A calorimetric study of the lanthanide aluminum oxides and the lanthanide gallium oxides: Stability of the perovskites and the garnets

High-temperature solution calorimetry using a 2PbO . B2O3 solvent at 977 K was performed for LnMO(3) perovskites and Ln(3)M(5)O(12) garnets (Ln = La-Lu, Y; M = Al, Ga), alpha-Al2O3, and beta-Ga2O3. The following four reactions were discussed from the viewpoint of thermodynamic parameters, Delta H, Delta S, and Delta V; LnMO(3) = 1/5Ln(3)M(5)O(12) + 1/5Ln(2)O(3), Ln(3)M(5)O(12) = 3LnMO(3) + M2O3, 1/2Ln(2)O(3) + 1/2M(2)O(3) = LnMO(3), and (3)(2)Ln(2)O(3) + 5/2M(2)O(3) = Ln(3)M(5)O(12) The stability of LnMO(3) against the disproportionation to garnet plus sesquioxide is controlled almost entirely by Delta H and P Delta V but not by T Delta S. On the contrary, the stability of Ln(3)M(5)O(12) against disproportionation to perovskite plus sesquioxide is controlled not only by Delta H and P Delta V but also by T Delta S, The P-T boundary between Ln(3)M(5)O(12) and 3LnMO(3) + M2O3 has a negative slope. The positive ILS and negative Delta V for the disproportionation are caused by an increase in coordination number and an increase in bond distance. Delta H of perovskite formation is mainly controlled by two factors, the strengthening of the ionic bond in Ln(2)O(3) with decreasing ionic radius of Ln(3+) and the weakening of the ionic bond between Ln and the distant four O atoms in LnMO(3) with decreasing ionic radius of Ln(3+). Delta H Of garnet formation is mainly controlled by two factors, the strengthening of the ionic bond in Ln(2)O(3) with decreasing ionic radius of Ln(3+) and the deviation of the ionic radius of Ln(3+) from the optimum size for the garnet structure. Delta S values of both perovskite formation and garnet formation are deduced to be negative, which suggests that Ln(2)O(3) phases possess relatively large entropies. (C) 1998 Academic Press.