PHASE-TRANSITION OF RARE-EARTH ALUMINATES (RE(4)AL(2)O(9)) AND RARE-EARTH GALLATES (RE(4)GA(2)O(9))

Lattice parameters of RE(4)Al(2)O(9) (RE = Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) prepared at 1600-1800 degrees C and those of RE(4)Ga(2 )O(9) (RE = La, Pr, Nd, Sm, Eu, and Gd) prepared at 1400-1600 degrees C were refined by Rietveld analysis for the X-ray powder diffraction p atterns. The parameters increased linearly with the ionic radius of th e trivalent rare-earth elements (r(RE)). High-temperature differential calorimetry and dilatometry revealed that both RE(4)Al(2)O(9) and RE( 4)Ga(2)O(9) have reversible phase transitions with volume shrinkages o f 0.5-0.7% on heating and thermal hystereses. The transition temperatu res (T-tr) decreased from 1300 degrees C (Yb) to 1044 degrees C (Sm) f or RE(4)Al(2)O(9), except for Y4Al2O9 (T-tr = 1377 degrees C), and fro m 1417 degrees C (Gd) to 1271 degrees C (La) for RE(4)Ga(2)O(9) with i ncreasing ionic radius of the rare-earth elements. These transition te mperatures were plotted on a curve against the ionic radius ratio of A l3+ or Gd3+ and RE(3+) (r(Al,Ga)/r(RE)) except for Y4Al2O9.