Intervalence charge transfer of Ti and Fe defects in blue kyanite

The electronic and magnetic properties of Ti-Fe defects substitution for two adjacent aluminum ions in blue kyanite with various spin configurations, charges, and alignments have been investigated using first-principles calculations based on density functional theory. Based on the total-energy calculations, we find two possible ground-state spin configurations: high-spin states of Ti-IV-Fe-II and Ti-III-Fe-III with anti-ferromagnetic alignment between Ti and Fe. Among all spin states and alignments, the high-spin state of Ti-IV-Fe-II type-IV alignment is the lowest energy configuration. The optical excitation energy of the high-spin state of Ti-IV-Fe-II calculated from the highest occupied Fe state to the lowest unoccupied Ti state is 1.48 eV, lying in the infrared region of the solar spectrum. Owing to the infrared absorption of Ti-Fe defects, these defects are unlikely to be responsible for the origin of the blue color in kyanite. We, therefore, suggest that other defects or mechanisms may be responsible for the blue coloration in kyanite.