Electron-spin-resonance study of Tl0 centers of the laser-active type structure in SrCl2.

Abstract

The electron-spin-resonance spectra and production properties of two 〈111〉-oriented ${\mathrm{Tl}}^{0}$ centers, called ${\mathrm{Tl}}_{\mathit{c}}^{0}$(1) and ${\mathrm{Tl}}_{\mathit{i}}^{0}$(1), have been carefully analyzed in x-ray-irradiated ${\mathrm{SrCl}}_{2}$:TlCl crystals. It is shown that these two centers possess essentially the laser-active ${\mathrm{Tl}}^{0}$(1)-type structure observed in the alkali-metal halides: The structural core is in both cases a ${\mathrm{Tl}}^{0}$ atom perturbed by a nearest-neighbor anion vacancy. In the ${\mathrm{Tl}}_{\mathit{c}}^{0}$(1) center, directly produced by x-ray irradiation at liquid-nitrogen temperature, the ${\mathrm{Tl}}^{0}$ occupies the cation site, whereas in ${\mathrm{Tl}}_{\mathit{i}}^{0}$(1) the ${\mathrm{Tl}}^{0}$ occupies the empty interstitial site of the fluorite lattice and is associated with a divacancy in a linear configuration along 〈111〉. Apart from the study of the production properties, the analysis of the isotropic part of the hyperfine interaction has proved to be very valuable in identifying the microscopic defect structures and to differentiate between the models for the two defects. Our measurements establish that at least part of the substitutional ${\mathrm{Tl}}^{+}$ dopant is charge compensated by an adjoining anion vacancy along 〈111〉. It is also found that in ${\mathrm{SrCl}}_{2}$ the anion vacancy becomes mobile above 140 K, and the thermal conversion of ${\mathrm{Tl}}_{\mathit{c}}^{0}$(1) to ${\mathrm{Tl}}_{\mathit{i}}^{0}$(1), which involves site switching from the cation site to the empty interstitial site, may be helped by the incipient mobility of the anion vacancy.