• journal_title：American Mineralogist
• Contributor：Yuanming Pan ; Rudolf I Mashkovtsev ; Dan Huang ; Mao Mao ; Anton Shatskiy
• Publisher：Mineralogical Society of America
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2011.3762
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：96
• issue：8-9
• firstpage：1331
• section：Articles

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 D_2h 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 D_2h 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.