Phase Equilibrium Relationships in the System Gd₂O₃-TiO₂

The phase equilibrium relationships for a major portion of the Gd₂O₃-TiO₂ system were determined in air, from a study of solid state reactions and from fusion characteristics. Three intermediate phases, a 1:2 compound, a 1:1 compound, and a face-centered cubic solid solution occur in the system. The solid solution phase is indicated on the phase diagram as existing from 33 to 40 mole percent TiO₂, at 1750 °C. This phase melts incongruently over a range of temperatures and compositions, from 1840 °C, the peritectic temperature at 35 mole percent TiO₂, to 1775 °C at 40 mole percent TiO₂, the incongruent melting temperature of Gd₂O₃∙TiO₂. The minimum temperature of stability for the phase is 1600 °C at 38 mole percent TiO₂. The compound Gd₂O₃∙2TiO₂ has a cubic pyrochlore structure type with a = 10.181 Å and melts congruently at 1820 °C. This phase apparently accepts up to about 3 mole percent TiO₂ in solid solution at 1460 °C. The compound Gd₂O₃∙TiO₂, melts incongruently at 1775 °C and has a reversible phase transition at 1712 °G. The x-ray powder diffraction pattern of the high temperature modification was indexed on the basis of a hexagonal cell a = 3.683 Å, c= 11.995 Å and is apparently related to the A-type rare earth oxide. The compositions Sm₂O:TiO₂ and Eu₂O₃:TiO₂ gave x-ray diffraction powder patterns with marked similarity one to the other as well as to both the high and low polymorphs of Gd2O₃∙TiO2. The composition Dy₂O₃:TiO₂ formed several phases one of which is apparently related to the high temperature polymorph of Gd₂O₃ •TiO₂.