Cu+andAg+Centers in Alkali Halides

The displacements of ${\mathrm{Ag}}^{+}$ and ${\mathrm{Cu}}^{+}$ ions in various alkali halides are calculated using a polarizable pointion model. It is found that ${\mathrm{Ag}}^{+}$ in RbCl lies about 0.54 Å along a direction, and that ${\mathrm{Ag}}^{+}$ in NaCl, KCl, and RbBr is on site, in agreement with experiment. ${\mathrm{Cu}}^{+}$ ions are found to be off center in NaCl, KCl, RbCl, and RbBr, also in directions, by 0.2, 1.36, 1.65, and 1.84 Å, respectively. Using a model of a free ${\mathrm{Cu}}^{+}$ ion in a crystalline electric field, the oscillator strength of the ${\left(3\mathrm{}\mathrm{d}\right)}^{10}\to {\mathrm{}\left(3d\right)\mathrm{}}^{9}4s$ transition is calculated using Hartree-Fock-Slater wave functions. It is found that the observed oscillator strengths of the order of 0.025 can be induced by fields of 2×${10}^8$ V/cm. This result suggests displacements in the directions about 50% bigger than those given above for the point-lattice model. It turns out also that for ${\mathrm{Cu}}^{+}$ in NaCl, oscillator-strength measurements alone are insufficient to decide whether the ion is on site.