Phonon density of states of Fe2O3across high-pressure structural and electronic transitions

Abstract

High-pressure phonon density of states (PDOS) of Fe${}_{2}$O${}_{3}$ across structural and electronic transitions has been investigated by nuclear resonant inelastic x-ray scattering (NRIXS) and first-principles calculations together with synchrotron M\ossbauer, x-ray diffraction, and x-ray emission spectroscopies. Drastic changes in elastic, thermodynamic, and vibrational properties of Fe${}_{2}$O${}_{3}$ occur across the Rh${}_{2}$O${}_{3}$(II)-type structural transition at 40--50 GPa, whereas the Mott insulator-metal transition occurring after the structural transition only causes nominal changes in the properties of the Fe${}_{2}$O${}_{3}$. The observed anomalous mode-softening behavior of the elastic constants is associated with the structural transition at 40--50 GPa, leading to substantial changes in the Debye-like part of the PDOS in the terahertz acoustic phonons. Our experimental and theoretical studies provide new insights into the effects of the structural and electronic transitions in the transition-metal oxide (TMO) compounds.