Single-crystal EPR study of three radiation-induced defects (Al-O ₂ ^3-, Ti ³⁺ and W ⁵⁺) 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 (Ti ³⁺ and W ⁵⁺). 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-O ₂ ^3- center, with the unpaired electron equally distributed on two equatorial oxygen atoms of a substitutional Al ³⁺ 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 Ti ³⁺ and W ⁵⁺ centers, which are confirmed by their diagnostic ⁴⁷Ti, ⁴⁹Ti and ¹⁸³W hyperfine structures, arise from electron trapping on substitutional Ti ⁴⁺ and W ⁶⁺ ions at the octahedral Si site.