Prevalence of pretransition disordering in the rutile-to- CaCl2 phase transition of GeO2

Abstract

The ability to tailor a material's electronic properties using density driven disordering has emerged as a powerful route to materials design. The observation of anomalous structural and electronic behavior in the rutile to ${\mathrm{CaCl}}_{2}$ phase transition in ${\mathrm{SnO}}_{2}$ led to the prediction that such behavior is inherent to all oxides experiencing such a phase transition sequence [Smith et al., J. Phys. Chem. Lett. 10, 5351 (2019)]. Here, the ultrawide band gap semiconductor ${\mathrm{GeO}}_{2}$ is confirmed to exhibit anomalous behavior during the rutile to ${\mathrm{CaCl}}_{2}$ phase transition. A phase pure rutile ${\mathrm{GeO}}_{2}$ sample synthesized under high-pressure, high-temperature conditions is probed using synchrotron diffraction and x-ray and optical spectroscopy under high pressure conditions. Density functional theory calculations show that the enthalpic barrier to displacing an oxygen along the ${B}_{1g}$ librational mode decreases with pressure leading up to the rutile to ${\mathrm{CaCl}}_{2}$ phase transition. The band structure of the distorted state shows that such oxygen displacements form small polarons.