High-Temperature Phase Transition of Cuspidine-Type Rare-Earth Aluminates

Abstract

Recent studies on the high-temperature martencitic phase transition of cuspidine-type rare-earth aluminates (RE₄Al₂O₉, RE: rare-earth elements) were reviewed. Calorimetry, dilatometry, thermal gravimetry and crystal structure analyses of RE₄Al₂O₉(RE=Y) revealed a reversible phase transformation at around 1370°C with thermal hysteresis. The cell volume of the high-temperature phase is approximately 0.4% smaller than that of the low-temperature phase. The high-tempera-ture phase crystallizes in a monoclinic cell, a space group P 2₁/c. This is identical to the space group of the low temperature phase. Phase transition temperatures of cuspidine-type rare-earth aluminates (RE₄Al₂O₉, RE=Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) and gallates (RE₄Ga₂O₉, RE=La, Pr, Nd, Sm, Eu, Gd) decreased from 1300°C (Yb) to 1044°C (Sm) for the aluminates and 1417°C (Gd) to 1271°C (La) for the gallates with increasing ionic radius of the rare-earth elements.