Thermodynamic study of orthorhombic T^x and tetragonal T′ lanthanum cuprate, La₂CuO₄

The enthalpies of transition among the T^x, T′, and T-La₂CuO₄ phases are obtained from a combination of differential scanning calorimetry, high temperature oxide melt solution calorimetry, and transposed temperature drop calorimetry. The enthalpy of transformation of T^x to T is 2.32 ± 0.07 kJ/mol and the corresponding entropy of transition is 4.38 ± 0.13 J/(mol K). The T′ modification, with an average of 1.40 kJ/mol, is less stable in enthalpy than T^x but at 0.96 kJ/mol, more stable in enthalpy than T. Although we cannot rule out a small stability field at temperatures near the T^x-T transition at 530 K, T′ is most likely metastable at all temperatures.