Single-crystal EPR study of three radiation-induced defects (Al–O2 3?, Ti3+ and W5+) in stishovite

Single-crystal electron paramagnetic resonance spectra of electron-irradiated stishovite, measured at temperatures from 3.5 to 294?K, reveal three S?=?1/2 radiation-induced defects: an aluminum-associated oxygen hole center and two nd ¹ centers (Ti³⁺ and W⁵⁺). The aluminum-associated oxygen hole center, characterized by an orthorhombic site symmetry, coaxial matrices of the electronic Zeeman g, nuclear hyperfine A(²⁷Al) and nuclear quadrupole P(²⁷Al), and the orientation of the g-minimum axis along an O–O direction and those of the unique A(²⁷Al) and P(²⁷Al) axes perpendicular to the O–O direction, is an Al–O₂ ^3? center, with the unpaired electron equally distributed on two equatorial oxygen atoms of a substitutional Al³⁺ ion at the octahedral Si site. Fully optimized Al-doped structure, theoretical ²⁷Al nuclear hyperfine and quadrupole coupling constants and directions, and 3D spin densities from periodic hybrid density functional theory calculations provide further support for this structural model. Spin Hamiltonian parameters of the Ti³⁺ and W⁵⁺ centers, which are confirmed by their diagnostic ⁴⁷Ti, ⁴⁹Ti and ¹⁸³W hyperfine structures, arise from electron trapping on substitutional Ti⁴⁺ and W⁶⁺ ions at the octahedral Si site.     

Theoretical modeling of the Al paramagnetic center and its precursors in stishovite

Previous electron paramagnetic resonance (EPR) spectroscopic study of gamma-ray-irradiated stishovite at 77 K detected an Al hole center, which was proposed to be an [O₂ ³⁻–Al³⁺] defect. First-principles quantum-mechanical calculations show that the unpaired spin is 85% localized on one of the six oxygen atoms at an AlO₆ octahedron, while the calculated ²⁷Al hyperfine constants are similar to those determined by EPR experiments. Theoretical results allow us to propose the Al center to represent an [AlO₆]⁰ defect, and hole hoping among equivalent oxygen atoms is responsible for its detection only at cryogenic temperatures. Theoretical calculations also show that the diamagnetic precursors [AlO₆/H⁺]⁰, [AlO₆/Li⁺]⁰, and [AlO₆/Na⁺]⁰ are stable in stishovite. The calculated OH bond distance and orientation are in excellent agreement with those inferred from FTIR spectra and previous theoretical calculations. The calculated [AlO₆/Li⁺]⁰ and [AlO₆/Na⁺]⁰ defects suggest that the monovalent cations such as Li⁺ and Na⁺ are potentially important in accommodating Al in stishovite in the lower mantle.     

Ab initio calculations on the O<Subscript>2</Subscript><Superscript>3−</Superscript>-Y<Superscript>3+</Superscript> center in CaF<Subscript>2</Subscript> and SrF<Subscript>2</Subscript>: its electronic structure and hyperfine constants

The O₂ ^3?-Y³⁺ center in fluorite-type structures (CaF₂ and SrF₂) has been investigated at the density functional theory (DFT) level using the CRYSTAL06 code. Our calculations were performed by means of the hybrid B3PW method in which the Hartree–Fock exchange is mixed with the DFT exchange functional, using Becke’s three parameter method, combined with the non-local correlation functionals by Perdew and Wang. Our calculations confirm the stability and the molecular character of the O₂ ^3?-Y³⁺ center. The unpaired electron is shown to be almost exclusively localized on and equally distributed between the two oxygen atoms that are separated by a bond distance of 2.47 Å in CaF₂ and 2.57 Å in SrF₂. The calculated ¹⁷O and ¹⁹F hyperfine constants for of the O₂ ^3?-Y³⁺ center are in good agreement with their corresponding experimental values reported by previous electron paramagnetic resonance and electron nuclear double resonance studies, while discrepancies are notable for the ⁸⁹Y hyperfine constants.     

Applications of Natural Radiation-Induced Paramagnetic Defects in Quartz to Exploration in Sedimentary Basins

INTRODUCTION Point defects in quartz have attracted intensiveand continuing research since the 1950s ( Weil ,2000 ,1984 ; Marfunin, 1979 ; Weeks , 1956 ; McClellandand Donoghue ,1953) because they are key factors inenhancing ( or reducing) device performance andquality of this i mportant piezoelectric and optical ma-terial (Beall ,1994) .Point defects in quartz are usu-ally dilute in concentration and,therefore ,are ame-nable to study by only a fewstructural techniques ,ofwhich electron…