Atomistic study of elastic constants and thermodynamic properties of zinc – blende CuBr

Elastic constants and thermodynamic properties of zinc blende CuBr are calculated using a molecular dynamics simulation based on Tersoff empirical interatomic potential. We find that the elastic modulus C₁₁ is bigger than the other theoretical and experimental data, while C₁₂ is somewhat small. The elastic modulus C₄₄ is in good agreement with the theoretical calculations and experiment. Thermal expansion coefficient, specific heat capacity at constant volume and thermal conductivity are in very well agreement with experimental data. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New insights on the thermodynamic barrier for nucleation in glasses: The case of lithium disilicate

An analysis is performed of the temperature dependence of the thermodynamic barrier to nucleation, W*(T), calculated from a fit of lithium disilicate glass data to the classical theory of nucleation. It is shown that, in order to obtain a satisfactory agreement between experimental and theoretical determinations of W*(T), lower values must be assigned to both the thermodynamic driving force and the surface energy as compared with the corresponding macroscopic values. This finding is consistent with theoretical considerations taking into account the effect that, in general, both the bulk and surface properties of the critical nuclei differ considerably from the respective properties of the newly evolving macroscopic phases. In addition, an anomalous increase of W*(T) with decreasing temperature is found near the glass transition interval. This increase is interpreted as a result of the effect of elastic strain on the thermodynamic driving force. The values of elastic strain energy estimated from the low temperature behavior of W*(T) are congruent with those calculated using the elastic constants of glass and crystal.     

Elastic,Piezoelectric and Dielectric Constants of Pentaerythritol

The elastic, piezoelectric and dielectric constants of pentaerythritol, C(CH₂OH)₄, have been computed employing the method of long waves and the unscreened rigid ion potential model developed by RAMAMOORTHY and KRISHNAMURTHY. Piezoelectric contribution is taken into account in the computation of elastic constants. This is the first report on the piezoelectric and dielectric constants of pentaerythritol. In the notations followed by LANDOLT-BORNSTEIN the computed piezoelectric constants are: _g14 = 4.82, _g15 = 4.37, _g31 = 16.21 and _g36 = 5.89 (in unit of 10⁻¹Vm/N) and the static dielectric constants are: K₁ = 2.38 and K₃ = 2.47. The hydrogen bonding in pentaerythritol has no effect on its dielectric behaviour. Calculation of acoustic velocities in piezoelectric crystals and as a special case to the crystals of class 4 is explained in detail. Sound velocities calculated from the elastic, piezoelectric and dielectric constants computed in the present investigation agree with the experimental values determined by the ultrasonic method.     

Diffraction Elastic Constants for Textured Materials — Different Methods of Calculation

A systematic review of different types of traditional averages for diffraction elastic constants in textured materials is given. In addition a new type of average, which recently was suggested, is presented. These averages are of vital importance for a correct interpretation of internal (e.g. residual) stress measurements. The theoretically predicted diffraction elastic constants are compared with the experimental ones for rolled steel samples; there exists good agreement between theory and experiment.     

Nematic Liquid Crystals with a Trifluoromethyl Group

Abstract

We have synthesised a new class of compounds incorporating a trifluoromethyl group in the terminal alkoxy or alkenyloxy chain. Compounds containing several aromatic rings were synthesised with a view to producing compounds of high birefringence. This also included the synthesis of compounds containing a carbon-carbon triple bond. Compounds with three 1,4-disubstituted rings were synthesised in order to produce materials with a high nematic clearing point and as low a melting point as possible. Laterally fluoro-substituted compounds were prepared to generate a high positive value of the dielectric anisotropy, a low melting point and no smectic mesophases. Substances containing a cyclohexyl ring, which tend to induce a lower viscosity and compounds incorporating a bicyclo[2.2.2]octane ring were also synthesised in order to produce a high nematic clearing point and to influence the elastic constants. The (E)-trifluorobut-2-enyloxy-compounds were prepared in attempts to produce compounds with appropriate elastic constants and a high birefringence.     

Electrooptical coefficients and temperature and pressure derivatives of the elastic constants of tetragonal Li2B4O7

New values for the following properties of tetragonal Li₂B₄O₇ are reported: dielectric, electrostrictive, electrooptic, elastic, thermoelastic and piezoelastic constants, coefficients of thermal expansion and indices of refraction. These values are considered to be of superior precision compared to earlier published data. Most elastic constants possess positive temperature derivatives. This anomalous behaviour is not reflected in the pressure derivatives. The electrooptic effects are not large enough to compete with other materials used for technical applications.     

Magnetic state, elastic constants, and longwave static displacements of atoms in γ-FeNi alloys

The experimental results on the concentration and temperature behavior of longwave (both transverse and longitudinal) static displacements of atoms, as well as the data on the elastic constants (derived from neutron and ultrasonic measurements) of γ-FeNi alloys, are reported. The relationship between the alloy magnetic state and the above-mentioned parameters is discussed and these parameters are classified with respect to the premartensitic phenomena and the Invar effect.     

Investigation into the seismoacoustic properties of specific polycrystalline materials used in nuclear reactors

The characteristics of the wave fields observed during the transmission of quasi-longitudinal ultrasonic waves through polycrystalline graphite samples have been studied. The specific features of propagation of elastic waves in a bilayer medium, where one of the layers (isotropic) is an acrylic glass hemisphere and the other (anisotropic) is a polycrystalline porous graphite hemisphere, are considered. The velocities and propagation times of quasi-longitudinal waves in polycrystalline graphite samples and a bilayer acrylic glass-graphite sample in different directions are experimentally measured by ultrasonic spatial sounding. The experimental results are compared with theoretical calculations using the data on graphite crystallographic texture obtained previously by neutron diffraction. The reasons for the discrepancy between the theoretical and experimental characteristics of elastic waves in the media under study are established and analyzed.     

一种利用实验测量和数值计算确定3m点群晶体弹性系数的方法

将实验测量与数值计算相结合确定低对称性晶体弹性系数的方法可以弥补常规方法测量时造成的弹性系数非对角元误差大的不足.本文将该方法由正交系晶体推广到了三方系3m点群晶体,在数值计算中选取了目前公认的无约束最优化方法中最稳定的两种算法-单纯形法和BFGS法,并增加了弹性系数的约束条件,提高了方法的适应性和正确性.在此基础上,本文利用3m点群晶体LiNbO3和LiTaO3的弹性劲度系数[cij]和顺服系数[sij]的实验数据对该方法进行了讨论及验证.结果表明这种方法对3m点群晶体弹性系数非对角元的计算是可行的.另外,本文还对数值计算时,初值的选取和检验结果合理性等关键性问题进行了详细的讨论.     

Piezoelektrische und elastische Eigenschaften von Triglycinselenatkristallen im Bereich des Phasenüberganges

The piezoelectric constants d₂₁, d₂₃, d₂₅ and the elastic constants S₁₁, S₁₅, S₃₃ and S₃₅ of triglycine selenate are measured by the resonance method in the region of phase transformation. The elastic and piezoelectric constants are shown to have sharp maxima in the Curie point. The influence of a constant electric field on the temperature dependence of the piezoelectric constant d₂₃ and of the elastic constant S₃₃ is studied as well as the dependence of the mentioned constants on the intensity of a constant electric field at temperatures below and above the Curie temperature. A considerable increase of the elastic constant S₃₃ is found in weak electric fields of the ferroelectric phase. The electrostrictional coefficients Q₂₁, Q₂₃ and Q₂₅ are determined by means of the piezoelectric constants, spontane polarisation and dielectric receptivity. The found experimental dependences are explained on the base of the character of the domain structure of this crystal.     

La3Ga5SiO14晶体电子结构及光学性质的第一性原理研究

采用基于密度泛函的第一性原理平面波赝势法对La3Ga5SiO14晶体基态的几何参量、能带结构、态密度和光学性质进行了系统的研究.优化了La3Ga5SiO14晶体中原子的内部坐标,利用精确计算的能带结构、态密度和电荷密度等值线分析了晶体的吸收谱、介电函数、折射率,计算结果与实验符合较好,为La3Ga5SiO14晶体材料的分子设计与应用提供了理论依据.     

The Effective Elastic Constants of Textured Polycrystals in Second Order Approximation

The effective elastic constants of textured polycrystalline materials are calculated in two approximations on the basis of a minimum energy assumption which is more realistic than the VOIGT and REUSS assumptions. The formulae are specified for the case of cubic crystal symmetry and orthorhombic specimen symmetry (sheet symmetry). Numerical values calculated for a copper and a steel sheet are compared with the corresponding experimental values.     

Neutron scattering perspectives for protein dynamics

After a double decade of intensive neutron scattering studies of the biological physics of protein dynamics on a few well-characterised model systems, the time has come to extend the method to address the vast biological diversity of proteins and their dynamics-function relationships. The time-scale and length-scale dependent mean square displacement and effective force constant, measured by neutron elastic intensity temperature scans, are proposed as relevant experimental parameters, and examples are given of their correlation with biological function. The parameters are directly calculable from molecular dynamics simulations, and their proposed deposit in an accessible data bank will greatly strengthen the links between experimental and theoretical approaches to protein dynamics.     

Elastic debye temperature of compounds with sphalerite structure

Elastic Debye temperatures θ_EL of the semiconducting compounds A^IIIB^V, A^IIB^VI and CuCl are evaluated from second order elastic constants C_ik available from the literature. The dependence of θ_EL on temperature is given. The results at 0 K are compared with those calculated from low temperature measurements of specific heat. Generally, the agreement between these values is better than found in the literature up to now.     

Proton NMR studies of smectic-C phases

We present proton NMR second moments studies of some compounds exhibiting smectic-C phases. From the comparison of experimental and theoretical angular dependences of the second moments the tilt angles δ are derived. The ratio f of the second moments of samples with statistical distribution of domain orientations and of perfectly aligned samples is calculated and compared with experimental data. For certain substances with constant tilt here are significant deviations from the theoretically expected values suggesting that the underlying theoretical model has still to be modified.     

Physical properties of single crystals of KTiPO4, KxRb1−xTiOPO4 (x = 0.85; 0.75), KGeOPO4 and KTiOAsO4

Single crystals of the title compounds of optical quality and dimensions up to 30 × 25 × 25 mm were grown by the flux method. During the growth process the formation of twins consisting of bicrystals was observed, even in the case of single-crystalline seeds. The twinning boundaries could be detected by etching with hot KOH solutions. The following physical properties were investigated: refractive indices, dielectric constants, thermal expansion, piezoelectric, and electrooptic effects, elastic and thermoelastic constants. The crystals exhibit a quite similar behaviour. The anisotropy of all properties is quasi-tetragonal. In respect to polar properties only small differences within the mixed crystals K_xRb_1–xTiOPO₄ are found. Replacement of P by As does only slightly affect the polar properties, whereas the replacement of Ti by Ge leads to a drastic reduction of polar effects.     

Pressure dependence of elastic properties of low-silica calcium alumino-silicate glasses

The hydrostatic and uni-axial pressure dependence of elastic properties of a low-silica calcium alumino-silicate glass (LSCAS) is determined by ultrasonic pulse-echo techniques at room temperature. The experimental results are used to obtain the third-order elastic constants (TOECs) of these glasses. The pressure dependence of fractal bond connectivity of these glasses is discussed. The normal behavior of positive pressure dependence of ultrasonic velocities was observed for the glass. The pressure dependence of both shear modulus and bulk modulus are positive for these glasses.     

Interatomic force constants in AIIBIVCV2 chalcopyrite compounds

Force constants of the A^II C^V and B^IV C^V bonds in the A^IIB^IVC^V₂ compounds with chalcopyrite structure are estimated from experimental lattice vibration data using a simplified version of the Keating model. It is shown that the force constants depend exponentially on the bond length. The parameters of this relation are practically the same as those found for the A^IV A^IV, A^III B^V and A^II B^VI bonds in the elemental semiconductors and binary compounds and for the B^III C^VI bond in the A^IB^IIIC^VI₂ semiconductors.