Solid phase transformation and metlting of the compounds Ln4Al2O9 (Ln Gd, Dy, Y)

High-temperature studies of the compounds Ln4Al2O9 (Ln Gd, Dy, Y) were performed using both DTA and dilatometry. A solid phase transition is observed in the three compounds at a temperature which increases from 1090°C for Ln Gd to 1367°C for Ln Y. This transition is reversible with a hysteresis as large as 190°C. The enthalpy of transformation is about 2 kJ mol−1. During the heating, the transition is associated with a linear expansion of about 0.2%. Gd4Al2O9 is shown to display an incongruent melting at about 1900°C. The congruent melting point of the other compounds increases from 1917°C for Dy4Al2O9 to 1987°C for Y4Al2O9.