Sn+centers and thermal motion of associated vacancies in KCl:SnCl2

Exposure of KCl:Sn${\mathrm{Cl}}_2$ crystals to x or $\gamma$ rays at 77 K results in the trapping of electrons by the ${\mathrm{Sn}}^{++}$ ions to form ${\mathrm{Sn}}^{+}$ centers and positive holes to form ${\mathrm{Cl}}_2^{-}$, $V_K$ centers, as shown by EPR spectra. Three distinct and well-defined ${\mathrm{Sn}}^{+}$ centers have been observed. Each consists of an approximately linear ${\mathrm{}\left(\mathrm{ClSnCl}\right)\mathrm{}}^{-}$ molecule ion with the molecular axis $z$ along . The ${\mathrm{Sn}}^{+}$ center formed upon irradiation at 77 K has orthorhombic symmetry with $x$ and $y$ along ; this $\mathrm{Sn}\mathrm{Cl}_2^{}{}_{}{}^{-}[v_{+},(\mathrm{nnn}\left)\right]$ has a positive-ion vacancy, $v_{+}$, in a nearest-neighbor (nn) position to ${\mathrm{Sn}}^{+}$ in the $xy$ plane. On warming to 190 K a second ${\mathrm{Sn}}^{+}$ center is formed which also has orthorhombic symmetry with $x$ and $y$ along ; this $\mathrm{Sn}\mathrm{Cl}_2^{}{}_{}{}^{-}[v_{+},(\mathrm{nnn}\left)\right]$ has a $v_{+}$ in a next-nearest-neighbor (nnn) position in the $xy$ plane. Near 240 K the $v_{+}$ moves out of the neighborhood producing a third ${\mathrm{Sn}}^{+}$ center, an isolated Sn${\mathrm{Cl}}_2^{-}$ with tetragonal symmetry, Sn${\mathrm{Cl}}_2^{-}$[tetrag]. The Sn${\mathrm{Cl}}_2^{-}$ molecule ion has three principal optical transitions near 200 nm which are perturbed by the $v_{+}$. Both the energies and the polarizations of these transitions depend on the location of the $v_{+}$ with respect to the Sn${\mathrm{Cl}}_2^{-}$....