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.     

Analysis of first and second order strain derivatives of the dielectric constants of alkali halides

Expressions for evaluating the first and second order strain derivatives of the electronic and static dielectric constants of alkali halides are derived. The strain derivatives of the electronic dielectric constant are calculated by taking account of the effect of the crystalline potential on the variation of electronic polarizabilities with volume. The strain derivatives of the static dielectric constant are evaluated adopting the exponential and the inverse power forms for the short range repulsive potential. Two sets of the Born repulsive parameters derived from ultrasonic data and dielectric data are used to evaluate the lattice contribution to the volume dependence of static dielectric constant of alkali halides.     

Phonon dispersion and dielectric behaviour of mixed alkali halide crystal NaCl0.5Br0.5

In the present paper we have calculated the phonon dispersion curves and dielectric constants, their volume derivatives and the Gruneisen parameters of the mixed alkali halide NaCl_0.5Br_0.5, using the extended three body force shell model. The results of dielectric constants, their volume derivatives and Gruneisen parameters are found to agree reasonably well with the experimental values, which establishes the validity of the three body charge transfer parameter. With this concept the phonon dispersion curves are obtained in all the three symmetry directions (q,o,o), (q,q,o) and (q,q,q), which will be of interest for experimental workers, since no experimental values have so far been obtained for phonon frequencies.     

Transverse optic mode Gruneisen parameter and dielectric properties of alkali halides

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.     

Dielectric relaxation studies of aqueous sucrose in ethanol mixtures using time domain reflectometry

Time domain reflectometry method has been used in the frequency range of 10 MHz to 10 GHz to determine dielectric properties of aqueous sucrose in ethanol. The dielectric parameters, i.e., static dielectric constant and relaxation time were obtained from the complex permittivity spectra using the non-linear least squares fit method. The Luzar theory is applied to compute the cross-correlation terms for the mixtures. It adequately reproduces the experimental values of static dielectric constants. The Bruggeman model for the non-linear case has been fitted to the dielectric data for mixtures.     

Lattice dynamics properties of XAs (X=Al,Ga and In) with zinc-blende structure from first-principle calculations

Band structures, density of states, dielectric and vibrational properties of XAs (X=Al, Ga and In) alloys with zinc-blende structure have been studied using the density functional theory (DFT). The calculated lattice constants, band gap, static dielectric constants and phonon frequencies are all in good agreement with the available experimental data and other theoretical results. The calculated results show that Born effective charges Z_B increase with cation mass. A similar tendency has been observed for phonon frequencies ω_TO and ω_LO. Calculation results prove that static dielectric constants ε(0) increase with atomic weight, i.e. in the sequences AlAs–GaAs–InAs, and show an inverse sequence for band gap.     

Use of Harrison's potential in ZnS,NaCl and CsCl-structure solids

Harrison's overlap repulsive potential has been used for evaluating cohesive energies, bulk modulus and its pressure derivatives, Gruneisen parameter, and the volume derivatives of Gruneisen parameter for cuprous halides, silver halides and thallous halides. This potential has replaced the old and widely used Born-Mayer potential in ionic crystals. The Harrison repulsive potential has been represented by a quantum mechanical analytical potential form derived from the tight binding theory. The parameters involved in this potential form have been determined in terms of valence state energies of outermost d-electrons of Cu⁺, Ag⁺ and Tl⁺ ions. The results calculated in the present study have been found in good agreement with experimental data and better than those predicted from the Born potential model.On leave fromDepartment of Physics, G. B. Pant University, Pant Nagar, U. P., India.     

Mechanical properties,Born effective charge tensors and high frequency dielectric constants of the eight phases of BaTiO<Subscript>3</Subscript>

We have preformed the first-principles calculations for the mechanical properties, Born effective charge tensors and high frequency dielectric constants of the eight phases of BaTiO₃. The independent elastic constants, bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio were obtained, which were consistent with the available theoretical and experimental values. The mechanical stability and brittle/ductile behaviors of the eight phases of BaTiO₃ have been discussed. The calculated results indicated that the eight phases were all mechanically stable and behaved in a brittle manner. The calculated Born effective charge tensors shown the covalent Ti–O bond and ionic Ba–O bond. Moreover, the high frequency dielectric constants have been given.     

Higher order elastic constants and generalized Gruneisen parameters of elastic waves and low temperature thermal expansion of gadolinium

Expressions for the higher order elastic constants are derived using the sublattice displacements to the second degree in strains. These expressions are used to obtain the higher order elastic constants and their pressure derivatives in gadolinium. The higher order elastic constants are used to find out the generalized Gruneisen parameters of the elastic waves propagating in different directions in gadolinium. The Brugger gammas are evaluated and the low temperature limit of the Gruneisen gamma is obtained. The results are compared with the available reported values.     

The dielectric and dynamical properties of zinc-blende BN, AlN and GaN from first-principle calculation

The ab initio calculations of the electronic structural, dielectric and lattice-dynamical properties of zinc-blende BN, AlN and GaN were presented. The ground-state properties, i.e., the lattice constant, the bulk modulus and band gap, were calculated using a plane-wave-pseudopotential method within the density-function theory. A linear-response approach to the density-function perturb theory was used to derive the Born effective charge, the high-frequency dielectric constants and interatomic force constants for these materials. The interatomic force contants (IFCs) are useful for interpolating the dynamical matrices through the whole Brillouin zone. Phonon frequencies along high-symmetry lines were also obtained by interpolating the dynamical matrices using the interatomic force constants. In this paper, we discussed the difference of dielectric and dynamical properties among zinc-blende BN, AlN and GaN, and meanwhile, also compared these properties with other experimental data available and theoretical values. Generally, the calculations were in good agreement with the other existing experimental data and theoretical values. Supported by the Hunan Provincial Natural Science Foundation of China (Grant No. 05JJ40135) and Xiangnan University Important Science Foundation (Grant No. 2007Z010)     

Mode Gruneisen parameters and thermal expansion of sodium nitrate

Using the generalised Gruneisen theory in the quasiharmonic approximation, the low and high temperature limiting values of the generalised Gruneisen parameters have been computed from the measured second-and calculated third-order elastic constants. The relevance of these calculations to the thermal expansion of sodium nitrate is discussed.     

Dielectric relaxation and hydrogen bond interaction study of diol-water mixtures

Complex dielectric permittivity measurements in the frequency range 10MHz-20GHz have been carried out in diol-water mixtures over the entire concentration range using a time domain reflectometry (TDR) method at 25°C. A hydrogen bonded theory is applied to compute the dielectric constant for the mixtures. It adequately reproduces the experimental values of static dielectric constants for the diol-water mixtures. The dielectric parameters confirm that the intermolecular homogeneous and heterogeneous hydrogen bonding vary significantly with the increase in concentration of the constituents of the diol-water mixtures.     

Microhardness and bulk modulus of binary tetrahedral semiconductors

Using the plasma oscillations theory of solids, the microhardness and bulk modulus of binary tetrahedral semiconductors have been calculated. New relations between plasmon energy and these parameters have been proposed. Our calculated values of microhardness and bulk modulus from two different equations are in excellent agreement with the experimental values and the values reported by other workers. In the present model, no experimental data are required except the plasmon energy, and one can predict the value of these parameters in the case of unknown semiconductors where experimental measurements have not yet been made due to a lack of availability of experimental melting points, dielectric constants or bond lengths.     

Lattice-dynamical calculation of the thermodynamic properties of NaCl,KCl and RbCl

Results of a lattice-dynamical calculation of the coefficient of thermal expansion, the heat capacity, and the macroscopie Grüneisen ratlo for NaCl, KC1 and RbCl are presented. Good agreement with the experimentai results is obtained without adjustable parameters. The needed normal mode frequencies and mode-Grüneisen parameters were caiculated with a deformation dipole model with nearest- and next-nearest-neighbor central-force interactions. Predicted values for the static elastic constants, the strain derivatives of the static potential energy, the mechanical equilibrium value of the nearest-neighbor distance, and the mode-Grüneisen parameters are given.     

Giant Static Dielectric Constant of Strained PbTiO3

First-principles density functional perturbation calculations are employed to study the dielectric and piezoelectric properties of strained tetragonal PbTiO3. Lattice distortion, static dielectric constant, Born effective charge, zone-centre phonons, and piezoelectric constant are obtained. For the strained tetragonal PbTiO3, we obtain a giant static dielectric constant （3600） under a strain 0. 77%. Moreover, the calculated piezoelectric constant e15 of strained PbTiO3 reaches about 203 C/m^2 which is about 20 times of that of unstrained system. The giant static dielectric constant is mainly due to the softening of the lowest-frequency phonon mode and the reduce of Ti-O bond length. This work demonstrates a route to a giant static dielectrics for electrically microwave and other devices.     

Ab initio lattice dynamics and piezoelectric properties of MgS and MgSe alkaline earth chalcogenides

We have studied structural, elastic, dielectric, vibrational, and piezoelectric properties of rock-salt and zinc-blende Mg alkaline earth chalcogenides (MgS, and MgSe) by using the plane-wave pseudopotential method within the local density approximation. The calculated lattice constant, bulk modulus, and elastic constants for these compounds are found to be in good agreement with experiment. The density functional perturbation theory has been employed to derive the Born effective charge and high-frequency dielectric constant and to calculate phonon dispersion curves and density of states. The piezoelectric constant has also been determined. The calculated results are compared with theory and experiment when available and showed reasonable agreement. In other cases, our results are predictions. The pressure dependence of the studied quantities has been examined and discussed. The pressure coefficients and mode Grüneisen parameters are determined.     

静压下Zn1-xCdxSe/ZnSe窄量子阱的激子和光跃迁

利用波恩公式近似建立了应变与介电常量的定量关系考虑应变对介电常量、有效质量、晶格常量(体积)等诸多物理量的影响,用变分法计算了静压下Zn_1-xCd_xSe/ZnSe窄量子阱中激子结合能和光跃迁能量随压力的变化理论计算结果与其他作者的实验和理论结果进行了比较和讨论.     

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.     

Dielectric susceptibility of a deuterated KDP crystal from experiment on Raman scattering and in the cluster approximation

The temperature dependence of the static dielectric susceptibility obtained from the experiment on Raman scattering in a deuterated KDP (DKDP) crystal has been described using the cluster approximation in terms of the microscopic mean-field theory. The optimum values obtained for parameters of the cluster approximation are in good agreement with experimental data over a wide temperature range. These values are found to be comparable to the estimates previously obtained in describing experimental data on the DKDP crystal.