Abstract
High-temperature solution calorimetry using a 2PbO · B2O3solvent at 977 K was performed forLnMO3perovskites andLn3M5O12garnets (Ln=La–Lu, Y;M=Al, Ga),α-Al2O3, andβ-Ga2O3. The following four reactions were discussed from the viewpoint of thermodynamic parameters, ΔH, ΔS, and ΔV;LnMO3=15Ln3M5O12+15Ln2O3,Ln3M5O12= 3LnMO3+M2O3, 12Ln2O3+ 12M2O3=LnMO3, and32Ln2O3+52M2O3=Ln3M5O12. The stability ofLnMO3against the disproportionation to garnet plus sesquioxide is controlled almost entirely by ΔHandPΔVbut not byTΔS. On the contrary, the stability ofLn3M5O12against disproportionation to perovskite plus sesquioxide is controlled not only by ΔHandPΔVbut also byTΔS. TheP–Tboundary betweenLn3M5O12and 3LnMO3+M2O3has a negative slope. The positive ΔSand negative ΔVfor the disproportionation are caused by an increase in coordination number and anincreasein bond distance. ΔHof perovskite formation is mainly controlled by two factors, the strengthening of the ionic bond inLn2O3with decreasing ionic radius ofLn3+and theweakeningof the ionic bond betweenLnand the distant four O atoms inLnMO3with decreasing ionic radius ofLn3+. ΔHof garnet formation is mainly controlled by two factors, the strengthening of the ionic bond inLn2O3with decreasing ionic radius ofLn3+and the deviation of the ionic radius ofLn3+from the optimum size for the garnet structure. ΔSvalues of both perovskite formation and garnet formation are deduced to be negative, which suggests thatLn2O3phases possess relatively large entropies.
Published Version
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