Abstract

Synthetic stishovite before and after electron- and γ-ray irradiation has been investigated by single-crystal 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) Cr3+ centers: one with D2h symmetry and another of triclinic symmetry. Quantitatively determined spin Hamiltonian parameters, including matrices g, D, and A(53Cr) and high-spin Zeeman term BS3, suggest that the D2h center represents a substitutional Cr3+ ion at the Si site without an immediate charge compensator. The triclinic center, which is characterized by a well-resolved 1H superhyperfine structure, also arises from a substitutional Cr3+ 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 1H 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 Cr3+ centers are both converted to an S = 1/2 Cr5+ center characterized by two 29Si superhyperfine structures arising from interactions with two nearest and eight second-nearest Si atoms, respectively. The spin Hamiltonian parameters of this Cr5+ center provide further support for the location of the two Cr3+ centers at the Si site.

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