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

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) Cr 3+ centers: one with D 2 h 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 2 h 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.