On the Criterions for F_A (II) Center Formation in Alkali Halide Crystals

Analysis of a qualitative criterion for forecasting off-centre displacement of impurity ions in ionic crystals

“…The system marked with a question mark was not explicitly checked, at that time [10], for on-centre behaviour which, however, was thought unquestionably true by the authors quoted in ref. [10].…”

On the possibility of a forecast for off-centre configuration of Ag+ and Cu+ in alkali halide crystals

“…It has been evidenced that the critical value of the ratio r*/r+ between impurity and host ion radius which allows off-centre configuration is dependent on the impurity ion mass. This model is able to give also prediction for FA(II) centre formation in alkali halides [5].The aim of this paper is to present an effort to verify the validity of our criteria for heavy monovalent isoelectronic impurity ions (Ag § Cu § which have been the most extensively investigated in the literature. In table I we present the data obtained for Ag § impurity.…”

On the Use of the Rabin and Klick Diagram

“…According to the Sanderson model the atoms contain partial charges whose values (k 6) can be calculated readily by using a new scale of electronegativities developed by Sanderson himself. Sanderson developed a simple empirical formula to quantify the variation of the ionic radius r with the partial charge,…”

“…The quantity S / D obviously is a measure of the space available for a halide atom to be inserted between two halide ions along a face diagonal: S is given by (a/@) -d, where a is the lattice constant and d ( = 2 r _ ) is the halide ion diameter (i.e. twice the anion radius r -) .Townsend Recently, we adopted 151 in color center as well as in defect physics the Sanderson model for non-molecular structures which utilizes a univocal scheme in determining the ionic radii; we obtained a forecast for off-center configuration of impurity ions in alkali halides and oxides [5], for FA (11-type) center formation [6], and for F-center luminescence quantum yield [7] in alkali halides (incidentally, we recall that we found [8] a correlation between the Sanderson partial charge and the Szigeti effective charge). Very recently our scheme was adopted by other authors [9] in an analysis of the anharmonicity of the OH-and OD-stretching mode in alkali halides.…”