Origin of stability of the high-temperature, low-pressure Rh2O3 III form of rhodium sesquioxide

We present a first principles study of the equilibrium structures and relative thermodynamic stability of the three observed polymorphs of rhodium(III) sesquioxide. The thermodynamic Gibbs free energies for each phase are calculated as a function of P and T based on the electronic total energy, as well as vibrational energy and vibrational entropy contributions in the local harmonic (LH) approximation. The results confirm that Rh₂O₃ I is a low-temperature, low-pressure form and Rh₂O₃ II is a high-pressure form. A breakdown in the LH approximation at high T is then discussed and to address this breakdown an empirically corrected local harmonic (ECLH) approximation is introduced. ECLH demonstrates that the high-temperature, low-pressure form Rh₂O₃ III is entropically stabilized and produces a partitioning of phase space that is consistent with published experimental investigations.