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Synthetic stishovite before and after electron- and γ-ray irradiation has been investigated by singlecrystal electron paramagnetic resonance (EPR) spectroscopy for the first time. Room-temperature single-crystal EPR spectra of as-is stishovite reveal two high-spin (S = 3/2) Cr 3+  centers: one with D 2h  symmetry and another of triclinic symmetry. Quantitatively determined spin Hamiltonian parameters, including matrices g, D, and A( 53 Cr) and high-spin Zeeman term BS 3 , suggest that the D 2h  center represents a substitutional Cr 3+  ion at the Si site without an immediate charge compensator. The triclinic center, which is characterized by a well-resolved  1 H superhyperfine structure, also arises from a substitutional Cr 3+  ion at the Si site but has an H +  charge compensator bonded to one of the four equatorial oxygen atoms. The magnitude and orientation of the  1 H superhyperfine structure yield the location of the H atom at (0.46, 0.12, 0). These structural models for Cr and H in stishovite also have been corroborated by periodic density functional theory (DFT) calculations using the Vienna ab initio simulation package (VASP), with 2 × 2 × 4 supercells, plane-wave basis sets and the projector augmented wave (PAW) potentials. In addition, 85 K EPR spectra of irradiated stishovite show that the two Cr 3+  centers are both converted to an S = 1/2 Cr 5+  center characterized by two  29 Si superhyperfine structures arising from interactions with two nearest and eight second-nearest Si atoms, respectively. The spin Hamiltonian parameters of this Cr 5+  center provide further support for the location of the two Cr 3+  centers at the Si site.

Mechanisms of Cr and H incorporation in stishovite determined by single-crystal EPR spectroscopy and DFT calculations