Difference in Structure and Electronic Properties of Oxygen Vacancies in alpha-Quartz and alpha-Cristobalite Phases of SiO2

alpha-cristobalite (alpha-C) is a polymorph of silica, mainly found in space exploration and geochemistry research. Due to similar densities, alpha-C is often used as a proxy for amorphous SiO2, particularly in computer simulations of SiO2 surfaces and interfaces. However, little is known about the properties of alpha-C and its basic oxygen defects. Using density functional theory (DFT) simulations we provide a comprehensive report on the properties of perfect structure and oxygen vacancies in alpha-C. The calculated properties of alpha-C are compared with those of the better-characterized alpha-quartz (alpha-Q). Our results demonstrated that the positively charged O vacancy in alpha-C is most stable in the dimer configuration, in contrast to alpha-Q, which favors the puckered configuration. A back-projected configuration was also predicted in both polymorphs. We calculated the optical transition energies and isotropic hyperfine constants for O vacancies in both alpha-Q and alpha-C, and compared our findings with the results of previous studies and experiments. This work, thus, offers one of the first in-depth investigations of the properties of oxygen vacancies in alpha-C.