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 CeZrO(4) is refined by Rietveld analysis of neutron diffraction data measured in situ over the temperature range 296-1831 K. The CeZrO(4) exhibits a tetragonal structure with the space group P4(2)/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 CeZrO(4) 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 CeZrO(4) phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching 0.0 A 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.