Metastability of corundum-type In2O3

The description of structural relations between bixbyite- and corundum-type structures is of particular interest because of the common occurrence of both structures. One of the representative examples of the bixbyite to corundum transition is the high-pressure high-temperature synthesis of the corundum-type indium oxide. The wet chemistry synthesis and stabilisation of the corundum-type In2O3 under ambient pressure conditions calls for a re-interpretation of the In-O phase diagram as well as for the clarification of the phase transitions in In2O3. One of the questions to be clarified is the stability of the corundum-type In2O3. In the present work we studied the stability of the corundum-type In2O3 both theoretically (by density-functional calculations) and experimentally. The synthesis of the corundum-type In2O3 was performed by the modified non-alkoxide sol-gel method based on the ammonia-induced hydrolysis of indium nitrate in methanol. The corundum-type In2O3 was subjected to thermal analysis (STA) as well as to structural studies, that is, it was examined using X-ray powder diffraction (XRPD) including in situ XRPD characterisation upon thermal treatment. For the first time we have undoubtedly demonstrated, both theoretically and experimentally, the metastability of the corundum-type In2O3 polymorph. The In2O3 Polymorph appears to be metastable throughout the entire enthalpy-pressure phase diagram. Upon heating, corundum-type In2O3 transforms irreversibly into cubic bixbyite-type In2O3 as shown by STA as well as in situ heating XRPD experiments. Computations indicate the existence of another high-pressure modification of In2O3 with orthorhombic structure, iso-typic to Rh2O3-II. We predict this new phase to form at pressures exceeding 15 GPa from both the cubic bixbyite-type and the corundum-type modification of In2O3.