Melting temperature and anharmonicity of lattice vibrations in solids

A correlation between melting points and Grüneisen lattice constants was found for a number of glass-forming oxides. It was established that, at the melting point, the mean energy of the thermal motion of a kinetic unit equaled the ultimate strain work of the interatomic bond corresponding to the quasi-elastic force maximum.     

Effects of lattice anharmonicity on spin-lattice relaxation in solids

A derivation is given for the formulas defining the operator for the spin-lattice interaction as averaged over the orbital motion of the unpaired electron in the solid. Simple formulas are derived for the constants of the spin-lattice interaction. It is shown that the relaxation time T₁ of this interaction may be substantially dependent on the concentration of lattice defects at helium temperatures; approximate relations between T₁ and the phonon mean free path are derived.     

First-order transverse phonon deformation potentials of tetragonal perovskites

A theoretical formalism is put forward with the aim of describing the softening of first-order transverse optical phonons in a strained tetragonal perovskitic lattice. On the basis of the dynamical equation for nondegenerate polar modes, the influence of oblique phonons could be first described by assuming a prevalence of short-range interatomic forces; then, the softening effect arising from external stress could be explicitly expressed as a function of orientation of the crystallographic texture. As a further step in the adopted formalism, the microstructure of a perovskitic polycrystal has been ideally modeled as an ensemble of mesocrystals, whose individual crystallographic directions corresponded to an average orientation over the unit volume of the probe. An experimental confirmation of the theoretical formalism is concurrently carried out, and phonon deformation potentials (PDP) have been directly measured for the first-order transverse phonon of a tetragonal PbZrTiO3 perovskite lattice.     

Athermal simulation of plastic deformation in amorphous solids at constant pressure

An algorithm is introduced for the molecular simulation of constant-pressure plastic deformation in amorphous solids at zero temperature. This allows to directly study the volume changes associated with plastic deformation (dilatancy) in glassy solids. In particular, the dilatancy of polymer glasses is an important aspect of their mechanical behavior. The new method is closely related to Berendsen's barostat, which is widely used for molecular dynamics simulations at constant pressure. The new algorithm is applied to plane strain compression of a binary Lennard-Jones glass. Conditions of constant volume lead to an increase of pressure with strain, and to a concommitant increase in shear stress. At constant (zero) pressure, by contrast, the shear stress remains constant up to the largest strains investigated (ε = 1), while the system density decreases linearly with strain. The linearity of this decrease suggests that each elementary shear relaxation event brings about an increase in volume which is proportional to the amount of shear. In contrast to the stress–strain behavior, the strain-induced structural relaxation, as measured by the self-part of the intermediate structure factor, was found to be the same in both cases. This suggests that the energy barriers that must be overcome for their nucleation continually grow in the case of constant-volume deformation, but remain the same if the deformation is carried out at constant pressure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2057–2065, 2004     

Adsorption-induced deformation of mesoporous solids: macroscopic approach and density functional theory

We present a theoretical study of the deformation of mesoporous solids during adsorption. The proposed thermodynamic model allows one to link the mechanical stress and strain to the solvation pressure exerted by the adsorbed molecules on the pore wall. Two approaches are employed for calculation of solvation pressure as a function of adsorbate pressure: the macroscopic Derjaguin-Broekhoff-de Boer theory of capillary condensation, and the microscopic density functional theory. We revealed that the macroscopic and microscopic theories are in quantitative agreement for the pores >8 nm diameter within the whole range of adsorbate pressures. For smaller pores, the macroscopic theory gradually deteriorates, and the density functional theory extends the thermodynamic model of adsorption-induced deformation to the nanometer scales.     

First principles computation of lattice energies of organic solids: the benzene crystal

We provide a first-principles methodology to obtain converged results for the lattice energy of crystals of small, neutral organic molecules. In particular, we determine the lattice energy of crystalline benzene using an additive system based on the individual interaction energies of benzene dimers. Enthalpy corrections are estimated so that the lattice energy can be directly compared to the experimentally determined sublimation energy. Our best estimate of the sublimation energy is 49.4 kJ mol(-1), just over the typical experimentally reported values of 43-47 kJ mol(-1). Our results underscore the necessity of using highly correlated electronic structure methods to determine thermodynamic properties within chemical accuracy. The first coordination sphere contributes about 90 % of the total lattice energy, and the second coordination sphere contributes the remaining 10 %. Three-body interactions are determined to be negligible.     

Exact statistical mechanical lattice model and classical Lindemann theory of melting of inert gas solids

A modified form of Lindemann’s model shows that the melting points of the heavy inert gases and other effectively spherical molecular species are proportional to the depths of their diatomic potential wells. The success of the model when compared with experiment seems to rely on the almost constant value of the ratio of the fractional volume and entropy changes during fusion. The Lindemann proposal can be incorporated into an exactly treated statistical mechanical lattice model utilising expandable clusters which reproduces the solid–liquid melting phenomenon for argon with a realistic volume change and melting line.     

Vibrational excitation of molecules in a lattice due to shock induced molecular deformation

A simple formalism is developed to calculate the rate of internal vibrational excitation of a molecule in a lattice due to abrupt deformation of the bonds of the molecule as a result of the application of a shock pulse to the lattice. The excitation rate is calculated as a function of rise time of the pulse and peak pressure for the case of 1,3,5-trinitro, 1,3,5-triazocyclohexane. It is shown that large vibrational excitation rates can be achieved if the rise time of the shock pulse is in the order of the period of vibration of the bond. The possible role of this process in shock induced chemical reactions in solids is considered.     

The optically detected coherent lattice oscillations in silver and gold monolayer periodic nanoprism arrays: the effect of interparticle coupling

Using femtosecond transient spectroscopy, we studied the optically detected laser-induced coherent phonon oscillation of monolayers of periodic arrays of prismatic-shaped silver and gold nanoparticles, assembled by using the technique of nanosphere lithography. In this method, the same size of polystyrene sphere and the same vacuum conditions are used. Under these circumstances, the gold nanoprisms formed are found to have sharper tips than the corresponding silver nanoprisms. For both gold and silver nanoparticles, the surface plasmon absorption maximum is found to depend linearly on size. The coherent lattice oscillation periods are also found to depend linearly on size. However, although the observed dependence for the silver nanoparticle is found to follow the calculated dependence of a single particle on size (based on a one-dimensional standing wave model), the gold nanoparticle deviates from this model, and the deviation is found to increase with the size of the nanoparticles. This deviation can be explained by considering interparticle coupling. A simple interparticle lattice oscillating dipolar coupling model of the dimer is found to qualitatively account for both the sign and the size dependence of the deviation. The absence of this deviation in the silver nanoparticle arrays is blamed on the weak interparticle coupling due to their rounded tips and the possibility of oxidation of their surfaces.     

Small-strain elastic moduli of quasi-isotropic semicrystalline eutectoid copolymers and aspects of universality

The small-strain elastic moduli of eutectoid random copolymers of ethylene are described in the total melting range. The approach is based upon the model of a cluster network constituted by the crystals which operate as active solid fillers. Number and average size of these fillers can be computed with the aid of a thermodynamic melting theory. Universal aspects of elastic small-strain behaviour in semicrystalline system will be discussed.     

Phenomenon of delayed reversible deformation (inverse creep) of solids in surface-active media. Steel-3 in gas hydrogen

The tensile test of steel samples within the stress range lower than the yield strength showed that, in a hydrogen gas medium, creep is developed, which is delayed with time and becomes completely reversible with the removal of a medium. It is found that the rate of reversible creep is controlled by the transport of hydrogen gas through structural defects into incipient subcritical cracks formed upon the action of stresses. Original Russian Text ? M.M. Lordkipanidze, 2006, published in Kolloidnyi Zhurnal, 2006, Vol. 68, No. 2, pp. 282–285.     

Propagating transverse electric and transverse magnetic modes in liquid crystal-clad planar waveguides

ABSTRACT

In a planar dielectric waveguide, weak confinement of a propagating mode in a high index core leads to a measurable evanescent interaction with the cladding. In this work, we study the effect of a reorientable anisotropic cladding on the behaviour of Transverse Electric (TE) and Transverse Magnetic (TM) mode polarisations using a liquid crystal (LC)-clad waveguide architecture. The polarised evanescent field of a guided mode interacts with a voltage-tunable birefringent LC cladding to deflect an out-coupled beam. Experimental measurements are coupled with a theoretical framework and show good consistency with simulation results. We isolate the effect of mode confinement by changing the thickness of the high index core. Interactions between the LC index ellipsoid and the mode polarisation are probed by changing the initial alignment of the LC. Finally, we examine the difference in deflection between TE and TM modes, which incorporates both a change in mode confinement and a difference in LC index components.     

Orientational environments in liquids and solids

Local orientational order in liquids and solids is examined using the average “distribution of cosines” of angles subtended at particles in the liquid by pairs of nearest-neighbour particles, a techninue first introduced by Scott and Mader. This average information is compared with recent, more complete theories and simulations of orientational order. The lowest-order approximation to the “distribution of cosines” is shown to fail for dense systems, but the true distribution may measured very efficiently in computer simulators.     

Vibronic coupling in molecules and in solids

We utilize the experience gained in our previous studies on the "chemistry of vibronic coupling" in simple homonuclear and heteronuclear molecules to begin assembling theoretical guidelines for the construction of potentially superconducting solids exhibiting large electron-phonon coupling. For this purpose we analyze similarities between vibronic coupling in isolated molecules and in extended solids. In particular, we study vibronic coupling along the antisymmetric stretch coordinate (Q(as)) in linear symmetric AAA molecules, and along the optical phonon "pairing" mode coordinate (Q(opt)) in corresponding one-dimensional [A]( infinity ) chains built of equidistant A atoms. This is done for a broad range of chemical elements (A). The following similarities between vibronic coupling in molecules and phonon coupling in solids emerge from our calculations: 1) The HOMO/LUMO electronic energy gap in an AAA molecule increases along Q(as), and the highest occupied crystal orbital/lowest unoccupied crystal orbital gap in [A]( infinity ) chain increases along Q(opt). 2) The maximum vibronic instability is invariably obtained for a half-filled, singly occupied molecular orbital in AAA molecules, and for a corresponding half-filled band in [A]( infinity ) chains. 3) The vibronic stability of an AAA molecule increases with a decrease of the AA bond length, as does the vibronic stability of [A]( infinity ) chains (external pressure may lead to a reversal of a Peierls distortion). 4) The high degree of s-p mixing and ionic/covalent forbidden curve crossing dramatically enhance the vibronic instability of both AAA molecules and [A]( infinity ) chains. We also introduce one quantitative relationship: The parameter log(R) (where R is molar refractivity, a parameter used by Herzfeld to prescribe the conditions for the metallization of the elements) correlates with a parameter f(AA) (defined as twice the electronegativity of A, divided by the equilibrium AA bond length), used by two of us previously to describe vibronic coupling in AAA molecules for a broad range of elements (A=halogen, H, or an alkali metal). We hope to illustrate that key chemical aspects of vibronic coupling in simple molecules may thus be profitably transferred to corresponding materials in the solid state.     

Computation of reequilibration data in solids

Reequilibration processes are often encountered within solids and they have long been described mathematically. However, computation of reequilibration data over entire processes is often difficult. An algorithm has been developed specifically for this purpose. It allows a fast and efficient computation of reequilibration parameters. Anisotropic diffusion can, moreover, be taken into account.     

Computer simulation of transformations in solids

Recent developments in molecular dynamics (MD) and Monte Carlo (MC) methods enable us to fruitfully investigate transformations in solids by employing appropriate potentials. The possibility of varying both the volume and the shape of the simulation cell in these simulation techniques is especially noteworthy. In this article we briefly describe some of the highlights of the recent MD and MC methods and show how they are useful in the study of transitions in monatomic solids, ionic solids, molecular solids (especially orientationally disordered solids), and glasses. The availability of reliable pair potentials will undoubtedly make these methods more and more useful for studying various aspects of condensed matter in the years to come.