Lattice Dynamical Model Calculations Research Articles

Crystal chemistry peculiarities of Cs 2Te 4O 12

The Raman and IR-absorption spectra of the Cs 2Te 4O 12 lattice are first recorded and interpreted. Extraordinary features observed in the structure and Raman spectra of Cs 2Te 4O 12 are analyzed by using ab initio and lattice-dynamical model calculations. This compound is specified as a caesium-tellurium tellurate Cs 2Te IV(Te VIO 4) 3 in which Te IV atoms transfer their 5 p electrons to [Te VIO 4] 3 6− tellurate anions, thus fulfilling (jointly with Cs atoms) the role of cations. The Te VI–O–Te VI bridge vibration Raman intensity is found abnormally weak, which is reproduced by model treatment including the Cs + ion polarizability properties in consideration.

Lattice dynamics ofCuGeO3: Inelastic neutron scattering and model calculations

The lattice dynamics in ${\mathrm{CuGeO}}_{3}$ has been analyzed by the combination of inelastic neutron scattering studies and lattice-dynamical model calculations. We report on an almost complete set of dispersion curves along the three orthorhombic directions and along [101]. Some modes exhibit rather low phonon frequencies in relation either with the almost layered character of the crystal structure or with an instability of the ${\mathrm{CuO}}_{2}$ ribbons and ${\mathrm{GeO}}_{4}$ chains against twisting. The dispersion of branches associated with the modes directly involved in the spin-Peierls transition allows to explain the particular propagation vector of the structural distortion in the dimerized phase.

Si diffusion in GaAs

Theoretical studies are carried out to ascertain the dominant mechanism of Si diffusion in GaAs. Lattice dynamical model calculations have shown that the most probable diffusion mechanism is through a single vacancy even though several experiments cannot fix the mechanism as substitutional, substitutional-interstitial pair or neutral defect pair.

Lattice vibrations and electron-phonon coupling in superconducting quarternary borocarbides: An inelastic neutron-scattering investigation

The phonon densities of states of the borocarbides ${\mathrm{RNi}}_{2}$${\mathrm{B}}_{2}$C (R=La, Y, and Lu) have been investigated using inelastic neutron-scattering techniques. These vibrational spectra are analyzed with the help of lattice dynamical model calculations. In addition experiments were performed on materials for which superconductivity has been suppressed by substituting 30% Ni by Co. The superconducting compounds display strong anomalous frequency shifts when compared to these Co-doped reference systems. This applies in particular to in-plane boron excitations found between 40 and 60 meV. Furthermore, the low-frequency part of the phonon spectra (up to 38 meV) of the superconducting systems shows a pronounced soft-mode behavior of the Ni-B vibrations and, unexpectedly, also of the heavy R-atoms involved excitations. The latter effect also appears upon cooling ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C from room temperature to below 30 K. In contrast to these findings, the high frequency ${\mathrm{A}}_{1\mathrm{g}}$ mode which Mattheiss et al. predicted to couple particularly strongly to the electronic system does not display any unusual frequency shift.

Study of phonons in semiconductor superlattices by Raman scattering spectroscopy and microscopic model calculation

Raman spectroscopy is used to study phonons in a series of thin (AlAs) m /(GaAs) n superlattices (SLs) grown by molecular beam epitaxy (MBE). The influence of buffer layer type on the interface roughness of heterostructures is carefully evaluated. The accuracy of optical phonons and the degree of peak sharpness of GaAs-like confined modes are examined via off-resonance Raman spectroscopy. Theoretical calculations of phonons in thin (AlAs) m /(GaAs) n superlattices (i.e. samples with m, n ≤ 12) are reported for various directions of propagation by using a rigid-ion model. Optical phonons acquire significant dispersive character when the wavevector q forming an angle θ with the growth axis of the superlattice is changed from θ = 0 to π/2, i.e. from [001] to [100]. The frequency gaps in the angular dispersions due to mode anti-crossing behavior observed recently by Zunke et al. using micro-Raman spectroscopy and studied by a continuum model are found to be in reasonably good agreement with our lattice dynamical model calculations.

Phonon density of states in Zr 2Ni

The phonon spectrum is needed to estimate the electron-phonon coupling strength leading to BCS superconductivity in some of the C16 structural intermetallics. The neutron-weighted phonon density of state in Zr 2Ni is obtained from neutron inelastic scattering at the Dhruva reactor. The experimental results are in fair agreement with independent lattice dynamical model calculations.

Coherent X-ray scattering by phonons

Interference effects arising from coherently coupled X-ray beams inelastically scattered from phonons have been used to determine phonon eigenvectors in Si and GaAs crystals. An outline of the method, experimental requirements, and theoretical background is presented. The experimental results are analyzed with the help of different lattice dynamical model calculations. Best overall agreement is found with phonon eigendata based on the bond charge model.

Intermolecular modes of solid carbon disulphide

In a recent paper Powell et al. (1982) compare CS2 lattice frequencies measured at 150K with lattice dynamical model calculations appropriate to 0K and 95K. Here it is shown that the comparison is greatly improved by using 150K lattice parameters in the calculations.

Invariance and stability conditions in lattices with rigid molecules

For performing lattice dynamical model calculations it is necessary to impose a number of constraints on the force constants, i.e. a number of invariance conditions and the Huang-conditions. For lattices with point masses, these have been derived by Born and Huang [1]. In this paper, the above mentioned conditions are obtained for lattices containing rigid molecules on a way somewhat different to that of W. Ludwig [11–13]. The formalism used here leads to shorter expressions for the final results which, moreover, satisfy the same symmetry relations as the corresponding results for lattices with point masses.