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

作者：Pan Yuanming^*; Mashkovtsev Rudolf I; Huang Dan; Mao Mao; Shatskiy Anton

来源：American Mineralogist, 2011, 96(8-9): 1331-1342.

DOI：10.2138/am.2011.3762

• Stishovite
• single-crystal EPR
• Cr-53 hyperfine
• Si-29 and H-1 superhyperfine
• site occupancy
• local structure
• substitution mechanism
• periodic DFT calculations

Synthetic stishovite before and after electron- and gamma-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(Cr-53) 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 H-1 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 H-1 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 x 2 x 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 Si-29 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.

• 出版日期2011-9
• 单位Saskatoon; Saskatchewan