A simple shell model calculation of the differential ionic relaxation of the MgO(100) surface

A model previously reported and applied to the calculation of the differential ionic relaxation of the (100) surfaces of the alkali halides has been extended and applied to MgO(100). In particular, mechanically induced polarisations of the surface ions due to asymmetry in the surface short range forces are now included. The new model is tested by repeating the calculation for selected alkali halides. The results for MgO(100), obtained assuming a fully ionic surface, are consistent with the results of a LEED theory-experiment comparison, and indicate that this surface is not relaxed but may be rumpled on a small scale.     

Approximate degeneracies of zone boundary phonons in alkali halide crystals

We point out the existence of a pervasive pattern of near degeneracies of phonon frequencies in isobaric alkali halide crystals (NaBr, KCl, RbBr, CsI) which strongly suggests that their dynamical matrices are almost invariant under transformations which exchange positive and negative ions. We extend this hypothesis to a relation between phonon properties of “mirror” alkali halides in which the ions of one crystal are replaced by the oppositely charged isobaric ions of the other, such as RbCl and KBr. Experimental evidence supporting this can also be adduced. Similar near degeneracies universally occurring in NaCl structure alkali halides and alkaline earth oxides are also noted and a possible dynamical basis for understanding these suggested.     

A new theory of compressible ions — structures of the alkali halides

Ions in ionic crystals are considered to exist in compressible space-filling polyhedral cells analogous to the Wigner-Seitz cell in metals. Repulsion arises from the compression energy of the ions written as a surface integral over the ionic cells. Two adjustable parameters are introduced per ion with the provision that the same parameters can be used in any crystal of any structure in which the ion occurs. The 18 parameters for the 5 alkali and 4 halogen ions have been determined from PV data on the 20 alkali halides. The important successes of the theory are: (i) All the twenty alkali halides are correctly predicted to occur in their observed structures (ii) The thermal transition in CsCl is explained (iii) The pressure transitions in the alkali halides are predicted well (iv) The calculated values of the variation of transition pressures with temperature agree well with experiment. These results are much better than those obtained by earlier theories.     

Analysis of the repulsive softness parameter in alkali halides

An analysis of the repulsive softness parameter for alkali halides has been presented on the basis of the additivity rule. The softness parameters for individual ions have been obtained using data from three different sources viz. overlap data, molecular data and crystal data. The softness parameters so derived are used to evaluate the bulk modulus and its pressure dependence for 20 alkali halides with NaCl and CsCl structures adopting Smith's distortion model for repulsive interactions. Some attractive features of the present approach have been discussed in order to demonstrate its superiority over the previous work.     

The fractional ionic character of alkali and silver halide crystals

The fractional ionic character of alkali and silver halide crystals is defined in terms of the deviations from the additivity rule for polarizabilities of ions. The electronic polarizabilities of ions are calculated using an empirical relationship according to which the electronic polarizability of an ion can be assumed to be directly proportional to the cube of its radius. The calculated ionicities indicate that the alkali halides are nearly or more than 90% ionic and silver halides are much less ionic which is also evident from the Phillips ionicity scale.     

The Born repulsive parameters and dielectric behaviour of alkali halides

The dependence of the Born repulsive parameters of alkali halides on elastic and dielectric data has been discussed. The values of hardness parameter in alkali halides have been recalculated using the revised values of van der Waals energies. It is observed that the two sets of hardness parameter corresponding to elastic and dielectric data differ from each other but become compatible if an effective charge parameter for the ions is introduced. Its usefulness has been demonstrated by calculating the strain derivative of static dielectric constant of alkali halides.     

A model for the metal-non-metal transition in metal-molten-salt solutions

Summary We outline a theory for the metal-non-metal transition occurring in solutions of alkali metals in molten alkali halides as governed by a balance between the binding of ionic clusters by localized electrons and the excess free energy of an ionic assembly screened by metallic electrons. In the model the transition is driven by the composition dependence of the screening length. The theory is amenable to an analytic solution within the mean spherical approximation when Thomas-Fermi screening is used and the ions are described by charged hard spheres.     

Single-ion parameters for interionic potentials in alkali halides

A deformation-dipole model for the interaction between ions in alkali halides is presented which is entirely based on single-ion properties. Each ion is characterized by a set of five crystal-independent parameters. With the use of only 32 adaptable parameters for eight ions, a remarkably good fit is obtained for 96 properties of 16 crystals. The single-ion parameter model is particularly useful in calculations of substitutional impurity systems.     

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.     

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.     

Rotational barrier height for diatomic molecular impurities in alkali halides

A general expression for the coupling constants according to Devonshire's model for the hindered rotation of diatomic molecules in cubic crystals is derived. The treatment is confined to alkali halides with polarizable point ions. The results are applied to the librational modes of OH^? and OD^? impurities in KCl-, KBr- and KI-lattices.     

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.     

Neutral beam sputtering of positive ion clusters from alkali halides

Neutral argon atom beams of 15 keV energy have been used to sputter alkali halides and the ejected positive ions have been analysed in energy, mass and angular distribution.

The use of a neutral beam, rather than an ion beam, minimizes surface charge and the deflection of ejected ions by electrostatic interaction with a charged incident beam.

A cluster component of the form K₂Cl⁺, K₃Cl⁺ ₂ and higher members of the series is found for all alkali halides studied.     

The generalised Huggins-Mayer form of born repulsive potentials for NaCl-type alkali halides

The generalised Huggins-Mayer form of Born repulsive potentials for NaCl-type alkali halides have been re-evaluated using more reliable, recently published thermodynamic data and van der Waals energy coefficients. As with older versions of these potentials excellent agreement between model and experimental values of interionic distance and cohesive energy is achieved. Further, the earlier shortcomings in the failure of the stability condition and predicted elastic constants are significantly reduced. For the majority of salts the new short-range interactions have stronger van der Waals attractions and slider repulsions. The model gives average crystal radii for the alkali ions about ~0.3 Å larger than traditional free ion radii and ~0.08 Å larger than Tosi-Fumi crystal radii. The predicted halogen crystal radii are correspondingly smaller by the same amounts.     

Analysis of melting based on the diffusional force theory for alkali halides

The theory of melting based on the concept of diffusional force given by Bosi is used for studying the melting of alkali halides. Values of thermal expansivity and the Anderson-Gruneisen parameter are used to predict the interionic separations in 16 alkali halides at melting temperatures with the help of the Anderson formula. A model for melting is developed by estimating the diffusional force from the knowledge of interionic potentials based on ultrasonic data for bulk modulus and its temperature and pressure derivatives. The model thus developed is found to yield satisfactory results in agreement with the experimental data on melting.     

First stage coloration of alkali halides doped with M2+ impurities that change their valence state by irradiation as a function of X-irradiation dose rate

The amount of room temperature first stage coloration of ten alkali halide systems, i.e., NaCl:Pb, NaCl:Co, NaCl:Mg, NaCl:Sn, NaCl:Fe, KCl:Eu, KBr:Eu, KI:Eu, RbCl:Eu and RbBr:Eu has been analyzed as a function of X-irradiation dose rate. In all systems the amount of first stage coloration was found to be proportional to the square root of irradiation dose rate. These results are in very good agreement with the prediction of the theoretical model recently developed by our group to account for the F-center production in alkali halides doped with doubly valent impurity ions that change their valence state by irradiation and give evidence of its validity.     

Interpretation of the mass distribution of ejected (CsI)nCs+ ion clusters by the non-cascade sputtering model

Large (CsI)_nCs⁺ cluster ions ejected by keV ion bombardment of CsI surfaces have been observed by various researchers, notably those from the Naval Research Laboratory and the University of Manitoba. We use the non-cascade sputtering model to describe the mass distribution of these cluster ions. Reasonable agreement between theoretical predictions and experimental observations is obtained. The energy deposited by the bombarding ion giving rise to the ejection process is shown to vary with the stopping powers of the bombarding ion species. the model is also shown to be applicable to cluster ions ejected from other alkali halides.     

Analysis of polarizabilities of alkali halides using Narayan-Ramaseshan repulsive potential

The repulsive potential in ionic crystals recently proposed by Narayan and Ramaseshan (NR) can be expressed as the sum of the contributions from the individual ions. In the present paper we show that using this repulsive potential it is possible to divide the polarizability arising from the relative displacement of ions into its ionic constituents. NR have also derived the ionic radii in alkali halides which we have used to estimate the electronic polarizabilities of ions with the help of polarizability-radius cube relation. The electronic polarizabilities of alkali and halogen ions thus evaluated show a good agreement with those deduced from the experimental refraction data.     

Electronic polarizabilities and sizes of ions in alkaline earth halides and alkali chalcogenides

An analysis of the electronic polarizabilities and sizes of ions in the crystals of alkaline earth halides and alkali chalcogenides has been performed using a relation between polarizability and ionic radius. The electronic polarizabilities and sizes of ions are calculated using the free state data reported by Pauling. The quantities obtained in the present study are found to vary from crystal and crystal, thus showing the deviations from the additivity rule. The results are discussed and compared with those obtained by other investigators.