Electron-spin-resonance study of Tl atom defects in KCl and relativistic many-body analysis of the hyperfine structure

Two thallium atom defects of tetragonal symmetry around $〈100〉$ are produced by x irradiation above 230 K in KCl: TlCl. Their electron-spin-resonance spectra are characterized by comparable and large g shifts but quite different hyperfine parameters. A relativistic many-body calculation of the Tl⁰ atom hyperfine interaction including the effect of a tetragonal crystal field permits a quantitative analysis of the spin-Hamiltonian parameters. It is found in particular that the hyperfine data of the two Tl⁰ defects can be explained by assuming the presence of one or two perturbing positive charges, respectively, and a corresponding 35 and 45% delocalization of the 6p¹ electron on the surrounding lattice. This analysis together with x-ray production, optical-excitation, and pulse-anneal data and the fact that both Tl⁰ defects can only be produced at temperatures where negative-ion vacancies are mobile (>230 K) allows one to propose precise defect structures: In one defect the Tl⁰ atom (on a positive-ion site) is associated with a single negative-ion vacancy in a nearest-neighbor position along $〈100〉$; in the other one, the Tl⁰ is flanked by two nearest-neighbor negative-ion vacancies along $〈100〉$. The data also provide strong evidence for the existence of a defect consisting of a Tl⁺ ion and a nearest-neighbor anion vacancy.