Kinetic control in the synthesis of metastable polymorphs: Bixbyite-to-Rh2O3(II)-to-corundum transition in In2O3

An example for kinetic control of a solid-state phase transformation, in which the system evolves via the path with the lowest activation barrier rather than ending in the thermodynamically most favorable state, has been demonstrated. As a case study, the phase transitions of indium sesquioxide (In2O3) have been guided by theoretical calculations and followed in situ under high-pressure high-temperature conditions in multi-anvil assemblies. The corundum-type rh-In2O3 has been synthesized from stable bixbyite-type c-In2O3 in two steps: first generating orthorhombic Rh2O3-II-type o′-In2O3 which is thermodynamically stable at 8.5 GPa/850 °C and, thereafter, exploiting the preferred kinetics in the subsequent transformation to the rh-In2O3 during decompression. This synthesis strategy of rh-In2O3 was confirmed ex situ in a toroid-type high-pressure apparatus at 8 GPa and 1100 °C. The pressure–temperature phase diagrams have been constructed and the stability fields of In2O3 polymorphs and the crystallographic relationship between them have been discussed