SOLID-PHASE TRANSFORMATION AND MELTING OF THE COMPOUNDS LN(4)AL(2)O(9) (LN=GD, DY, Y)

High-temperature studies of the compounds Ln(4)Al(2)O(9) (Ln = Gd, Dy, Y) were performed using both DTA and dilatometry. A solid phase trans ition is observed in the three compounds at a temperature which increa ses from 1090 degrees C for Ln = Gd to 1367 degrees C for Ln = Y. This transition is reversible with a hysteresis as large as 190 degrees C. The enthalpy of transformation is about 2 kJ mol(-1). During the heat ing, the transition is associated with a linear expansion of about 0.2 %. Gd4Al2O9 is shown to display an incongruent melting at about 1900 d egrees C. The congruent melting point of the other compounds increases from 1917 degrees C for Dy4Al2O9 to 1987 degrees C for Y4Al2O9.