UV Absorption and Electro‐Optical Alignment Behavior of Off‐Center AG⁺ in RbI

Abstract

The polarization of four uv-absorption bands due to Ag+ in RbI can be determined from the sign of the zero-moment absorption change under applied electric field. The oscillator strength of these bands shows a temperature dependence very much like that of Ag+ in RbCl and RbBr, with a minimum around 150 K. Assuming fully polarized uv-transitions the electro-optical behavior can be fitted to a dipole alignment model with electric dipole moment values (uncorrected for local fields) of I, = 1.6 or 2 eA. Two models, suggested earlier by Jimenez et al., a (111)

Vibrations of off-centre Ag+in RbI

“…For RbI : Ag', we find A by putting A equal to b exp[(r+ + r-)/p]; r+ and r-are the ionic radii of cations and anions, respectively, and on the assumption that, because of the presence of Ag+, only r+ changes from 1.47 A (Rb+) to 1.26 A (Ag+) and r-and b remain unchanged. The static off-centre displacement d of Ag' is 0.916 A which is deduced from the observed value of the electric dipole moment p of Ag+ in RbI of 2.0 e A (Kapphan 1982). It is assumed that the local field is given by the Lorentz local field Eloc = [ ( E~ + 2)/3]E, c0 being the static dielectric constant, and the charge on the Ag+ ion is equal to the electronic charge e .…”

“…We have developed an impurity model for the (11 1) off-centre substitutional impurity in an alkali halide crystal of rocksalt structure. We have used the static off-centre Kapphan (1982) and the short-range potential developed by Catlow et a1 (1977) to estimate the force constant changes between the impurity and its nearest neighbours. The model is applied to calculate the local density of states of Ag' in RbI.…”

“…Mokross and Dick (1977) and Mokross et a1 (1977) have developed a general model for (1 10) 'off-centre' substitutional impurities in alkali halides and applied the theory for the calculation of infrared absorption and first-order Raman scattering. Recently, Kapphan (1982) has measured ultraviolet absorption and electro-optical behaviour of off-centre Ag+ in RbI and has concluded that the most probable direction of off-centre displacement should be (111). This and the planned work on infrared lattice absorption (Kapphan 1986) have been the reason for our interest in the theory of resonant vibrations of Ag+ in RbI.…”

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

“…[3] have been presented in table I of our previous works [4,5] together with the values for the partial charges. [7] (*) The systems marked with a question mark were not explicitly checked, at that time [6], for off-centre behaviour which, however, was thought unquestionably true by the authors quoted in ref. [6].…”