Hyper‐Raman and Raman scattering in paratellurite TeO2

Raman spectra of the tetragonal structure of paratellurite TeO₂ have been revisited avoiding anomalous polarization‐selection‐rules violations previously observed and due to optical activity. We present a complementary hyper‐Raman scattering study of paratellurite. Wavenumber and symmetry assignments are given for all expected 21 Raman active optical branches, except one LO component (out of the eight expected TO–LO pairs) of the polar doublet E modes. Also, the four expected hyper‐Raman active A₂ (TO) modes have been observed. Moreover, we have observed a strong Kleinman‐disallowed hyper‐Rayleigh signal, which is tentatively assigned as a first evidence of hyper‐Rayleigh optical activity. Copyright © 2013 John Wiley & Sons, Ltd.     

Temperature dependences of Raman scattering in different types of GaN epilayers

First-order Raman scatterings of hexagonal GaN layers deposited by the hydride vapour phase epitaxy and by metal-organic chemical vapour deposition on SiC and sapphire substrates are studied in a temperature range between 303 K and 503 K. The temperature dependences of two GaN Raman modes (A₁ (LO) and E₂ (high)) are obtained. We focus our attention on the temperature dependence of E₂ (high) mode and find that for different types of GaN epilayers their temperature dependences are somewhat different. We compare their differences and give them an explanation. The simplified formulas we obtained are in good accordance with experiment data. The results can be used to determine the temperature of a GaN sample.     

Raman scattering in the Bi2TeO5 single crystal

Polarized Raman spectra of the Bi₂TeO₅ single crystal have been investigated for the first time. The group-theoretic analysis of the first-order vibrational spectra is performed. The number of the experimentally observed bands is less than the predicted number of normal modes. The spectral ranges with similar bands are revealed. Some ranges in the spectra of Bi₂TeO₅ are identified from the spectral data for the materials containing bismuth-oxygen and tellurium-oxygen complexes.     

Temperature dependences of the dielectric properties of lithium-titanium ferrite ceramics

Temperature dependences of the real ?′ and imaginary ?″ parts of the complex permittivity of lithium-titanium ferrite ceramics are measured in the frequency range 10²?10⁶ Hz at different test-signal amplitudes and dc bias voltages. It is found that the dielectric characteristics of the ceramic samples drastically change in narrow temperature ranges. The assumption is made that relaxators whose reorientation is due to tunneling transitions of electrons inside “bivalent iron ion-trivalent metal ion” pairs are involved in polarization processes. Under certain conditions, the reorientation of relaxators can have collective character.     

Raman scattering studies on phase transition in barium sodium niobate at high pressure

Phase transition in barium sodium niobate at high pressure is investigated by Raman scattering experiments. Polarized Raman spectra from polished single domain crystal plates of definite orientation are obtained under pressure produced by a diamond anvil device. Anomalous hardening and intensity decrease of 32 cm^-1B₂ mode are found with increasing pressure. A remarkable pressure dependence of the frequencies of the bending modes of NbO₆ octahedrons is observed.     

A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.     

Raman scattering in sodium nitrite crystals near the phase transition

Optical Raman spectra of a ferroelectric sodium nitrite crystal have been detected in a wide spectrum range at various temperatures, including the region of the ferroelectric phase transition. A manifestation of a transverse soft polar mode of the A ₁(z) type responsible for the ferroelectric phase transition has been discovered in the spectrum at room temperature. This mode has been found to become overdamped even far from the ferroelectric phase transition temperature. This mode also appears as a central peak under heating. It has been found that the pseudoscalar mode of the A ₂ type has the highest intensity in the Raman spectrum of sodium nitrite. The frequency corresponding to the maximum intensity of this mode in the Raman spectrum varies from 130 cm^–1 at 123 K to 106 cm^–1 at T = 513 K. A fair agreement of the experimental data for the A ₁(z) mode with the Lyddane–Sachs–Teller relation has been established. The polariton curves for the A ₁(z) polar mode and the dispersion curves for axinons has been plotted.     

Vibrational spectroscopy at very high pressures—XX Raman and far-IR study of the phase transition in paratellurite

Raman and far-IR spectra have been obtained up to ca. 45 kbar [4.5 GPa] for paratellurite (Te0₂) in a diamond anvil cell. At the D₄⁴ to D₂⁴ phase transition ca. 9 kbar an E symmetry mode initially at 122cm^?1 was found to split into two components in both IR and Raman spectra: their coincidence in both types of spectra shows that one component in the Raman spectrum cannot be an LO mode. No other E modes were seen to split. A further phase transition claimed ca. 30 kbar was not confirmed.Paratellurite is exceptionally sensitive to the presence of non-hydrostatic stresses, which greatly influence the phonon frequencies. This behaviour is illustrated by Raman spectra obtained under varying degrees of shear stress.     

In2（MoO4)3晶体的高压拉曼振动研究

钼酸铟(In2(MoO4)3)由于其独特的负热膨胀性质,已被广泛应用于燃料电池、光学器件、激光材料等方面. 为了进一步探讨其晶体结构和物理性质,本工作在金刚石对顶砧上原位测量了In2(MoO4)3 的高压拉曼光谱,最高压力达到18 GPa. 在研究的压力范围内,本工作观察到了两次相变. 首先,In2(MoO4)3在压力为1.2 GPa 时, 发生了 由从P21/a相到低对称结构的相转变. 在压力为5.8 GPa 时, 样品又发生了从晶相到非晶相的第二次相转变. 卸压后样品仍为非晶相,说明In2(MoO4)3 在高压下的非晶相变为不可逆相变.      

Raman scattering and phase changes of CulnSe2 and LiInSe2 at high pressure

Abstract

We have investigated the effect of pressure on the Raman spectra of the ternary chalcogenides CulnSe₂ (chalcopyrite structure) and LiInSe₂ (β-NaFeO₂ structure) for pressures extending well above their first pressure-induced phase transitions. Sign and magnitude of Griineisen parameters are discussed by comparing to related tetrahedrally coordinated chalcogenides. Discontinuous changes of the Raman frequencies indicate pressure-induced phase transitions at 8.0±0.3 and 4.2±0.2 GPa in CuInSe₂ and in LiInSe₂, respectively. The Raman spectra of the low-pressure phases are not recovered after pressure release but a highly disordered structure is induced. In the case of LiInSe₂, the effect of laser heating on the Raman spectra of the high-pressure phase is investigated and discussed in light of recent high-pressure x-ray investigations.     

Raman scattering study of ferroelectric phase transition in ammonium sulphate

The interesting features of the temperature and pressure dependence of the Raman spectrum of ammonium sulphate are discussed with respect to its ferroelectric transition at 223 K. These include, the observation of quasi-elastic wing scattering and rapid line-width broadening of both internal and external phonon modes. A model of the transition based on these and previous results is indicated.     

Low-frequency Raman scattering study of the ferroelectric phase transition in the DKDP crystal

The low-frequency Raman scattering spectra of the DKDP ferroelectric crystal are studied in the temperature range 30–393 K. At temperatures above 150 K, the Raman spectra exhibit a central peak which reflects the lattice relaxation susceptibility. The width and integrated intensity of the central peak are derived from the experimental spectra. The critical slowing down of the relaxation response predicted by the Ginzburg-Landau-Devonshire theory is observed throughout the temperature range in which the central peak persists. Its integrated intensity does not, however, follow the predictions of the theory and reveals a strong temperature dependence in the ferroelectric phase and a weaker dependence in the paraphase. It is shown that the thermal activation law describes well the temperature dependence of the intensity of the central peak. An interpretation is proposed according to which the intensity of order parameter fluctuations is related to the activation barrier whose height is proportional to the deviation from the phase transition temperature.     

Investigation of ferroelectric phase transition for modified barium titanate in multilayer ceramic capacitors by in situ Raman scattering and dielectric measurement

The ferroelectric phase behaviors of modified BaTiO₃ in X7R multilayer ceramic capacitors (MLCCs) were investigated by dielectric measurements and in situ Raman scattering. The in situ thermo-Raman scattering shows that because of a residual stress existing in the MLCCs, the tetragonal to cubic phase transition for modified BaTiO₃ in MLCCs takes place over a wide temperature range of 373 K to 473 K, suggesting a diffuse-like characteristic which can be well explained by a modified phenomenological thermodynamic model, while the dielectric measurement indicates that the tetragonal to cubic transition occurs at 393 K. A disagreement exists between the two experimental results. Furthermore, a dc field-induced paraelectric to ferroelectric transition was identified by the two measurement techniques, but their phase-transition mechanisms are different. The dielectric measurement reveals the polar-micro-region to macro-domain transformation in the shell part of the core–shell structure while the in situ Raman scattering shows the cubic to tetragonal transition under a dc bias field. PACS 74.20.De; 77.22.Ch; 77.84.Dy; 77.80.Bh; 78.30.-j     

Temperature and pressure dependence of Raman active lattice mode frequencies of cubic C8H6(Br)2, evidence of a phase transition

The temperature dependence of the Raman active frequencies of the lattice modes of dibromocubane is measured below room temperature. The frequencies are linearly dependent on the temperature decreasing with increased temperature with a marked change in slope occurring at 200?K indicative of a structural phase transition. The pressure dependence of the frequencies measured at room temperature up to 24?KBar shows no evidence of a phase change. Density functional calculations of the structure and Raman frequencies of the internal modes of an isolated molecule of C₈H₆(Br)₂ indicate the molecule is slightly distorted from a cubic carbon structure.     

Brillouin scattering study of phase transitions in T1Ins2

The temperature dependence of the elastic coefficients of T1InS₂ has been investigated by Brillouin scattering measurements. For longitudinal phonons propagating in the (001) plane the variation of ΔC_∼∼(T_c−T) was found below the ferroelectric transition point (T_c=189 K). For propagation perpendicular to the (001) plane (C₃₃ mode) clear anomalies were observed at T=213 K and T=195 K. Between these temperatures the Brillouin peaks show broadening, which is attibuted to a coupling between the acoustic mode and an overdamped mode.     

Angle- and polarization dependences of the two-photon Raman scattering via biexcitons in CdS

Two-photon Raman scattering via virtually excited biexcitons is reported for CdS, in different scattering configurations and polarizations. If the crystallographic $\text{c}$-axis is perpendicular to the scattering plane, the results correspond to those found in cubic CuCl¹, whereas for $\text{c}$ in the scattering plane, the influence of mixed-mode polaritons in the final state is obvious.     

Brillouin and raman scattering studies of the melting transition in salol

Brillouin and Raman scattering studies of salol from room temperature to within 5 mK of the melting transition at T_m = 40.97°C are reported. Changes in the Brillouin shifts and linewidths were accurately determined by nonlinear least-squares fitting and deconvolution. A marked increase in the deconvoluted Brillouin linewidth (~400%) and a gradual softening (~20%) of the transverse acoustic modes were observed very close to T_m. The increase of the Brillouin linewidths was analyzed by a simple dislocation model assuming the hypersonic attenuation to be proportional to the concentration of thermally generated defects near T_m. The defect formation energy E_D(T) was computed from the temperature-dependent linewidth data, and was found to decrease significantly (~60%) near T_m, suggesting a cooperative effect producing a catastrophic growth of defects which brings about melting by destroying the long range order of the crystal. The conclusion that melting is mediated by a sudden growth of defect concentration near T_m was further strengthened by Raman scattering experiments in which 13 new Raman modes appeared close to T_m. These new modes are believed to be defect activated through breaking of the local symmetry of the crystal. A slight softening of the Raman modes (~5%) was observed close to the melting point.     

Temperature dependence of Raman scattering and anharmonic properties in LiNbO_3

The temperature dependence of the Raman spectrum in LiNbO \(_3\) is investigated from 100 to 700 K. The various sources of asymmetry of Raman bands and artefacts are discussed before analyzing the temperature dependence of A \(_1\) and E first-order phonon lines. The phonon frequency downshift and damping increase on heating are interpreted in terms of normal volume expansion and third- and fourth-order anharmonic potentials. Anharmonic contributions are highly anisotropic and mainly explain the temperature dependences of both frequency and damping of A \(_1\) optical vibrational modes along the ferroelectric axis. Results are consistent with Caciuc et al. (Phys Rev B 61:8806, 2000) predictions.