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. - Graphical abstract: In situ energy-dispersive XRD patterns in multi-anvil assemblies show the sequence of phase transition c-In₂O₃→o′-In₂O₃→rh-In₂O₃ under particular pressure and temperature conditions. The tick marks refer to the calculated Bragg positions of bixbyite-type (c-In₂O₃), Rh₂O₃-II-type (o–-In2O₃) and corundum-type (rh-In₂O₃). - Highlights: • The solid-state synthesis methods can be employed for obtaining metastable phases. • The phase transition of In₂O₃ was guided by DFT calculations. • The phase transition of In₂O₃ was followed in situ under HP–HT conditions. • Orthorhombic o′-In₂O₃ polymorph was synthesized from c-In₂O₃ at 8.5 GPa/850 °C. • Metastable rh-In₂O₃ was obtained from o′-In₂O₃ at 5.5 GPa during decompression