Second order elastic anomalies in barium titanate from cubic to tetragonal phase transition

The anomalies of the second order elastic constants have been derived for barium titanate for the phase transition from cubic to tetragonal. The equilibrium values of the components of the order parameter and the strain variables have been obtained from the stability conditions. The fluctuations in the order parameter have been derived from the Landau-Khalatnikov equations. Expression for the shift in the zero point energy in the tetragonal phase is obtained and is shown to be proportional to (T −T _c)². The anomalies for all the second order elastic constants have been derived and relations among them reported. It is shown that the second order elastic anomalies suffer a discontinuity at the transition temperature.     

Anomalies in second order elastic constants and gyrotropic constants of triglycine sulphate near phase transition

The anomalies in second order elastic constants and gyrotropic constants have been considered for the phase transition of triglycine sulphate. Expressions have been derived for the equilibrium values of order parameter and strain variables in both phases. Using Landau-Khalatnikov equation the fluctuation in order parameter is expressed in terms of fluctuations in strain variables. Substitution of these in free energy gives anomalies arising from Landau and coupling energies in second order elastic constants. The real part of the anomalies decreases steeply across the transition temperature and thereafter flatly tend to ferroelectric values. The anomalies in the components of the gyrotropic tensor have been derived and their temperature variation discussed.     

Elastic anomalies in strontium titanate

The anomalies of the second and third-order elastic constants have been considered for the phase transition of strontium titanate within the framework of Landau’s theory. All the anomalies of the second-order elastic constants have been obtained in a single formula using Kronecker delta functions and relations among them have been established. The real parts ofC*₁₁ andC*₄₄ decrease steeply across the transition temperature and thereafter flatly tend to their asymptotic values in the low temperature phase agreeing qualitatively with experimental observations. We have also derived expressions for the third-order elastic anomalies and discussed the temperature variation of the real part ofC*₁₁₁. We have derived expressions for the attenuation of the longitudinal and transverse waves along certain simple symmetry directions and have shown that there is nearly good agreement with experimental observations.     

Third order elastic constants of V3Si and V3Ge

General expressions have been derived for the second order elastic constants and third order elastic constants of the A-15 structure compounds with the nearest neighbour central interaction. The second order elastic constants, the third order elastic constants and the pressure derivatives of the second order elastic constants of V₃Si and V₃Ge are reported and compared with the available experimental measurements.     

Analysis of anharmonic and thermoelastic properties of non-centrosymmetric crystals

In the present paper an analytical potential form is used for overlap repulsive energy, derived by Harrison from quantum mechanical considerations, along with the composite effect of three-body forces and intersublattice displacement. The short-range overlap parameters in Harrison's potential form have direct correlation with the valence state energies for outermost electrons. The potential model is applied to calculate the third and fourth order elastic constants, first and second pressure derivatives of second order elastic constants, Grüneisen parameter and its volume dependence, Anderson parameter, and thermal expansion coefficient for three non-centrosymmetric crystals, viz. CaF₂, SrF₂ and BaF₂. The calculated values of various physical quantities are found to be in good agreement with experimental data.The authors are grateful to Dr. Mansour Khalef, the Head of Physics Department, TNRC, Tajura (Tripoli) for the facilities and encouragements.     

Third-order elastic constants and pressure derivatives of the second-order elastic constants of β-tin

The second- and third-order elastic constants and pressure derivatives of second-order elastic constants of tetragonal β-tin have been obtained using the deformation theory. The strain energy density derived using the deformation theory is compared with the strain dependent lattice energy obtained from the elastic continuum model approximation to get the expressions for the second- and third-order elastic constants. Higher order elastic constants are a measure of the anharmonicity of a crystal lattice. The 12 non-vanishing third-order elastic constants and the six pressure derivatives of the second-order elastic constants in tetragonal β-tin are obtained in the present work and are compared with the available experimental values. The second-order elastic constant C₃₃ obtained in the present study is in reasonable agreement with the experimental values. The third-order elastic constants are generally one order of magnitude greater than the second-order elastic constants as expected of a crystalline solid. The third-order elastic constant C₃₃₃ is higher in magnitude than all other values. This shows a greater anharmonicity of β-tin along the c-axis direction of the crystal.     

The effect of hydrostatic and uniaxial pressure on the elastic constants of cadmium

The third order elastic constants of the hep metal cadmium have been determined from measurements of the hydrostatic and uniaxial pressure dependences of the velocities of ultrasonic waves propagated through single crystals. The hydrostatic pressure dependences of the second order elastic constants have also been obtained. The compression of cadmium has been estimated by extrapolation of the data up to high pressures by using the Murnaghan equation of state. Using the generalised Grüneisen theory in the quasiharmonic approximation, the long wavelength acoustic mode Grüneisen parameters have been obtained; the mean acoustic mode parameter is compared with the thermal Grüneisen parameter. In general the anisotropy of the vibrational anharmonicity of the acoustic modes is found to be consistent with the deviation of the c/a ratio of cadmium from that expected for an ideally close-packed metal.     

Single crystal elastic constants and magnetoelasticity of holmium from 4·2 to 300 K

The five independent elastic coefficients of holmium single crystals have been determined by means of an ultrasonic pulse technique at a frequency of 10 MHz, between 4·2 and 300 K. From the elastic constants the temperature variation of the directional adiabatic compressibilities, the limiting Debye temperature and the elastic anisotropy ratio were calculated. The elastic coefficients exhibit anomalies at the magnetic ordering transitions known to occur in holmium. Anomalous behavior in the elastic constants was also observed at about 80 K. The limiting value of the Debye temperature was found to be 191·5 K. The present measurements of the elastic constants, and the reported magnetostriction and thermal expansion data, enabled the calculation of the magnetoelastic contribution to the total Hamiltonian of holmium in the magnetically ordered states. A very small discontinuity in the temperature dependence of the magnetoelastic energy was observed at the Curie point of holmium. Below the Neel temperature, the magnetoelastic energy varies smoothly with decreasing temperature, attaining a value of ?2·13 J cm^?3 at liquid helium temperature. The temperature dependence of the magnetoelastic energy in the vicinity of the Curie point in holmium suggests that the magnetic transition from the antiferromagnetic arrangement into the ferromagnetic state is of second order.     

The second and third order elastic constants of alkali metals

The second and third order elastic constants of the alkali metals have been calculated on the long wave method using the Heine-Abarenkov lacal model potential with different exchange-correlation corrections. It is found that the use of an exchange correlation correction which satisfies the compressibility sum rule leads to a good agreement between the calculated and measured second order elastic constants of the alkali metals Na, K, Rb and Cs. The shear elastic constants however come out correct even if the compressibility sum rule is violated by the exchange-correlation correction. The third order elastic constants and the pressure derivatives of the second order elastic constants and the pressure derivatives of the second order elastic constants calculated on the HA local potential are lower than the experimental values at room temperature. The discrepancy is pronounced for the heavier alkali metals. Similar calculations using the Wallace potential for Li, Na and K and the Schneider-Stoll potential for Rb give the pressure derivative in good agreement with experiment. In view of the important role by the exchange correlation correction, Suzuki's results calculated without taking this correction into account can only be accepted with some reservation.     

Thermoelastic behaviour of mixed ammonium halides

A theory for the temperature derivatives of second order elastic constants of ammonium chloride and ammonium bromide, crystalizing in ceasium chloride structure has been developed incorporating contributions due to the thermal phonons and three body interactions in the lattice energy in the framework of Hildebrand approximation. The expressions for the temperature derivatives have been derived by differentiating the expressions of the second order elastic constants. The repulsive potential which is taken to be of the Born-Mayer type is assumed significant up to the first neighbours only. Computations have been made for pure and mixed crystals NH₄Cl_1–xBr_x for x=0, 0.13, 0.23 and 1.00. The results have been found in agreement with the experimental data wherever these are available.The authors are thankful to the referee for his valuable comments and suggestions which helped them in the revision of the paper.     

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.     

Temperature dependence of elastic constants for ionic solids

In this paper the thermal variation of volume for NaCl, KCl, MgO and CaO has been investigated on the basis of Anderson model. We have evaluated the values of elastic constants C₁₁, C₄₄ and K_T at different temperature on the basis of Murnaghan and Tallon models. Tallon's model with second approximation presents slightly better agreement with experimental results which shows that the Anderson–Grüneisen parameter is directly proportional to the volume ratio. Tallon's model can be used to evaluate the values of elastic constants for solids at different temperatures.     

Valence force field theory, elastic, acoustic and thermodynamic properties and a potential function of TeO2 glass

A short survey has been made on the extensive work that is being done on the pressure derivatives of the second order elastic constants (SOEC) to ascertain various properties of substances. Hence an attempt has been made to correlate the pressure derivatives to some properties of the substances. Thus some equations have been derived to correlate the Grüneisen parameter which is evaluated from Schofield's equations and Bhatia-Singh's (BS) parameters. They have been used to compute the longitudinal (γ_g^L) and transverse (γ_g^T) Grüneisen constants. γ_g^L calculated by different methods agree well with experiment. γ_g^T obtained from BS parameters gives rather higher value while Schofield's equations give results in agreement with experiment. The DeLaunay-Nath-Smith (DNS) equation has been used to derive a relation to compute γ_g^el (elastic). A method has been extended to calculate the third order elastic constants (TOEC) and it is found to give excellent values of TOECs in agreement with experiment. The absorption band position of TeO₂ has been predicted to occur at 276 cm⁻¹. The phonon dispersion curves have been calculated through BS equations for TeO₂. Several other properties of TeO₂ have been computed such as thermal Grüneisen parameter γ_g^th, its pressure derivatives (γ_g^th)′≡(dγ_g^th/dP), the pressure variation of bulk modulus C₁≡(dK_T/dP)_T and its pressure derivatives that is (dC₁/dP)_T which is in turn related to (γ_g^th)′, the heat capacity at constant volume C_V, and the second Grüneisen constant Q. In some cases we calculated these quantities by different methods and the agreement between them is good. Besides we evaluated δ_T^AG the Anderson Grüneisen parameter. Another important aspect of the present investigations is the formulation of the potential function (PF) of TeO₂ from which we calculated SOECs and these are found to be in excellent agreement with experiment. All other properties mentioned already have also been calculated through the use of the newly formulated PF and the calculated values obtained through various other equations are in good agreement with those obtained from PF. According to valence force field (VFF) all atomic forces can be resolved into bond bending β and bond stretching α forces. It is shown that TeO₂ does not satisfy Martins unity rule. Hence it is concluded that there is an effective dynamic charge on Te in TeO₂. Using the experimental elastic constants the bond bending force β and bond stretching force α and also their pressure derivatives have been evaluated. In addition the reststrauhlen optic frequency ω has been calculated. A self consistent check has been made by evaluating C₄₄ through the calculated values of α and β.     

The sublattice displacements and the elastic constants of A-15 compounds

The nature of the sublattice displacements in A-15 compounds has been discussed. Expressions have been derived for the elastic constants in terms of the force constants for these compounds. The numerical values of the force constants for Nb₃Sn have been evaluated.     

α-AlPO4晶体弹性温度特性的研究

研究了用缓慢升温法生长的α-AlPO₄晶体的弹性温度特性。测量采用传输法,测温范围为-30—150℃;用最小二乘法拟合弹性-温度曲线;用电子计算机处理数据。给出了弹性常数c_ij和s_ij(i,j=1,2,…,6)的温度特性方程以及T₀=25℃时的一级、二级和三级温度系数。 关键词：     

Symmetry-relations and dynamics of molecular crystals in the long wave limit

Symmetry relations for macroscopic constants are derived. Especially it is shown that the Voigt-symmetry holds for the indices of the elastic constants. This has been doubted several times for lattices of particles with additional degrees of freedom. In the second part a simple model for Br₂- and J₂-lattices is discussed. The intermolecular forces are assumed to be van-der-Waals-forces. The influence of the internal degrees of freedom on lattice dynamics is calculated especially for the elastic constants. Further the limiting frequencies forq?0 are given and compared with experimental values.     

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.     

Computation of collective modes and acoustic investigations at different temperatures of vitrous silica

This paper is mainly concerned with elastic and acoustic properties of vitrous silica besides the computation of phonon frequencies. Thus the phonon frequencies of vitrous silica have been calculated assuming the electronic bulk modulus,K _e, as equal to zero. New equations have been derived to relate the pressure derivatives of second order elastic constants to the acoustic Gruneisen’s parameters using both Bhatia-Singh’s parameters and Schofield’s equations. The calculated longitudinal and transverse Gruneisen’s parameters and the predicted absorption band spectra from Nagendranath’s equation and Bhatia Singh’s parameters are in good agreement with experiment. The calculated mean acoustic mode Gruneisen’s parameter evaluated from the pressure derivative of Nagendranath’s equation is also in good agreement with experiment. An erratum to this article is available at .     

Elastic nonlinearity parameters of tetragonal crystals

The equations of motion for the propagation of finite amplitude elastic waves in crystals of tetragonal symmetry have been derived starting from the expression for the elastic strain energy. The equations have been solved for a finite amplitude sinusoidal wave propagating along the pure mode directions which are [100], [110] and [001] for the tetragonal group TI. The solutions corresponding to longitudinal wave propagation yield expressions for the amplitudes of the fundamental and generated second harmonic for these directions in terms of certain combinations of second and third order elastic constants of the medium. The results will aid the experimenter to determine these constants using ultrasonic harmonic generation technique.     

Dispersion relations and lattice thermal expansion of dysprosium

The lattice dynamics, second and third order elastic constants and the lattice thermal expansion of dysprosium have been calculated using Keating's approach. The ten third order elastic constants are calculated using four anharmonic parameters. The present model reproduces the measured pressure derivatives of the second order elastic constants of dysprosium well. The low and high temperature limits γ?_I and γ?_H of the lattice thermal expansion are evaluated and the agreement between the calculated γ?_H and that obtained from the thermal expansion and specific heat data is satisfactory.