Effect of three-body interactions on electronic polarizabilities and photoelastic behaviour of partially covalent crystals

Summary We have investigated the effect of three-body interactions on the photoelastic behaviour and the electronic polarization of silver, thallium and copper halides, which are partially covalent in character and possess three different crystall structures. The cation, anion and molecular polarizabilities calculated in their crystalline state seem to be reliable as they follow a systematic trend. Our calculated values of the strain derivatives of the electronic dielectric constants are much closer to their experimental data than those obtained by earlier workers. To speed up publication, the authors of this paper have agreed to not rceive the proofs for correction.     

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.     

Effect of many body forces on the photoelastic constants of the alkali halides

A simple phenomenological lattice theory based on the Lundqvist potential of ionic cohesion (many body theory of elastic dielectric), to explain photoelastic behaviour of the solids with rock-salt structure, is presented and shown to apply within reasonable errors to the known experimental constants. The predicted values of the photoelastic constants of the alkali halides are very close to the experimental values and are better than obtained by all earlier workers.     

Dielectric behaviour and optic mode Gruneisen parameters of the halides of silver,caesium and thallium

The optic mode Gruneisen parameters in silver, caesium and thallium halides are calculated using the Born model for interionic forces and the Szigeti theory of dielectric constants. The strain derivatives of the electronic and static dielectric constants are also evaluated and compared with experimental data. The strain derivative of static dielectric constant reveals the inadequacy of the Born model for the crystals under study. Possible modifications have been suggested to improve the situation. The theoretical values of the optic mode Gruneisen parameters closely agree with recent experimental data. An appropriate process has been adopted to evaluate the average values for the Gruneisen parameter.     

Theory of strain derivative of electronic dielectric constant of ionic crysts

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.     

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.     

Study of the anharmonic properties of copper halides

Summary We have investigated the anharmonic properties of cooper halides by means of a three-body interaction potential extended to include the van der Waals effects. For this purpose, we have computed the thirdorder elastic constants and the pressure derivatives of second-order elastic constants after deriving their correct expressions from the present potential following the lines of Sharma and Verma. Our results have, generally, followed a systematic trend and given a satisfactory prediction of the available experimental data on the anharmonic properties of copper halides. This potential has as scope of its application to describe the dynamical and dielectric properties of these halides. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

电子-声子作用对卤化铊中Wannier激子结合能的影响

一些作者对卤化铊中Wannier激子的结合能所作的计算值比实验值大一个数量级。我们用过去得到的电子-空穴有效作用势,计算了卤化铊中Wannier激子的结合能,比其他作者所得的结果有了很大的改进。     

Evaluation of the photoelastic constants of ionic crystals

A theoretical model, for evaluating the photoelastic constants of ionic crystals, has been developed by considering the short range repulsive interactions between nearest and next nearest neighbours, the van der Waals potentials and three body interactions as well as taking into account the electronic polarizabilities of cations and anions. The model has been applied to calculate the photoelastic constants of RbC1, RbBr and RbI crystals. The results obtained in the present study are in good agreement with experimental data.     

DFT Study of HFI in Halogen-Containing Gold, Silver and Copper Complexes

We have analyzed the nuclear quadrupole coupling constants in copper, silver and gold halides and related compounds an the basis of the calculations with use of pseudo-potentials. The geometrical parameters and NQR halogen quadrupole constants obtained by these calculations substantially corresponded to the data of microwave spectroscopy in the gas phase. The analysis of the quality of the calculations with use of pseudo-potentials and the expanded basis set for the copper compounds was carried out. The ZORA model is shown to be a viable alternative to the computationally demanding B3LYP/SDD model for the calculation of halogen coupling constants in molecules. Besides the ZORA model as against BXYP/SDD model have been caused to realistic values of gold nuclear quadrupole coupling constants. In this case of the gold compounds the main contribution of the chemical Mössbauer shift comes from the 6s-orbiral population of the gold atoms.     

Two-fold interferometric measurements of piezo-optic constants: application to β-BaB2O4 crystals

We present the interferometric technique which allows to measure piezooptical and photoelastic characteristics of crystal materials of any symmetry. The offered two-fold interferometric method enables to determine all independent non-zero piezooptic and photoelastic constants by measuring pressure induced changes of optical path. As advantage to known acoustooptical techniques this method allows to measure both the absolute magnitude and sign of photoelastic constants. In general case the determination of 36 components of piezooptic tensor needs to carry out 57 measurements on 16 samples. The corresponding relationships are derived. As an example we apply here the interferometric technique to measure the piezooptic and photoelastic constants in trigonal β-BaB₂O₄ crystals.     

Theoretical study of temperature dependent lattice anharmonicity in TlCl and TlBr

Effect of temperature on ultrasonic attenuation in BCC structured (CsCl-type) thallium halides (TlCl and TlBr) have been investigated in a wide temperature range 50–500 K for longitudinal and shear modes along [1 0 0], [1 1 0] and [1 1 1] directions of propagation. Starting with nearest neighbour distance and repulsive parameter and taking interactions up to next nearest neighbours, second and third order elastic moduli have been evaluated, which in turn have been used for evaluating thermal relaxation time, Gruneisen numbers, acoustic coupling constants, ultrasonic velocity and ultrasonic attenuation. Ultrasonic attenuation in these bcc structured crystalline materials has been found less than the fcc crystalline materials. The results have been discussed and compared with available data.     

Enhancement of photoelastic constant of optical thin film waveguide assisted by surface acoustic wave

In the present research, we proposed a method of controlling the photoelastic constant using surface acoustic waves, which had not previously been reported, and carried out experimental studies thereof. A Bragg diffraction was carried out to determine the photoelastic constants of Ta2O5. As a result, it is confirmed that the photoelastic constant of a Ta2O5 thin film undergoing a sputtering process, during which surface acoustic waves were excited on the substrate, was about 2.19-2.27 times larger than those of thin films on which surface acoustic waves were not excited.     

Modes of vibrations due to (U-center + additive cation impurities) in alkali halides

Local and resonant modes due to hydride ions in various alkali halides containing additive cation impurities have been computed by the Green function technique. Local vibrations due to U-centers in alkali halides have O_h symmetry. When one of the six nearest neighbour cations is replaced by an additive impurity, the site symmetry of the system is lowered from O_h to C_4v. The phonon Green functions matríx is analysed according to the irreducible representation of the point group symmetry pertaining to the substitutional impurity. We have considered the vibrations of the hydride ion and all its six nearest neighbours. Analytical expressions have been derived for various modes of vibrations. Using group theory the 21 × 21 matrix has been block diagonalized into various irreducible representations. The effect of mass changes and the changes in short-range force constants have been taken into account. The computed results of the localized modes have been compared with the available experimental results. Good agreement has been found. Theoretical results on resonant modes are also displayed, which will be of use in future experiments on these systems.     

Effective dielectric and photoelastic tensor components in thin layered superlattices

The effective photoelastic constants of a superlattice composed of thin alternating layers of orthorhombic symmetry are calculated as functions of dielectric, elastic and photoelastic constants of the constituents. In a preliminary step to this calculation, we also obtain the effective dielectric tensor for any symmetry of the layers. It is shown that appropriate combinations of these effective constants are arithmetic averages of the corresponding quantities in the constituents, each layer having a weight equal to its relative thickness.     

Theoretical investigations on structural, elastic and electronic properties of thallium halides

Theoretical investigations on structural, elastic and electronic properties, viz. ground state lattice parameter, elastic moduli and density of states, of thallium halides (viz. TlCl and TlBr) have been made using the full potential linearized augmented plane wave method within the generalized gradient approximation (GGA). The ground state lattice parameter and bulk modulus and its pressure derivative have been obtained using optimization method. Young's modulus, shear modulus, Poisson ratio, sound velocities for longitudinal and shear waves, Debye average velocity, Debye temperature and Grüneisen parameter have also been calculated for these compounds. Calculated structural, elastic and other parameters are in good agreement with the available data.     

Relationship between proton shielding constants and electric dipole moments in the hydrogen halides

The proton shielding constants of the hydrogen halides have been calculated employing a molecular wave function built from gauge-invariant atomic orbitals. Each of the molecular wave functions contains a single parameter which is determined from the observed dipole moment. Calculated and experimental values of the shielding constants for HF, HCl, HBr and HI agree to within approximately 10 per cent.     

Anisotropic breathing shell model and phonons in NaCl structures

Callow, Diller and Norgett, in their investigations on the dynamics of alkali halides, have developed analytic potentials for the short range interactions between the anions and cations. These potentials consisting of Buckingham and van der Waals contributions have been determined from the elastic, dielectric constants and the equilibrium conditions of the lattice and are found to satisfactorily explain the defect properties of many halides. We employ the analytic potentials of Catlow et al. and use anisotropic breathing corrections for the computation on phonon dispersions and Debye-Waller factors of all NaCl structure alkali halides. The results of our study show that good agreement can be obtained with these more physical parameters.     

Higher order elastic constants of alkali halides

The Coulomb, van der Waals and repulsive lattice sums occurring in the higher order elastic constants up to sixth order have been calculated for the rocksalt and cesium chloride structures. Numerical values of the static elastic constants up to sixth order based on a rigid ion model with van der Waals and Born-Mayer type central force interaction between first and second nearest neighbors are calculated for several alkali halides representing both structure types. Fair agreement with the available experimental third and fourth order elastic constant data is found.