Abstract
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 In~2~O~3~ under ambient pressure conditions calls for a re‐interpretation of the InO phase diagram as well as for the clarification of the phase transitions in In~2~O~3~. One of the questions to be clarified is the stability of the corundum‐type In~2~O~3~. In the present work we studied the stability of the corundum‐type In~2~O~3~ both theoretically (by density‐functional calculations) and experimentally. The synthesis of the corundum‐type In~2~O~3~ was performed by the modified non‐alkoxide sol–gel method based on the ammonia‐induced hydrolysis of indium nitrate in methanol. The corundum‐type In~2~O~3~ 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 In~2~O~3~ polymorph. The In~2~O~3~ polymorph appears to be metastable throughout the entire enthalpy–pressure phase diagram. Upon heating, corundum‐type In~2~O~3~ transforms irreversibly into cubic bixbyite‐type In~2~O~3~ as shown by STA as well as in situ heating XRPD experiments. Computations indicate the existence of another high‐pressure modification of In~2~O~3~ with orthorhombic structure, iso‐typic to Rh~2~O~3~‐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 In~2~O~3~.