Electron-trapped centers in x-irradiated LiCl:Fe crystals.

Abstract

Single crystals of LiCl containing 140 ppm ${\mathrm{Fe}}^{2+}$ exhibit after x irradiation at 77 K ESR signals attributed to ${\mathrm{Fe}}^{+}$ centers. Designated ${\mathrm{Fe}}^{+}$${\mathrm{v}}_{\mathrm{c}}$(NN), ${\mathrm{Fe}}^{+}$${\mathrm{v}}_{\mathrm{c}}$(NNN), and ${\mathrm{Fe}}^{+}$, they are identified with respectively a substitutional ${\mathrm{Fe}}^{+}$ ion perturbed by a nearest- and a next-nearest-neighbor cation vacancy and an isolated ${\mathrm{Fe}}^{+}$ ion without any near defect. Simultaneous thermal decay of the first two types, present in a 5:1 ratio, occurs between 120 and 150 K as the perturbing vacancy moves away. A corresponding increase in the concentration of ${\mathrm{Fe}}^{+}$ centers with the perturbing vacancy located farther away, and finally of unperturbed ${\mathrm{Fe}}^{+}$, is observed. An activation energy of 0.46 eV for the free movement of the cation vacancy in LiCl is estimated. The crystal-field theory of Tinkham and the point-charge approximation is successfully applied to the noncubic ${\mathrm{Fe}}^{+}$ centers in LiCl:Fe, NaCl:Fe, and NaF:Fe. The presence of other ESR lines, tentatively attributed to ${\mathrm{Fe}}^{+}$ ions perturbed by near silicon ions, incorporated in the lattice during the crystal growth, is reported. No ${\mathrm{Fe}}^{+}$ centers were observed in KCl:Fe and RbCl:Fe crystals.