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Lattice dynamics of KPb 2 Cl 5 and KPb 2 Br 5 crystals is simulated in the framework of ab initio model, taking into account multipole moments of electron envelopes of ions. Results are compared with experimental polarized Raman spectra; good agreement with results of simulations is shown. Extremely low frequencies of phonon spectra are explained by big mass of anions together with considerable participation of heavy cations in the eigenvectors of upper frequency lattice modes.

Raman scattering spectra and elastic constants of KPb2Cl5 crystals have been studied. The results obtained are interpreted in terms of the ab initio lattice dynamics model taking into account multipole moments of ionic electron envelopes. The experimental results have been found to be in good agreement with numerical simulation; the narrow phonon spectra are shown to be due to a considerable contribution of heavy cations into the eigenvectors of the higher frequency lattice modes. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-frequency Raman spectra of normal and pressure-densified silica are investigated. Strong suppression of fast relaxation intensity is found in densified silica in comparison with the normal type, the dependence of the fast relaxation intensity versus the density changing its behaviour near the density of the low-density crystalline SiO2 polymorphs. The light–vibration coupling coefficients extracted from the comparison of Raman spectra and the vibrational density of states are different for densified and normal silica. In the case of the most densified sample, the position of the boson peak maximum marks the transition from the square frequency behaviour of the coupling coefficient to the linear one.

Low-frequency Raman spectra of LiNbO 3 are studied at temperatures far below of the ferroelectric phase transition. Nevertheless, the central peak is well observed in the spectra. It has been shown that this central peak is inherent property of crystal. The central peak found in the Raman spectra reveals a relaxation which is different from the relaxational part of a soft mode. Experimental features of the central peak are similar to ones in glasses caused by so-called fast relaxation. Interrelation of the central peak and acoustic attenuation is demonstrated.

Vibrational spectra of self-assembled GaAs and AlAs islands grown by molecular-beam epitaxy on InAs (001) substrates have been investigated by Raman spectroscopy. We observed large strain-induced shifts of optical-phonon frequencies in GaAs and AlAs islands with respect to the bulk materials. The values of shifts are 36 and $24 {\mathrm{cm}}^{\ensuremath{-}1}$ for GaAs longitudinal-optical (LO) and transverse-optical (TO) phonons and 55 and $28 {\mathrm{cm}}^{\ensuremath{-}1}$ for AlAs LO and TO phonons, respectively. Comparison of experimental data with calculated optical-phonon frequencies in strained GaAs and AlAs islands of different shape demonstrates that the structures studied are coherently strained, i.e., they do not contain dislocations that could lead to strain relaxation. The features of interface phonons were observed in the polarized Raman spectra between the peaks of InAs TO and LO phonons. The frequency positions of interface phonon lines are well described by the dielectric continuum model and also give evidence of three-dimensional island formation. Doublets of folded acoustic phonons appear in the low-frequency region of Raman spectra of multilayer structures with GaAs and AlAs islands. These doublets are very similar to those typically observed in planar superlattices and well described by the elastic continuum model.

Raman scattering spectra of CsScF4 crystal has been studied in both tetragonal phases, and lattice dynamics of the upper phase has been simulated numerically. Soft mode condensation at Brillouin zone boundary was obtained in the model simulations, and observed in experimental spectra below the tetragonal- tetragonal phase transition.

Primary (syn-growth) fluid inclusions in synthetic diamonds, produced within the field of their thermodynamic stability in a high-pressure split-sphere apparatus using the temperature gradient method, have been studied by optical microscopy and Raman spectroscopy. Inclusions in tabular diamond crystals contain CH 4 +other higher-molecular hydrocarbons, whereas inclusions in octahedral and dodecahedral diamonds contain only CH 4 . All inclusions in octahedral and dodecahedral diamonds contain graphite, which covers the walls of vacuoles as an opaque film. The formation of this film is most likely associated with precipitation of carbon from fluid on cooling. The obtained results emphasize the possible role of hydrocarbons in the process of diamond crystallization.

Glassy, heterocyclic copolymers of an epoxy oligomer and hexamethylene diisocyanate were characterized by low-frequency Raman scattering spectra at room temperature. The experimental data were treated according to the formalism of the theory of inelastic light scattering from fractons to derive structural information in the scale range below 10 nm. The samples studied were shown to behave essentially fractal-like in the range of characteristic scales (on an order-of-magnitude basis) spanning roughly the chain persistence length (ca. 0.7 nm) as a lower structural limit and the network mesh size (ca. 3 nm) as an upper structural limit.

Lattice dynamics of incommensurate ferroelectrics thiourea are investigated both theoretically and experimentally. Intermolecular interactions parameters are shown to change considerably during paraelectric-incommensurate (P-I) and incommensurate-ferroelectric (I-F) phase transitions. Acoustic modes activation and strong interaction of one of them with the soft lattice mode are observed in the I phase.

Raman scattering spectra are studied for Se-rich amorphous semiconductors AsSe. By comparing peak width, peak position, and Raman intensity in the range of bond modes it is derived that the changes occur not monotonically with increasing As content. The change of the spectra is explained as due to cross linking Se chains. [Russian Text Ignored].

Optical absorption and Raman spectra in a number of a-Si, a-Si:H and a-(Si:H):B samples have been investigated. It is shown that the optical gap changes are weakly correlated with the changes in TO peak width in Raman spectra but are well correlated with the changes in the TA peak intensity. The correlation between the amplitude of optical gap fluctuations, determined from the optical absorption spectra, and the TA peak intensity has been observed. These results suggest that the optical gap and the amplitude of its fluctuations in amorphous silicon and its alloys depend not only on bond-angle dispersion but also substantially on the degree of structural disorder on a scale of about 4-6 AA or greater.

Raman scattering measurements are reported for amorphous selenium. It is found that the molecular structure of a-Se on the scale of medium-range order differs from the structure of most inorganic glasses. A laser-induced glass-crystalline transition is also studied. The experimental results suggest the existence of successive phases in such transition with pronounced threshold behaviour. [Russian Text Ignored]

Optical absorption and Raman spectra in a-Si, a-Si:H and a-Si:H:B samples have been investigated. It is shown that the optical gap is weakly correlated with the TO-peak width in Raman spectra, but is well correlated with the TA-peak intensity. The conclusion was made that the degree of structural disorder at a scale of ≥4–6 A influences considerably on optical properties of amorphous silicon and its alloys.

GaAsInAs and GeSi strained superlattices (SL) with high quality interfaces were obtained by molecular beam epitaxy. A Raman study was performed on SL GaAsInAs grown on (001)-oriented substrates, and SiGe SL grown on (111)-oriented substrates. Both theoretical and experimental analyses of phonon spectra were carried out. The most striking feature of the low-frequency Raman spectrum on SL with a large thickness ratio is an observation of both odd and even folded-acoustic doublets of equal intensity. The additional peaks in the optical region of the GaAsInAs system are interpreted in terms of phonon confinement in the GaAs layers. The existence of confined acoustic phonons in GaAsInAs SL is predicted. In the GeSi system either folded TA or LA phonons were observed in the same polarization geometry. The analyses of intensities of FA phonons provided the oportunity of obtaining the photoelastic constants in both types of SL.

The terahertz density of vibrational states (i.e., the boson peak) for several B 2 O 3 glasses with different thermal histories was evaluated from specific heat data. It was found that the boson peak density of states is lower for more annealed glasses, the position of the boson peak maximum v B P shifting to higher frequencies. Low-frequency Raman spectra have been recorded for the same set of glasses. By comparison of Raman and low-temperature specific heat data the light-vibration coupling coefficient has been extracted. The coupling coefficient C(v) can be described by a linear dependence C(v)=A(v/v B P +0.5) in the spectral range 0.5v B P -2V B P for the whole set of B 2 O 3 glasses, the amplitude A being independent of annealing.

Raman scattering spectra of CsScF 4 crystal has been studied under high temperature and pressure. Soft mode condensation was observed below tetragonal-tetragonal phase transition under heating, and lattice dynamics simulation of the upper phase has been performed to clarify the form of the unstable mode. New first order phase transition has ben found under high pressure, new high pressure phase is supposed to be of low, C 2h 2 , Z equals 4 symmetry, the same as in KAlF 4 crystal.

Raman scattering spectra, optical and electronic properties of a-Si:H films, prepared by HF decomposition of silane-argon mixture at various substrate temperatures Ts (160

A Raman scattering method is used to investigate structures containing nanosize GaAs and AlAs clusters, which were grown by molecular-beam epitaxy on InAs substrates by the mechanism of self-organized growth under mechanical stress. A large shift of the phonon lines of GaAs and AlAs clusters with respect to the phonon frequencies in the bulk materials (36 and 24 cm−1 for GaAs LO and TO phonons and 55 and 28 cm−1 for AlAs LO and TO phonons, respectively) is observed in the spectra. This fact is explained by the presence of strong mechanical stresses in the GaAs and AlAs clusters. A comparison of the experimental data with the computed strain dependences of the phonon frequencies shows that the GaAs and AlAs clusters are pseudomorphic, i.e., they do not contain dislocations, which lead to relaxation of the mechanical stresses. In the interval between the InAs TO and LO phonon frequencies, the Raman scattering spectra contain features associated with interfacial phonons. The position of these features also attests to the formation of three-dimensional GaAs and AlAs islands and are described well by a continuum dielectric model.

We present the forward Raman scattering study of zone-centre optical phonon anisotropy in short-period GaAs/AlAs superlattices. Experiments were performed on specially prepared superlattice structures having anti-reflection dielectric coatings and removed substrates. The experimental data are compared with the angular dispersion of superlattice optical phonons calculated within the dielectric susceptibility model. We have found a good agreement between the experimental data and the calculations taking into account interface disorder.

A theoretical analysis is made of the condition for single-frequency stimulated emission at the center of a line of a gas laser with a short resonator. A calculation is given of the maximum length of the resonator in which interfering modes are still below the threshold. This calculation is carried out for a wide range of values of the pumping rate in excess of the threshold and for a wide range of ratios of the homogeneous and Doppler broadening. The results obtained may be useful in the design of lasers with interference-type mode selection.