Static,anharmonic and phonon properties of ND4I

A detailed investigation of the phonon dispersion and static and anharmonic properties has been carried out for ND₄I using an extended three-body force shell model (ETSM) developed by Singh and Chandra. The ETSM results for phonon dispersion show better agreement with the experimental data than those obtained from the deformation dipole and rigid shell models. Calculated values have also been obtained for some other properties.     

Shell model lattice dynamics of transition metal oxides

The lattice dynamics of transition metal oxides has been investigated in detail with threebody force shell model which is a successful extension of the shell model and includes the effect of electron shell displacements as well as deformations. The phonon dispersion relations along three principal symmetry directions obtained from this model have been compared with neutron data and found to give better agreement as compared to other models. The complete phonon spectra have also been computed and used to derive the temperature dependence of Debye temperature, and the second-order Raman and infrared spectra. The derived data agree fairly well with the observed ones wherever available.     

Lattice dynamics of BaClF and SrClF

The lattice dynamics of tetragonal BaClF and SrClF have been worked out in detail on the basis of a shell model. The shell model parameters obtained from the transfer of corresponding parameters from the respective fluorides gives rise to long wavelength optic phonons in good agreement with the experimental results. Calculations have been made of the phonon dispersion relations, elastic constants, phonon density of states, temperature variation of specific heats and the lattice phonon moments.The mean square displacements of the various ions in these systems have also been computed over a wide range of temperature. Comparison with the available X-ray experimental data on thermal vibration parameters shows the agreement to be satisfactory.     

Lattice dynamics and inelastic neutron scattering experiments on andalusite,Al2SiO5

We report coherent inelastic neutron scattering measurements of the phonon dispersion relations and lattice dynamics shell model calculations of several microscopic and macroscopic properties of andalusite, Al₂SiO₅. Andalusite has an orthorhombic structure with 32 atoms/unit cell. The inelastic neutron scattering measurements (up to energy transfers of 45 meV) were carried out using the triple axis spectrometer at Dhruva reactor, India using a single crystal of andalusite and the phonon dispersion relations along the [100] direction have been measured. The shell model calculations have been used to compute the crystal structure, elastic constants, phonon frequencies, dispersion relations, density of states and the specific heat. The calculated results are in good agreement with available experimental data. The computed one-phonon neutron scattering structure factors based on the shell model have been very useful in the planning and analysis of the inelastic neutron scattering experiments.     

Lattice dynamics of molecular solids by a phenomenological model

A quasiharmonic central-force rigid-atom model has been used to study the dispersion of phonon waves in molecular (rare-gas) solids. The model derives the interatomic forces by Buckingham-Corner potential and takes account of all neighbor interactions. Zero-point anharmonic effects have been taken into account through the potential parameters by a method of iteration. It is found that out of all the conventional power and exponential potentials the Buckingham-Corner potential function is best suited to this class of molecular solids. Further, the possibilities of the improvement of the present model have been discussed.     

Interatomic force constants and lattice vibrations of AlP,AlAs and AlSb

The interatomic force constants and the phonon dispersion curves of AlP, AlAs, and AlSb are obtained from the electronic theory of solids by using our presented binding force, which includes mainly covalent interactions in the pseudopotential formalism and partially ionic interactions. The potential-parameter, effective ionic charge and ionic fraction of AlP and AlAs are estimated from those of AlSb and other III–V tetrahedrally- bonded compounds in spite of no experimental information on the band- calculation and the neutron scattering data of AlP and AlAs. Numerical data for the crystal energy of AlP and AlAs are in good agreement with the observed data, and the obtained results for the phonon dispersion curves and bulk modulus are useful to study the lattice dynamics and anharmonic properties of these compounds.     

Many-body effects in molecular solids

The effects of many-body interactions in the molecular (rare-gas) solids have been investigated, on the basis of Axilrod-Teller approximation, by a rigid-atom model. It is found that the 3-body interaction is the most dominant of all and the rest may be safely ignored. The discrepancy seen in the phonon dispersion curves is expected to be removed by the inclusion of appropriate anharmonic effects.     

Calculation of the lattice dynamical properties of Ge

The lattice dynamical properties of Ge are studied employing a frozen phonon approach within an ab initio density-functional pseudopotential formalism. Using the atomic number, atomic mass, and crystal structure as the only input information, we calculate phonon frequencies and mode Grüneisen parameters at Γ and X and the third-order anharmonic parameter for the zone center optical mode. The results are in excellent agreement with available experimental data. An analysis of various energy contributions to the TA (X) mode shows that the proper inclusion of the exchange-correlation energy is crucial for accurate microscopic studies of lattice dynamics.     

Lattice dynamics of alkali cyanide crystals

The recent microscopic model of Michel and Naudts for the interaction of ionic vibrational displacements and molecular rotational displacements is combined with the lattice dynamical shell model to calculate phonon dispersion curves of KCN and NaCN. Very good agreement with the existing experimental data is obtained. In particular, the upward dispersion of the TA phonon branches, characteristic of the alkali cyanide crystals, is well reproduced.     

Lattice vibrational properties of TmTe

We have investigated the lattice dynamical properties of a TmTe compound by using a breathing shell model suitable for this compound. The calculated phonon dispersion curves (PDC) reveal that this compound does not show any anomaly in their phonon properties. Our results on PDC, phonon density of states and lattice specific heat reveal that the phonon properties of this compound are like the other rare earth chalcogenides, particularly Eu-chalcogenides. We emphasize the need of measurements of the complete PDC of TmTe to support the present results on the calculated phonon properties.     

一维分子链中激子与声子的相互作用和呼吸子解

运用连续极限近似，求解声子与激子相互作用的运动方程，得到了简谐分子链和 非简谐分子链中晶格振动的孤子解，在考虑三次非简谐势的情况下，一维分子链晶格振动具 有扭结孤子解，在考虑具有四次非简谐势的情况下，得到一维分子链晶格振动具有呼吸子解 . 关键词： 一维分子链 激子 声子 孤子 呼吸子 非线性效应     

Phonon dispersion in NiO

The phonon dispersion relations in NiO have been measured using coherent inelastic neutron diffraction. Good fits to the data were obtained using various shell models. The room temperature phonon density of states was calculated and used to determine the temperature dependence of the lattice specific heat and the Debye temperature.     

On the spin-phonon interaction in anharmonic crystals

In this paper, a theory is developed for examining the influence of the spin-phonon interaction on the phonon dynamics in anharmonic crystals. In contrast to a harmonic approximation such an approach allows for comparing the role of the direct interaction between lattice vibrations with that of the indirect interaction via spins in the propagation and attenuation of sound as well as in the behaviour of thermodynamic quantities. Explicit expressions are derived for the phonon self-energy using successively higher approximations obtained from a cumulant expansion of the density of interparticle distances. It is shown that even in the harmonic approximation the two-spin correlation function and the four-spin correlation function enter into the expression of the generalized dynamical matrix in deviation from previous results. The modification of this harmonic dynamical matrix due to the renormalization of the harmonic phonons and due to phonon-phonon scattering processes is given employing a “weak coupling” approximation between the localized spins and the interparticle-distance density of the crystal. The resemblance of the present approach to an earlier microscopic treatment of electronic contributions to phonon dynamics in anharmonic crystals is pointed out.     

Anomalies of the phonon properties in multiferroic BiFeO3 thin films near the magnetic phase transition temperature

Size, substrate, doping and magnetic field effects on the phonon properties in multiferroic BiFeO₃ thin films are studied based on a microscopic model. We obtain an anomaly near the magnetic phase transition temperature TN which can be attributed to the magnetoelectric nature of BiFeO₃ and strong anharmonic spin-phonon interaction. It is shown that due to crystal lattice distortion for dopants with ionic radius smaller than that of the host ions the phonon energy decreases (for example Tb or Ti), whereas for the opposite case (larger radius of the doping ions, for example Co or Ni) it increases. The phonon damping is always enhanced compared to the undoped thin film.     

The effect of 3-body forces and anharmonicity on the phonon spectrum of solid argon

The phonon spectra of solidified argon have been computed by a phenomenological rigid-atom-model. This model, which takes the constituent atoms as rigid-hard spheres, assumes that the potential energy of the solid is the sum of central and non-central interactions, and derives the same from the Buckingham-Corner potential function together with the Axilrod-Teller interaction term. The zero-point quantum and anharmonic effects, have been included. The effect of many-body forces as well as anharmonicity on the frequency spectrum and the lattice heat capacities of the solid is seen to be appreciable. The agreement between theoretical and the experimental results is not very satisfactory.     

Thermodynamics of impure anharmonic crystals

The vibrational thermodynamic properties of doped anharmonic solids are investigated using thermodynamic Green's functions. The study uses cubic and quartic anharmonic interactions, and different masses and force constants for the impurity and host lattice atoms. The explicit expressions so derived for the partition function, free energy, entropy, and lattice heat capacity for these solids in the low-impurity-concentration limit are discussed for various situations. Our results are modified due to the presence of interaction terms between the anharmonic and localized fields and reduce to those obtained by earlier workers as limiting cases.     

Reduction of intensities of paramagnetic resonance lines by lattice vibrations

Using a simple model of spin-phonon interaction we find that the intensities of paramagnetic resonance lines are reduced by lattice vibrations in solids. The intensity missing from the paramagnetic resonance line goes into phonon sidebands. If the phonon spectrum is a continuum, which it is in the frequency region of interest, the sidebands appear as a broad background.