InterstitialTl0atoms in alkali halides: ESR study of a -orientedTl2+center

A -oriented ${\mathrm{Tl}}_2^{+}$ center is produced above 220 K by x-ray irradiation of ${\mathrm{Tl}}^{+}$-doped KCl and RbCl. A careful analysis of the $X$-band electron-spin-resonance spectra establishes that the two Tl nuclei are equivalent, which implies that the center possesses inversion symmetry. The model proposed is a ${\mathrm{Tl}}_2^{+}$ center on a single cation site. Its production is believed to involve the trapping of a mobile anion vacancy between a distant pair of substitutional ${\mathrm{Tl}}^{+}$ impurities and the trapping of an electron in whatever order, an asymmetric relaxation of a ${\mathrm{Tl}}_2^{+}$ center towards a cation site and the diffusing away of a divacancy. In a formal sense this ${\mathrm{Tl}}_2^{+}$ center may be viewed as being an interstitial ${\mathrm{Tl}}^0$ atom trapped by a substitutional ${\mathrm{Tl}}^{+}$ impurity, although no mobile interstitial ${\mathrm{Tl}}^0$ as such is produced by the x-ray irradiation. Its structure is formally analogous to the interstitial halogen atom center, the $H$ center. The possible effect of the presence of -oriented ${\mathrm{Tl}}_2^{+}$ on the lasing properties of ${\mathrm{Tl}}^0$(1) centers is briefly discussed.