CU+ AND AG+ CENTERS IN ALKALI HALIDES

The displacements of Ag+ and Cu+ ions in various alkali halides are calculated using a polarizable pointion model. It is found that Ag+ in RbCl lies about 0.54 along a <111> direction, and that Ag+ in NaCl, KCl, and RbBr is on site, in agreement with experiment. Cu+ ions are found to be off center in NaCl, KCl, RbCl, and RbBr, also in <111> directions, by 0.2, 1.36, 1.65, and 1.84 respectively. Using a model of a free Cu+ ion in a crystalline electric field, the oscillator strength of the (3d)10→(3d)94s 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 2×108 V/cm. This result suggests displacements in the <111> directions about 50% bigger than those given above for the point-lattice model. It turns out also that for Cu+ in NaCl, oscillator-strength measurements alone are insufficient to decide whether the ion is on site. © 1969 The American Physical Society.