Thermal Expansion and Athermal Phase Transition of Y₄AI₂O₉ Ceramics

Thermal expansion of Y₄A1₄O₉ ceramics prepared at 1600° and 1800°C was measured from room temperature to 1500°C in air. Volume changes at the phase transition of Y₄A1₂O₉, along with thermal hysteresis, were observed around 1400°C. The volume of the high‐temperature phase was about 0.5% lower than that of the low‐temperature phase. The hysteresis width for the sample prepared at 1600°C was 56°C, wider than that (14°C) for the sample prepared at 1800°C. The averages of phase transition start temperatures on heating and on cooling for these samples were, however, almost the same at 1377°C. The phase transitions did not occur at fixed temperatures, and the proportions of the high‐temperature phase and the low‐temperature phase did not change with time as long as the temperature remained constant (athermal character). The sample prepared at 1800°C also showed another thermal hysteresis behavior from room temperature to about 1000°C.