The microscopic structure of the oxygen–aluminium hole center in natural and neutron irradiated blue topaz

The dominant O⁻ hole center in natural and neutron irradiated blue topaz with general formula Al₂SiO₄(F,OH)₂ has been investigated with electron paramagnetic resonance (EPR) and optical absorption. The analysis of the EPR spectra of the O⁻ center yielded superhyperfine interactions with two equivalent Al neighbors in four magnetically inequivalent positions. From the g-factors it is found that the O⁻ hole center is located in (8d) Wyckoff positions on fluoride sites that were substituted by hydroxyl molecules prior to the irradiation. The spin Hamiltonian parameters were successfully interpreted with the model of transferred hyperfine interaction by exchange polarization supporting the localization of O⁻ in (F,OH)⁻ sites. The O⁻ center has the same thermal stability as the absorption band centered at 620 nm, which is responsible for the blue color. The intensity of the absorption band correlates linearly with the O⁻ concentration. Its correlation with the blue color is discussed.