In situ observation of the tetragonal–cubic phase transition in the CeZrO4 solid solution – a high-temperature neutron diffraction study

The crystal structure of the compositionally homogeneous ceria–zirconia solid solution CeZrO4 is refined by Rietveld analysis of neutron diffraction data measured in situ over the temperature range 296–1831 K. The CeZrO4 exhibits a tetragonal structure with the space group P42/nmc at temperatures from 296 to 1542 K (Z = 1), and a cubic fluorite-type form with the space group Fm\overline 3 m at 1831 K (Z = 2). The isotropic atomic displacement parameters of Ce and Zr atoms B(Ce,Zr) and O atoms B(O) are found to increase with temperature, with B(O) being larger than B(Ce,Zr), suggesting the higher diffusivity of oxygen ions. The ratio of the c axial length to the a length of the pseudo-fluorite lattice (c/a F axial ratio) for the tetragonal CeZrO4 phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching unity between 1542 and 1831 K. The displacement of O atoms along the c axis in the tetragonal CeZrO4 phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching 0.0 Å between 1542 and 1831 K. These results indicate that the cubic-to-tetragonal phase transition between 1542 and 1831 K is accompanied by oxygen displacement along the c axis and the increase of the c/a F axial ratio from unity.