Influence of the isotopic constitution of the O2 −-center in alkali halides on its reorientation time

The theory of simple phonon induced reorientation of an O₂ ^?-center in alkali halides is refined to include the effect of the symmetry of its wave function. It is shown that the diagonal terms of the strain matrix and the off-diagonal ones lead to different coupling mechanisms between phonons and the impurity. For a rotation byπ/2 of a homonuclear molecule, either one or the other of the coupling mechanisms becomes ineffective, depending upon the isotopic constitution of the molecule and its nuclear spin state. Expressions for the reorientation time are obtained.     

Thermally stimulated depolarization of Eu2+-cation vacancy dipoles in alkali halide single crystals

Ionic thermocurrent (ITC) measurements have been performed on eight alkali halide single crystals doped with divalent europium. In all cases, the observed ITC peaks were fitted with a mono-energetic model without to appeal to any dipole-dipole interaction. Values for the reorientation parameters have been calculated. The relationship T_M1nτ^?1 α E previously found for I–V complexes in alkali halides has been found to be very well obeyed for the experimental data obtained in this investigation. It is also reported that the logarithm of the experimentally determined energies for free dipole reorientation shows a linear dependence on the interaction distance between the Eu²⁺ ion and the surrounding halogen ions in the distorted cubic site occupied by this impurity in the alkali halides.     

Study of strain optical constants of mixed lithium halides

The theory of photo-elasticity of mixed alkali halides has been used to analyse the strain optical constants and volume derivative of the electronic dielectric constant of the mixed crystal, LiF_0.5Cl_0.5.     

Paraelastisches Verhalten von S2 −-Zentren in Alkalihalogeniden

The reorientation of S₂ ^? molecule ions on anion sites in alkali halide crystals under mechanical stress has been investigated by means of electron spin resonance and optical measurements. The reorientation rate is given by an Arrhenius relation. Tunneling in the ground state of libration is unimportant contrary to the case of the O₂ ^? center. The stress-induced dichroism permitted identification and assignment of two optical absorption bands of the S₂ ^? center.     

Punktdefekte im kubischen elastischen Kontinuum

The determination of the displacement and strain fields of a point defect in a cubic crystal requires even in the framework of continuum elasticity theory numerical calculation. These fields of elastic dipoles are expanded in suitable vector and tensor fields. The coefficients of this expansion are calculated up to polynomials of 5th and 4th order in the direction cosines using the ratios of elastic constants as parameters. With this expansion the interaction of elastic dipoles in a cubic medium can be calculated. The results have been applied to the interaction of F-centres and of O₂ ^?-centres in alkali halides.     

The three reorientation processes of O-2 centres in unstressed alkali halide crystals

Measurements of the reorientation of O^-₂ centres in RbI and RbBr at low temperatures together with earlier measurements in different alkali halides suggest the following sequence of relaxation mechanisms in an unstressed srystal: Below about 3°K a one phonon tunneling process, then an activated tunneling process through the first librational state, and above 10°K a classical jumping process over the potential well.     

Dreikörperkräfte und die Stabilität einfacher Kristallgitter

Observed stability of crystal structures for rare-gas atoms, alkali halides and for those II–VI and III–V compounds whose ions are isoelectronic with rare-gas atoms is correlated with atomic and ionic interactions in dense media. On the basis of central forces essential discrepancies between theory and experiment occur. Further analysis indicates that strong, simultaneous interactions of exchange type between three atoms or three ions play an important role in determining the stable crystal structure. An evaluation of such three-body interactions is carried out in first and second orders of perturbation theory, using effective-electron wave functions of Gaussian form. It is shown that the theory accounts for all observed stability relations on a quantitative basis. The stability of the face-centered cubic configuration for solids of heavy rare gases is explained. Also, the occurrence of the cesium chloride structure for some of the heavy alkali halides and the magnitude of observed transition pressures are reproduced by the theory. In addition, it is shown that three-ion interactions are responsable for the occurrence of the sphalerite (zincblende) and wurtzite configurations with II–VI and III–V compounds, without covalent bonding between the ions being involved. General laws are given which govern the relative stability of the sodium chloride, cesium chloride, sphalerite and wurtzite configurations for these types of compounds.     

Dynamics of replacement sequences and of crowdions

The theory of the formation of point defects by ionization in alkali halides has been extended to include the dynamics of the subsequent motion of replacement sequences and of crowdions (H-centers) at low temperatures. Particular attention is paid to the stochastic aspect of the motion of these centers, which appears to play a crucial role in interpreting recent experimental studies of interaction proesses of H-centers with other defects in alkali halides. It turns out that at not too low temperatures the rate controlling reaction is the rotation of the centers. A detailed analysis of the kinetics of the various competing processes is given.     

Ultraviolet reflectivity of NH4Cl and NH4I in ordered and disordered phases

Near-normal incidence reflectivity spectra below 10 eV of NH₄Cl in phases II and IV and of NH₄I in phases II and III are reported. An interpretation is given by comparing these spectra with those of alkali halides. A temperature shift of the first exciton peak of NH₄I due to ordering of the crystal was found as was the case with NH₄Br. The absence of this effect in NH₄Cl strongly implies the relation to the transition II–III and the tetragonal distortion of the lattice.     

Analysis of the photoelastic effect in ionic crystals

An analysis of the photoelastic effect in ionic crystals has been presented within the framework of Clausius-Mossotti theory of the dielectric constant. The values of the strain derivative of the electronic dielectric constant have been calculated in alkali halides and MgO crystals by taking into account the variation of electronic polarizabilities with compressive stress. The results obtained are found closer to the experimental values. The photoelastic behaviour of MgO crystal is predicted to be of opposite nature to that of alkali halides, in conformity with the experimental observations.     

Slow relaxation,confinement, and solitons

Millisecond crystal relaxation has been used to explain anomalous decay in doped alkali halides. We attribute this slowness to Fermi-Pasta-Ulam solitons. Our model exhibits confinement of mechanical energy released by excitation. Extending the model to long times is justified by its relation to solitons, excitations previously proposed to occur in alkali halides. Soliton damping and observation are also discussed.     

Off-centre effects in the triplet relaxed excited state of impurity ns2 ions in alkali halides

Abstract

An off-centre displacement of an excited ns² ion from a crystal lattice site is studied for Ga⁺, In⁺ and TI⁺ centres in alkali halides at 0.09-5 K by the time-resolved polarization spectroscopy method. A theory is developed, which considers the off-centre effects as a result of the mixing of the impurity ³nsnp and ³npnp excited states by T_1u vibrations.     

Interstitial-exciton interaction in KBr:Na

The intensity of luminescence of the alkali halides submitted to an ionizing radiation decreases when color centers are created. The action of the interstitial as a quenching center is directly shown in KBr:Na. At LNT, the crystal has an emission band (430 nm), due to the exciton localized close to a sodium, which disappears according to a kinetics anticorrelated with that of the formation of the H_A (interstitials stabilized by Na) unde ionizing radiation.     

Photoelasticity in polycrystalline aggregates

A theory for the photoelastic behaviour of transparent polycrystalline aggregates consisting of randomly oriented anisotropic crystallites has been developed. Such an aggregate is isotropic but it becomes birefringent under the influence of a uniaxial load. The photoelastic constants of the aggregate are given by the components of the spatial average of the photoelastic tensor of the single crystal, and are worked out by assuming either the strain to be continuous (Voigt approximation) or the stress to be continuous (Reuss approximation). The components of the average photoelastic tensor are very different for these two limits. The elastic and the photoelastic constants of alkali halide aggregates have been evaluated for both the stress continuity and the strain continuity conditions. The maximum variation of the elastic constants in going from the Voigt to the Reuss condition is 50 per cent while the photoelastic birefringence can vary by as much as 300 per cent in alkali halides. In the case of KI and rubidium halides even the sign of the photoelastic birefringence is different for the two limits.     

Evaluation of the spectroscopic constants of the modified rittner potential by the Simon-Parr-Finlan technique

A critical examination is shown of the modified Rittner potential by use of the Simon-Parr- Finlan (SPF) technique, including contributions from the polarisability of ions and the van der Waals term. We develop an equation for the SPF constants, which are related to spectroscopic constants and the polarisability of the constituent ions. Our analysis indicates excellent agreement for the dissociation energies of 20 alkali halides with available experimental data. We also cite theoretical values of the SPF coefficients Y₂₀ and Y₁₁ for the 20 alkali halides; these also compare favourably with experimental data.     

Paraelectric alignment of NH 2 - centres in alkali halide crystals

The low temperature paraelectric alignment of substitutional NH ₂ ^- dipole centres in alkali halide crystals is studied measuring the electric field induced variation of the UV absorption and the electro-birefringence (Kerr-effect). We obtain the magnitude of the electric dipole moment and of the anisotropy of the oscillator strength. The reorientation of the dipoles between their equilibrium orientations is investigated by measuring the time dependence of the electro-birefringence after rapid changes of the applied electric field. The results indicate the predominance of one-phonon assisted tunneling processes.     

Absorption spectra of excitons in alkali-halides in the vacuum-ultra-violet region

On the basis of the localized excitation model, the spectra of K⁺ - 3p electron excitation in potassium halides are studied theoretically by using methods of the ligand field theory and a good agreement with the experiment is obtained. Some discussions are given on the applicability of the Frenkel exciton model to the VUV spectra of other alkali halides.     

Charged dislocations in ionic crystals

Experiments and theories concerning charged dislocations in alkali halide crystals are reviewed in detail, with particular attention to the way in which the experiments should be interpreted and to the range of applicability of the sweep-up and various forms of thermal-equilibrium models. Possible effects on mechanical properties and internal friction are analysed. The transient and steady-state effects of plastic deformation on ionic conductivity are described and new interpretations involving charged dislocations are proposed. A survey of results on the scattering of light by alkali halide single crystals leads to the conclusion that charged dislocations do not play an important role.

Evidence about charges on surfaces and on dislocations in AgCl and AgBr is reviewed and compared with that for alkali halides. Comparisons are also made with MgO, the CsCl and CaF₂ structures, semiconductors and ice.