Site-switched Tl0 atoms in Tl+-doped NaCl and KCl

The properties of two new Tl0-atom defects with orthorhombic symmetry ([110], [001], [110]) and which occur together with the well-known Tl0(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 Tl0 atom on an anion site (possibly perturbed by one or more cation vacancies), and that their production involves a site switching of the Tl0 atom in the Tl0(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 In0 and axial Ga0 centers in KCl, and it is shown that these centers possess many features in common with the orthorhombic Tl0 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. © 1986 The American Physical Society