1 +/-0.000 04 from the estimated standard deviation of the peak positions and to be 1 +/-0.002 from the full width at half maximum of the 400(f) 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 c(f) axis from the ideal fluorite position, does.