Theoretical modeling of the Al paramagnetic center and its precursors in stishovite

Abstract

Previous electron paramagnetic resonance (EPR) spectroscopic study of gamma-ray-irradiated stishovite at 77 K detected an Al hole center, which was proposed to be an [O2 3−–Al3+] defect. First-principles quantum-mechanical calculations show that the unpaired spin is 85% localized on one of the six oxygen atoms at an AlO6 octahedron, while the calculated 27Al hyperfine constants are similar to those determined by EPR experiments. Theoretical results allow us to propose the Al center to represent an [AlO6]0 defect, and hole hoping among equivalent oxygen atoms is responsible for its detection only at cryogenic temperatures. Theoretical calculations also show that the diamagnetic precursors [AlO6/H+]0, [AlO6/Li+]0, and [AlO6/Na+]0 are stable in stishovite. The calculated OH bond distance and orientation are in excellent agreement with those inferred from FTIR spectra and previous theoretical calculations. The calculated [AlO6/Li+]0 and [AlO6/Na+]0 defects suggest that the monovalent cations such as Li+ and Na+ are potentially important in accommodating Al in stishovite in the lower mantle.