OXYGEN-INDUCED STRUCTURAL-CHANGE OF THE TETRAGONAL PHASE AROUND THE TETRAGONAL-CUBIC PHASE-BOUNDARY IN ZRO2-YO1.5 SOLID-SOLUTIONS

In the ZrO2-YO1.5 solid solutions, the phase changes among the cubic p hase (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 d efined as tetragonal phases (P4(2)/nmc, Z = 2) with axial ratios of c/ a(f) = 1 and c/a(f) > 1, respectively, which were prepared by a diffus ionless transition from the high-temperature cubic phase during quench ing. a(f) is the lattice parameter of the pseudo-fluorite cell. The cr ystal structure of the tetragonal phase of Zr1-XYXO2-X/2 [= (ZRO(2))(1 -X)(YO1.5)(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 colle cted at 293 K [lambda = 1.5301 (3) or 1.5314 (2) Angstrom]. The crysta l structure of Zr1-XYXO2-X/2 (X = 0.18 and 0.20) has also been refined assuming either tetragonal (P4(2)/nmc, Z = 2) or cubic symmetry [Fm3m , Z = 4] by the Rietveld analysis of neutron powder diffraction data c ollected at 293 K [lambda = 1.5301 (3) Angstrom]. The lattice paramete rs were determined by profile-fittings of Cu K alpha X-ray powder diff raction data. The oxygen displacement from the ideal anion coordinate of the cubic fluorite-type structure, 0.25 - z, decreased with an incr ease of YO1.5 content, where z is the atomic coordinate of oxygen. The axial ratio c/a(f) also decreased with an increase of YO1.5 content. The Zr0.84Y0.16O1.92 sample, whose axial ratio c/a(f) is equal to unit y within experimental error, has clearly exhibited oxygen displacement along the c axis from the ideal site (8c) of the fluorite-type struct ure (t'' form). The space group of the t'' form was finally assigned t o be P4(2)/nmc after the examination of various space groups which are subgroups of Fm3m and supergroups of P4(2)/nmc. The coexistence of th e t' and t'' forms in the Zr0.86Y0.14O1.93 Sample suggests the existen ce of an energy barrier between them and that the high-temperature cub ic phase transforms into the t'' form and then a part of the t'' form transforms into the t' form.