Interband Electronic Structure of α‐Alumina up to 2167 K

Abstract

The electronic structure and interband transitions of α‐Al2O3 have been studied using temperature‐dependent vacuum ultraviolet spectroscopy from 5 to 43 eV, at temperatures ranging from 293 up to 2167 K, which is approaching the melting point of 2327 K. The energy range of the spectra spans the full range of interband electronic transitions. Kramers–Krönig analysis has been employed to recover the phase information, and the interband transition strength (Jcv) of the valence to conduction band transitions. Critical point (CP) analysis of Jcv, a modeling technique based on band structure topology, is then applied to the study of temperature‐induced changes in the interband electronic structure. This approach offers newinsights into the nature of the electronic structure of α‐Al2O3—by allowing us to decompose the interband transitions into subsets associated with states of O 2p nonbonding and Al = O bonding character, and by allowing us to apply partial optical sum rules to these subsets to determine changes in their electron occupancy as a function of temperature. Up to 1700 K the temperature dependence of the electronic structure is linear, corresponding to the linear behavior of the thermal lattice expansion and the vibrational Debye–Waller factors. Below 1700 K the absorption edge shifts at −1.1 meV/K while the exciton and band gap, decomposed through CP modelling, shift at 0.93 and 0.85 meV/K with an exciton binding energy of 0.13 eV. The electron occupancy of the O 2p nonbonding CP set decreases and the occupancy of the Al = O bonding CP set increases. Above 1700 K the temperature dependence of the electronic structure is nonlinear, reflecting the interaction of electrons with phonons in the anharmonic regime, and related to the nonlinearity observed in the vibrational Debye–Waller factors. Also above 1700 K, the O 2p nonbonding and Al = O bonding CP sets merge and this is discussed in the context of temperature‐induced changes in the interatomic bonding.