Electron spin resonance study of Fe+ centers in NaCl:Fe++ and motion of the associated vacancies

X-irradiation at 77 K of a NaCl:Fe++ crystal produces orthorhombic and axial Fe+ ESR spectra of comparable intensities. The corresponding Fe+ (ortho) and Fe+ (tetra) centers are identified with, respectively, a substitutional Fe+ perturbed by a nearest and a next nearest cation vacancy. At 170 K the cation vacancy of Fe+ (ortho) jumps to the next nearest neighbour position enhancing the Fe+ (tetra) intensity. Above 215 K the vacancy moves away completely and the unperturbed Fe+ (cubic) is formed. It is concluded from this study that before X-irradiation Fe++ perturbed by either nearest or next nearest cation vacancies occur in roughly comparable amounts and that the activation energy for cation vacancy motion in NaCl is 0.72 eV. (author)