Fluoride Borates with [(BO3)F]4– ↔ [F4]4– Anionic Isomorphism and X-ray Sensitivity

Abstract

Crystals of Ba7(BO3)4–yF2+3y and Ba4–xSr3+x(BO3)4–yF2+3y solid solutions display a phenomenon that has never been described for borates before—X-ray irradiation induces strong absorption in the visible range and, as the result, coloring of the crystals up to dark purple. The induced absorption can be removed by illumination with a wavelength of 300–400 nm. Owing to this phenomenon, these crystals are promising sensors applicable to fixing X-ray images. With the use of electron spin resonance and optical spectroscopy, it has been found that the formation of color centers is connected with the [(BO3)F]4– and [F4]4– tetrahedral groups, which are the distinguishing feature of Ba7(BO3)4–yF2+3y and Ba4–xSr3+x(BO3)4–yF2+3y structure. Partial replacement of Ba2+ to Sr2+ in the crystal lattice of Ba4–xSr3+x(BO3)4–yF2+3y leads to the formation of the stable electron–hole pairs under X-ray irradiation. The opportunity to vary the cationic and anionic sublattice composition within the same crystal structure becomes a...