Vibration spectra and the valence force field of the LiCuVO<Subscript>4</Subscript> crystal

Polarized Raman spectra of single-crystal LiCuVO₄ containing quasi-one-dimensional cuprate chains are measured. The force constants of the valence force field are calculated for the phonon modes at the center of the Brillouin zone with an accuracy sufficient to describe the experiment. From the results, it follows that the crystal contains rigid Cu-O-V-Cu-O-V structural rings with a charge distributed along their bonds.     

Electron-phonon coupling in intrinsic trans-polyacetylene

We consider a valence force field model for the phonon spectrum of (CH)_x and find that phonon coupling to extended π electron states must be included to describe the observed Raman active modes of the polymer. The calculated phonon spectra are shown to possess the zone center dispersion anomalies characteristic of a condensed Peierls ground state.     

Vibrational properties of single-wall BC3 nanotubes

The phonon dispersions of single-wall BC₃ nanotubes with any chirality are calculated within a symmetry-based force constant model of the lattice dynamics. Based on the non-symmorphic symmetry group of the BC₃ tubes, the symmetries and number of the Raman- and infrared-active modes at Γ point of the one-dimensional Brillouin zone are given. The neighbor atom-atomic interaction force constants are recalculated by fitting them to the experimental phonon energy-dispersion curves of honeycomb BC₃ sheet. The frequencies of the optically active modes are presented as the function of diameters and chiralities for BC₃ tubes. The obtained phonon density-of-states spectra, phonon dispersion relations, and vibrational patterns of the zone-center phonons are presented and discussed in detail. The calculated frequencies of infrared-active modes are compared with the experimental values reported in the literature. The results provide comprehensive information about the vibrational properties of the BC₃ tubes and shed light on the interpretation of Raman scattering and infrared spectroscopies.     

Effects of the phase transition in Hg2(Br,I)2 crystals

The effects of the phase transition in Hg₂(Br,I)₂ crystals have been investigated over a wide range of temperatures by the Raman scattering spectroscopy and X-ray diffraction analysis. The overtones (at the X point of the Brillouin zone boundary) and the fundamental tones (at the center of Brillouin zone) of soft modes are found in the Raman spectra of these crystals and studied in detail. The density of one-phonon states of the soft TA branch manifests itself in the Raman spectra of mixed crystals. The potentialities of the soft-mode spectroscopy are realized in full measure. Analysis of the ratio between intensities of overtones and fundamental tones of the soft modes has demonstrated the applicability of the Landau phenomenological theory of phase transitions. The orthorhombic splitting of the reflections corresponding to the basal plane is revealed in the X-ray diffraction patterns and thoroughly explored. The temperature dependences of the isotropic and shear spontaneous strains are obtained. It is shown that the shear spontaneous strain plays a decisive role. The critical indices are determined and the model of the improper ferroelastic phase transition D _4h ¹⁷ → D _2h ¹⁷ in the vicinity of the tricritical point is corroborated.     

Hydrogen storage: Lattice dynamics of orthorhombic NaMgH3

In this work, we investigate the structural, dynamic and thermodynamic properties of NaMgH₃, devoted for hydrogen storage. Density functional theory using pseudopotential methods and generalized gradient approximation has been used. A good agreement between the calculated structural parameters and the experimental data was found. A linear-response approach for the density functional theory is used in order to derive the Born effective charge tensors, the dielectric permittivity tensors, the phonon frequencies at the center of the Brillouin zone, the phonon-dispersion curves and the corresponding density of states for NaMgH₃ material. The obtained phonon frequencies at the zone center (Γ point) for the Raman-active and infrared-active modes are analyzed. Thermodynamic functions using the phonon density of states are also calculated.     

Observation of nuclear wave packets in the excited state of the F-center in alkali halides

Oscillations of nuclear wave packets (NWPs) in the excited state of the F-center in four alkali halides, potassium chloride (KCl), potassium bromide (KBr), potassium iodide (KI), and rubidium chloride (RbCl), are investigated. In KCl, the dominant mode is the LO mode near the zone center in the Brillouin zone. In KBr, the NWP oscillation consists mainly of the LO mode near the zone center and the LA modes near the singularity points in the [1 0 0] and/or [1 1 0] crystal directions. In KI, the totally symmetric local mode is dominant in the NWP oscillation. In RbCl, the LO mode near the zone boundary [1 0 0] predominantly forms the NWP oscillation. We comprehensively elucidate correspondences and differences between the present results and the resonance Raman spectra by considering the spatial extension of the electronic wave function and the perturbed phonon density of states.     

Ab initio calculations of the vibrational spectra of AgInSe2 and AgInTe2

The phonon spectra and densities of states of AgInSe₂ and AgInTe₂ semiconductor crystals with a chalcopyrite structure have been calculated from first principles by the linear response method. The frequencies calculated at the center of the Brillouin zone are in agreement with the experimental data obtained using IR and Raman spectroscopy. According to the atomic contributions to the vibrational modes, the spectra of the AgInSe₂ and AgInTe₂ crystals exhibit three groups of bands: the vibrations in the low- and medium-frequency ranges are mixed in character with approximately identical contributions of all sublattices, and the bands at higher frequencies are associated with the contributions of Ag, C ^VI and In, C ^VI (C ^VI = Se, Te) atoms. The position of these bands allows us to make the inference that, in the crystals under investigation, the In-C ^VI bonding is stronger than the Ag-C ^VI bonding.     

First-principles study of lattice dynamics of LiFePO4

Lattice dynamics of lithium iron orthophosphate (LiFePO₄) isostructural with olivine have been investigated using the first-principles calculations taking into account the on-site Coulomb interaction within the GGA + U scheme. Born effective charge tensors, phonon frequencies at the Brillouin zone center and phonon dispersion curves are calculated and analyzed. The Born effective charge tensors exhibit anisotropy, which gives a convincing evidence for the one-dimensional Li migration tunnel along the [010] direction in LiFePO₄, which has been proposed by other theoretical calculations and experimental observation. The calculated phonon frequencies at the Γ point of the Brillouin zone show good agreement with the available experimental observations.     

Dispersive phonon modes by resonant Raman scattering in AgBr

Dispersive Raman modes are observed in AgBr at low temperatures when the incident laser light is tuned across the indirect Γ-L^′₃ exciton absorption. These modes occur as sidebands of the resonantly enhanced 2 TO(L), TO+LA(L) and 2 LA(L) Raman peaks and are interpreted as being due to three- and four-phonon processes, respectively. They involve longitudinal acoustic phonons near the Brillouin zone center that exhibit strong dispersion. From analysing the spectra the relaxation of the intermediate exciton state can be studied in detail. The model applied enables us to obtain directly the average effective mass of the ls indirect exciton as 1.5 electron masses.     

First-principles study of the phonon spectrum of ɛ-GaSe

The phonon spectrum and phonon density of states of ?-GaSe layered semiconductor have been studied from the first principles in the linear-response approximation. The elastic constants and acoustic velocities along and across layers have been determined. The study of the equilibrium structure and phonon spectrum of the (0001) surface of ?-GaSe has demonstrated that the volume and surface structural dynamic properties of these crystals differ insignificantly. The calculated frequencies and symmetries of the phonon modes in the center of the Brillouin zone are in good agreement with the experimental data obtained from the Raman scattering and infrared spectra.     

First order phonon and magnon Raman scattering in KCoF3

At T=2°K, a magnon and a phonon Raman line is observed respectively at 38.5 and 56.7 cm^?1. The last one arises from an acoustic phonon at R point in the paramagnetic Brillouin zone, coupled to the magnon in the magnetically ordered phase.     

Structural and phonon dynamical properties of perovskite manganites: (Tb, Dy, Ho)MnO3

A shell model has been used to study the structure, phonon dynamics and phase coexistence of perovskite manganites RMnO₃ (R=Tb, Dy, Ho). The calculated crystal structure, Raman and IR frequencies and specific heats are found to be in good agreement with the available experimental data. The phonon density of states, elastic constants, elastic stiffness, shear constants and phonon dispersion curves have been computed for these manganites. A zone center imaginary A_u mode is revealed in these phonon dispersion curves, which indicates the occurrence of the metastability of the perovskite phase. The Gibbs free energy values calculated as a function of temperature and pressure for RMnO₃ in the orthorhombic phase, when compared with those of the hexagonal phase, reveal the possibility of coexistence of these two phases in the present multiferroic manganites under ambient conditions.     

Zone-folding effects on phonons and excitons in polytypic BiI3 single crystals

Resonant Raman scattering (RRS) of new mode phonons in BiI₃ crystals containing stacking faults is reported. Under excitation with laser light tuned to a characteristic sharp absorption line, new mode Raman lines show the resonance behavior. The new phonon modes and the origin of the sharp absorption line can be interpreted in terms of folding back effects of the Brillouin zone for phonons and excitons in a polytypic structure     

First-principles investigation of the phonon spectrum of β-GaS

This paper presents the results of the first-principles density functional theory calculation of the phonon spectrum of the ??-GaS semiconductor with a layered structure. The elastic constants and velocities of sound along and across the layers of the ??-GaS semiconductor have been determined. Investigation of the equilibrium structure and the phonon spectrum of the (0001) surface of the ??-GaS crystals has demonstrated that the bulk and surface structural and dynamical properties of these crystals differ only slightly. The calculated frequencies and symmetries of phonon modes at the center of the Brillouin zone of the semiconductor are in satisfactory agreement with the experimental data obtained from the Raman and infrared spectra.     

Phonon instabilities and the ideal strength of aluminum

We have calculated the phonon spectra of aluminum as a function of strain using density functional perturbation theory for <110>, <100>, and <111> uniaxial tension, as well as relaxed <112>[111] shear. In all four cases, phonon instabilities occur at points away from the center of the Brillouin zone and intrude before the material becomes unstable according to elastic stability criteria. This is the first time the ideal strength of a metal has been shown to be dictated by instabilities in the acoustic phonon spectra. We go on to describe the crystallography of the unstable modes, all of which are shear in character. This work further suggests that shear failure is an inherent property of aluminum even in an initially dislocation-free perfect crystal.     

Raman detection of phonon-phonon coupling in GaxIn1−xP

The coupling between the zone center phonons and continua has been observed and interpreted from the changes in the line shape of the TO Raman spectra and the existence of a side band on the low energy side of the LO phonon in mixed Ga_xIn_1?xP.     

Brillouin scattering investigation of paraelectric KH2PO4 under applied uniaxial stress

The Brillouin scattering spectra of KH₂PO₄ under an applied uniaxial stress have been studied in the neighborhood of the ferroelectric transition. Dependence of shear acoustic phonon on the uniaxial stress is similar to dependence on electric field. The acoustic phonon is overdamped near T_c, but become underdamped under the applied shear stress. In the case of ΔT(= T — T_c) < ～ 0.05 K, application of stress caused a transition not to have any soft acoustic phonon.     

The effects of incommensurability on the phonon modes in the one-dimensional superionic conductor K-hollandite

Polarized Raman and infrared spectra of the one dimensional (1-D) superionic conductor (K_2xMg_xTi(_8?x)O₁₆; x = 0.77) have been measured over the phonon frequency region 10–1000 cm^?1 as a function of temperature and pressure. The majority of the IR and Raman active modes predicted by group theory for the (Ti, Mg)O₆ framework were observed. The frequencies and their IR and Raman scattering cross-sections for the incommensurate lattice of K⁺ ions were calculated using a one dimensional linear chain model. This model assumes Coulomb interaction between nearest neighbors that are located in a sinusoidal potential due to the framework lattice. Several broad Raman bands were attributed to amplitudon type modes from the one dimensional incommensurate K⁺ ion sublattice. The IR active phason modes could not be identified unambiguously due to the underlying TiO framework vibrations which are known to possess large anharmonicity and oscillator strenghts.     

Lattice dynamics of room temperature orthorhombic SrHfO3

The lattice dynamics of room temperature orthorhombic phase of SrHfO₃ has been computed, using density functional perturbation theory. The full set of zone center phonon frequencies and infrared reflectivity spectra were reported, providing benchmark theoretical results for the interpretation of future experimental phonon measurements. It is found that the static dielectric constant is relatively anisotropic. This anisotropy has been discussed in relationship to the oscillator strengths and contributions of the lowest IR active modes.     

Vibrational spectra and elastic,piezoelectric, and magnetoelectric properties of HoFe3(BO3)4 and HoAl3(BO3)4 crystals

Raman spectra of light are obtained for HoFe₃(BO₃)₄ and HoAl₃(BO₃)₄ crystals at various temperatures and are used for determining the frequencies of crystal lattice vibrations at the center of the Brillouin zone. It is also found that the HoFe₃(BO₃)₄ crystal exhibits a phase transition at T _c ≈ 366 K. The magnetoelectric effect in the paramagnetic phase of these compounds is studied experimentally. The lattice vibration frequencies, elastic and piezoelectric moduli, Born dynamic charges, and the high-frequency permittivity are calculated using the density functional method. A peculiar behavior of the transverse acoustic vibration branch is observed in the Γ → Z direction of the Brillouin zone of the HoFe₃(BO₃)₄ crystal. The electric polarization induced by an external field is estimated using the calculated values of piezoelectric moduli and experimental values of magnetostriction.