Site-switchedTl0atoms inTl+-doped NaCl and KCl

The properties of two new ${\mathrm{Tl}}^0$-atom defects with orthorhombic symmetry ([11¯0], [001], [110]) and which occur together with the well-known ${\mathrm{Tl}}^0$(1) center are studied in KCl:TlCl and NaCl:TlCl using electron-spin-resonance and optical-absorption techniques. On the basis of their production and their thermal and optical properties and taking into account the results of the hyperfine-interaction analysis it is concluded that these centers consist essentially of a ${\mathrm{Tl}}^0$ atom on an anion site (possibly perturbed by one or more cation vacancies), and that their production involves a site switching of the ${\mathrm{Tl}}^0$ atom in the ${\mathrm{Tl}}^0$(1) center. A Jahn-Teller distortion determines the symmetry of these defects and yields a ground-state p orbital oriented along a 〈110〉 direction. Similar site-switched models were proposed earlier for orthorhombic ${\mathrm{In}}^0$ and axial ${\mathrm{Ga}}^0$ centers in KCl, and it is shown that these centers possess many features in common with the orthorhombic ${\mathrm{Tl}}^0$ centers. One of the latter centers is optically very stable and forms the dominant defect in NaCl:TlCl while it is not produced in KCl. Optical-absorption bands at 430 and 560 nm are attributed to this center.