Thermal Restructuring of F+2:S- and F+2:O- Centers in NaCl Crystals between 205 and 220 K

Single crystals of NaCl containing Na 2 S or NaOH exhibited an absorption band at 1.18 eV under illumination of light absorbed by the F band at room temperature after additive coloration. Further illumination at 80 K changed the peak position from 1.18 eV to 1.14 eV. The bands arise from both F 2 + :S - and F 2 + :O - centers, in which the F 2 + center is loosely bound by a S - or O - ion (1.18 eV:type A) or most compactly bound (1.14 eV:type B). Keeping the sample at a desired temperature between 205 and 220 K resulted in recovery of the peak position to 1.18 eV. Energy for recovery was estimated to be 0.50±0.01 eV and 0.54±0.02 eV for F 2 + :S - and F 2 + :O - centers, respectively. Comparison of the energies with that for rotation of the dipole axis of the F + +O - pair, or for migration of an anion vacancy suggests that 60 degree rotation of the F 2 + axis in the F 2 + :S - or F 2 + :O - center causes restructuring of the center from type B to A.