ESR study of the structure and production ofSn−(5p3,S4) defects in alkali halides

Several complex ${\mathrm{Sn}}^{-}$ defects are produced in KCl: ${\mathrm{Sn}}^{2+}$ by x irradiation, followed by suitable thermal and optical treatments. The ${\mathrm{Sn}}^{-}$($5p^3$,${}_{}{}^4{}_{}{}^{}S$) ions which occupy anion sites are identified by their characteristic $S=\frac{3}{2}$ ESR spectra exhibiting dominant zero-field splittings. In the ${\mathrm{Sn}}^{-}$ (cubic) center the ${\mathrm{Sn}}^{-}$ occupies an unperturbed anion site. The ${\mathrm{Sn}}^{-}$(tetrag) defect has tetragonal symmetry around $〈100〉$, and the ${\mathrm{Sn}}^{-}$ is associated with either a cation vacancy or a cation-anion divacancy along $〈100〉$. The z axes of the orthorombic defects ${\mathrm{Sn}}^{-}$(ortho, 1) and ${\mathrm{Sn}}^{-}$(ortho, 2) make small angles, 2.1° and 1.4°, respectively, with the $〈110〉$ directions in the {100} planes. Each of these two defects must be associated with at least two vacancies, inducing a dominant tetragonal field component nearly along $〈110〉$. Production, pulse-anneal, and selective optical $F$- and $V_K$-center bleaching experiments are performed in order to try to establish the production mechanism of these complex ${\mathrm{Sn}}^{-}$ defects, in particular the change to a negative-ion position of the tin. These experiments indicate that the production proceeds via the high-temperature ${\mathrm{Sn}}^{+}$ defects and via a defect consisting of a ${\mathrm{Sn}}^0$ atom occupying an anion-cation vacancy pair. Subsequent trapping of an electron and of vacancies results in the formation of the complex ${\mathrm{Sn}}^{-}$ defects.