δ-Phase to defect fluorite (order–disorder) transition in the R2O3–MO2 (R = Sc, Tm, Lu; M = Zr, Hf) systems

We have studied the δ-phase to defect fluorite F* (order–disorder) transition in the R4M3O12 (R = Sc, Tm, Lu; M = Zr, Hf) compounds. The temperature of the δ–F* phase transition in Tm4Zr3O12 is ∼1600 °C. The rate of this transition in R4Zr3O12 (R = Sc, Tm, Lu) decreases markedly with decreasing difference in ionic radius between the R3+ and Zr4+, leading to stabilization of the δ-phases R4Zr3O12 with R = Sc and Lu at high temperatures (∼1600 °C). During slow cooling (5 °C/h), the high-temperature defect fluorites F*-R2Hf2O7 (R = Tm, Lu) decompose reversibly to form the δ-phases R4Hf3O12. Some of the materials studied exhibit microdomains formation effects, typical of the fluorite-related oxide compounds in the R2O3–MO2 (M = Ti, Zr, Hf) systems of the heavy rare earths. The high-temperature defect fluorites F*-R4M3O12 (R = Tm, Lu; M = Zr, Hf) as a rule contain antiphase microdomains of δ-R4Zr3O12. After slow cooling (5 °C/h), such microdomains are large enough for the δ-phase to be detected by X-ray diffraction. The conductivity data for R4M3O12 (R = Sc, Tm, Lu; M = Zr, Hf) and Ln2Hf2O7 (Ln = Dy, Lu) prepared by different procedures show that the rhombohedral phases δ-R4M3O12 (R = Sc, Tm, Lu; M = Zr, Hf) are poorer conductors than the defect fluorites, with 740 °C conductivity from 10−6 to 10−5 S/cm. The conductivity drops with decreasing rare-earth ionic radius and, judging from the Ea values obtained (1.04–1.37 eV), is dominated by oxygen ion transport. The highest conductivity, ∼6 × 10−4 S/cm at 740 °C, is offered by the rapidly cooled F*-Dy2Hf2O7. In the fluorite homologous series, oxygen ion conductivity decreases in the order defect pyrochlore > defect fluorite > δ-phase.