A structural phase transition between the cubic (space group, Fm3m) and tetragonal (space group, P4,lnmc) phases in a zirconia-ceria solid solution (Zr, -,Ce,O,) has been observed by Raman spectroscopy. The cubic-tetragonal (c-t") phase boundary in compositionally homogeneous samples exists at a composition X , (0.8 < X , c 0.9) at room temperature, where t" is defined as a tetragonal phase whose axial ratio c/a equals unity. The axial ratio c/a decreases with an increase of ceria concentration and becomes 1 at a composition Xg (0.65 < Xh c 0.7) at room temperature. The sample with a composition between X , and X i is t"-ZrO,. By Raman scattering measurements at high temperatures, the tetragonal (t") + cubic and cubic + tetragonal phase transitions occur above 400°C in Zro,2Ceo.802 solid solution.

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In the ZrO2-YO~.5 solid solutions, the phase changes among the cubic phase (Fm3m, Z = 4), the t" form and the t' form were investigated by neutron and X-ray powder diffraction, where the t" and t' forms are defined as tetragonal phases (P42/nmc, Z = 2) with axial ratios of c/ay = 1 and c/a s > 1, respectively, which were prepared by a diffusionless transition from the high-temperature cubic phase during quenching, ay is the lattice parameter of the pseudofluorite cell. The crystal structure of the tetragonal phase of Zrl _ xYxO2-xn [ = (ZrO2)l--x(YOl.5)x; X = 0.10, 0.12, 0.14 and 0.16] has been refined both by the direct estimation of the integrated intensity ratio I(102)/I(101) and by the Rietveld analysis of neutron powder diffraction data collected at 293 K [A = 1.5301 (3) or 1.5314 (2)/k]. The crystal structure of Zrl-xYxO2-x/2 (X = 0.18 and 0.20) has also been refined assuming either tetragonal (P4Jnmc, Z--2) or cubic symmetry [Fm3m, Z--4] by the Rietveld analysis of neutron powder diffraction data collected at 293 K [A--1.5301 (3)A]. The lattice parameters were determined by profile-fittings of Cu Ka X-ray powder diffraction data. The oxygen displacement from the ideal anion coordinate of the cubic fluorite-type structure, 0.25-z, decreased with an increase of YO~.5 content, where z is the atomic coordinate of oxygen. The axial ratio c/a I also decreased with an increase of YO~.5 content. The Zro.84Yo.16Oi.92 sample, whose axial ratio c/afis equal to unity within experimental error, has clearly exhibited oxygen displacement along the c axis from the ideal site (8c) of the fluorite-type structure (t" form). The space group of the t" form was finally assigned to be P42/nmc after the examination of various space groups which are subgroups of Fm3m and supergroups of P42/nmc. The coexistence of the t' and t" forms in the Zro.86Yo.~40~.93 sample suggests the existence of an energy barrier between them and that the high-temperature cubic phase transforms into the t" form and then a part of the t'" form transforms into the t' form.

The Raman spectra of hydrous‐zirconia fine particles produced by the hydrolysis of various ZrOCl2 solutions were investigated. The Raman spectra of hydrous zirconia synthesized at HCl concentrations below 1 mol/L were similar to those of monoclinic, crystalline ZrO2; those of hydrous zirconia synthesized at HCl concentrations greater than 1 mol/L showed a crystal structure change. The line width of the Raman bands increased with increasing H+ ion concentration. Analyzing the relationship between Raman band width and particle size revealed that the primary particle size of hydrous zirconia was controlled by the H+ and Cl− ions, because these ions interfered with the polymerization in a hydrolysis reaction. Based on the experimental results, the formation mechanism for primary particles of hydrous zirconia was determined.