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1.
H.P. Sharma Jai Shanker M.P. Verma 《Journal of Physics and Chemistry of Solids》1976,37(12):1077-1079
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. 相似文献
2.
3.
The rumpling of anions and cations in the (100) surface layer of various rockSalt-structure alkali halides has been calculated using a simple extension of an original model due to Verwey. It is shown that reasonable agreement with the results of more sophisticated calculations can be obtained when the interionic potentials used in the calculations are derived from shell model calculations based on the elastic properties of the various materials. The idea of a “Surface ionic polarisability” is proposed, and the results of its use in rumpling calculations compared with those obtained using conventional “bulk” polarisabilities. 相似文献
4.
First-principle local-density calculations of the total energy of ions confined within a sphere were performed The results for positive alkali and negative halogen ions are used to test the validity of the compressed-ion model for the cohesion of alkali halides recently proposed by Narayan and Ramaseshan It was found that the model is not realistic since the calculated ionic compression energies are by far too large to account for the observed equilibrium lattice constants and bulk moduli of alkali halides. 相似文献
5.
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. 相似文献
6.
A theoretical method for evaluating the strain derivative of the electronic dielectric constant of ionic crystals has been developed. The analysis presented here is based on the shell model and takes account of the exchange charge polarizations. Values of strain derivative of the electronic constant dielectric calculated for 6 alkali halides and MgO show a remarkable agreement with experimental data on photoelastic constants. 相似文献
7.
Semiempirical pseudopotential surfaces for chemical reactions: The (AB)+ X - dialkali halide systems
W.S. Struve 《Molecular physics》2013,111(4):777-803
A generalization of the Roach-Child semiempirical pseudopotential calculation for K + NaCl to several analogous dialkali halide systems has been used to elucidate the chemical interactions governing the reaction dynamics. The Li + LiF ground-state potential surface, which exhibits a ~ 20 kcal/mole basin for isosceles Li2F, is qualitatively similar to one obtained in a recent configurational interaction calculation. It is shown that regions of the Na2Cl ground-state surface corresponding to Na2 + interacting with Cl- can be described in terms of an ion-pair Rittner potential model similar to that employed for the alkali halides. Chemical trends in the triangular complex well depths satisfactorily account for the experimentally observed transition between the collision complex mechanism (Rb + KCl) and the osculating complex model (Li + KBr) for the alkali-alkali halide exchange reactions at thermal energies. For collinear configurations with the alkalis on opposite ends, avoided intersections between the lowest two potential surfaces are characterized in terms of diabatic surfaces computed from truncated basis sets. Crossings of these surfaces account for the vibrational-electronic energy transfer between alkali atoms and vibrationally excited alkali halides. The ionic X - + A 2 + potential surfaces are used to predict the product electronic excitation and partitioning of exoergicity in reactions of halogen atoms with alkali dimer molecules. 相似文献
8.
R.W. Whitworth 《物理学进展》2013,62(2):203-304
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. 相似文献
9.
We show that the quadrupolar deformation of surface ions gives appreciable contribution to the potential energy between the atom and an ionic surface. Calculations are presented for a number of alkali halides. The validity of Fumi and Tosi crystal radii in the surface region is discussed in connection with the depth and the corrugation parameter. 相似文献
10.
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. 相似文献
11.
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. 相似文献
12.
O. Aikala 《Solid State Communications》1979,32(8):699-701
The general and individual features of the anisotropies in the directional Compton profiles computed recently by the author and a coworker for several alkali halides are discussed and concluded to depend mainly on the structure of the crystals and on the ionic radii of the constituents. Measurements of the Compton profiles for LiF, NaF, and KF in the directions [100] and [110] are suggested. 相似文献
13.
A continuum approach based on the Poisson-Boltzmann equation for the self-consistent electrostatic potential is used to describe the periodic spatial distributions of aliovalent impurities in ionic crystals. Some thermodynamic properties of the model are discussed and compared with experimental data on the Suzuki phase in alkali halides. 相似文献
14.
《Applied Surface Science》2005,239(3-4):273-278
In this paper, we consider a simple model, based on the electronegativity concept, which makes it possible to calculate the surface binding energy of any particles, sputtered from a solid surface by ion bombardment. The model contains empirical equations for calculation of “ionic” and “covalent” parts of the binding energy happened between a surface atom and its close neighbors. The model is strongly desirable in the case of the ion sputtering of any strong electronegative (C, N, O, F, Cl, etc.) or electropositive elements (alkali metals, rare earth elements) from semiconductor surfaces, when the strong ionic binding between these elements and the surface atoms can be estimated. 相似文献
15.
The well known Szigeti relations for the dielectric theory of ionic crystals have been critically examined by investigating the volume dependence of dielectric behaviour of alkali halides. It has been predicted that the strain derivative of effective ionic charge parameter should be nearly equal to zero. Values of transverse optic mode Gruneisen parameter for NaCl structure alkali halides have been estimated. 相似文献
16.
After an outline of work on rare-gas systems, which serves as a target for parallel work on alkali halides, and an initial brief survey of those parts of this parallel work for which results have been obtained, interionic potential models for alkali halides are considered in some detail. The rigid ion potentials of Fumi and Tosi are discussed and then a major part of the section is devoted to deriving a new set of polarizable ion potentials, which incorporate the ideas behind the lattice dynamical shell model. Extensions which include many-body terms in the potentials are considered briefly and finally the information which can be obtained from alkali halide diatomic molecules is discussed. In the third section methods of computer simulation for ionic liquids are outlined, concentrating on the molecular dynamics method, and some of the properties which can be obtained by analysing the ion trajectories are listed. Results from simulations, including some new work on LiF, NaCl and RbI, are reviewed. 相似文献
17.
We have evaluated the second order strain derivatives of the electronic dielectric constant of alkali halides employing two different theories viz. the Clausius—Mossotti theory of polarizability and the Penn model of energy gap. The results obtained from the two theories are in fair agreement with each other. It has been emphasised that the present calculations can be used to separate the electronic and ionic contributions to the second order strain derivative of the static dielectric constant. 相似文献
18.
Some studies on the relative stabilities of the NaCl,CsCl and cubic ZnS structures in ionic crystals
Ramesh Narayan 《Pramana》1979,13(5):571-579
The problem of the relative stability of ionic structures is still unsolved current semi-empirical theories wrongly predict
the caesium halides to have the NaCl structure. We point out here that these theories also predict some of the other alkali
halides to occur in cubic ZnS structure. To understand these discrepancies, we study the effect of various interactions (such
as second neighbour repulsion, van der Waals interaction and differences in ionic compressibilities) on the relative stability
of simple structures. The results throw into question the radius ratio approach. It is suggested that one could allow for
the presence of three-body interactions by relaxing the requirement that the repulsion interaction should be strictly proportional
to the number of neighbours. Such an approach might explain the relative stability of simple ionic structures. 相似文献
19.
Summary A rereading of the Fumi and Tosi paper on the ionic sizes and Born repulsive parameters in the alkali halides family led us
to draw a parallelism with recent total-energy calculations for semiconductors, where the Coulombic interactions are substituted
by a semi-empirical tight-binding estimation of the covalent attraction and the repulsive term is still of Huggins-Mayer form.
We find that for crystalline silicon the hardness parameter ρ is surpirisingly the same as for the alkali halides.
In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching. 相似文献