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.

Calculations of the Gibbs free energies for three phases of Rh sesquioxide, including estimates of the vibrational energy and vibrational entropy contributions, show that this Rh2O3 III phase is entropically stabilized.Figure optionsDownload as PowerPoint slide