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, Sin, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) prepared at 1600–1800°C and those of RE 4 Ga 2 O 9 (RE = La, Pr, Nd, Sm, Eu, and Gd) prepared at 1400–1600°C were refined by Rietveld analysis for the X‐ray powder diffraction patterns. The parameters increased linearly with the ionic radius of the trivalent rare‐earth elements ( r RE ). High‐temperature differential calorimetry and dilatometry revealed that both RE 4 Al 2 O, and RE 4 Ga 2 O, have reversible phase transitions with volume shrinkages of 0.5–0.7% on heating and thermal hystereses. The transition temperatures (7 tr ) decreased from 1300°C (Yb) to 1044°C (Sm) for RE 4 A1 2 O 9 , except for Y 4 Al 2 O 9 ( T tr = 1377°C), and from 1417°C (Gd) to 1271°C (La) for RE 4 Ga 2 O, with increasing ionic radius of the rare‐earth elements. These transition temperatures were plotted on a curve against the ionic radius ratio of Al 3+ or Gd 3+ and RE 3+ ( r A1 Ga /r RE ) except for Y 4 Al 2 O 9 .