Thermodynamics of glass formation and metastable solidification of molten Y₃Al₅O₁₂

Drop solution calorimetry was used to determine the enthalpy of solution in lead borate for three different materials with the chemical composition Y₃Al₅O₁₂. Crystalline yttrium aluminum garnet (YAG), a crystalline mixture of 3 YAlO₃ (perovskite) plus Al₂O₃ (α-alumina), and a glass, were all synthesized by containerless melting and cooling. The enthalpies of drop-dissolution per mole of Y₃Al₅O₁₂ were 472.49 ± 4.19, 440.35 ± 3.59, and 196.02 ± 3.39 kJ/mol, respectively, for the garnet, the crystalline mixture, and the glass. The garnet phase is thermodynamically stable with respect to the corresponding mixture of α-alumina and perovskite confirming that the two phase mixture is metastable. The enthalpy of vitrification of YAG is 276.47 ± 5.40 kJ/mol (13.82 ± 0.27 kJ/g atom). This high value for the enthalpy of vitrification is related to the reluctant glass forming ability of the YAG composition. The heat of fusion of Y₃Al₂O₁₂ is estimated to be 25.8 kJ/g atom or 516 kJ/mol at its melting point, 2240 K.