Raman scattering in low-dimensional HfTe5

Nine long-wavelength phonons have been observed by Raman scattering in the quasi-one-dimensional crystal HfTe₅. The assignment of these phonon modes is made on the basis of symmetry arguments using the correlation method. The similarity between the HfTe₅ and the ZrTe₃ chain-like crystal structures instigates the attribution of the most characteristic modes like the high-frequency chalcogen diatomic-mode and the low frequency rigid-chain mode. Although no new line appears even at 14 K, the irregular temperature dependence of the spectra suggests the possibility of a structural instability connected with a large peak in the electrical resistivity and with the X-ray diffraction anomalies.     

Lattice dynamics of the transition metal pentatellurides ZrTe5 and HfTe5

Fresh Raman data collected from the (001) face of a ZrTe₅ single crystal and from polycrystalline powders of ZrTe₅ and HfTe₅ are presented. A simple model assuming only central interactions between nearest neighbour atoms is used for lattice dynamical calculations at $\text{k}\text{=}\text{0}$. The model accounts for the observations and places emphasis on the virtually lacking dynamical coupling between the two basic structural units in these pentatellurides [trigonal-prismatic (MTe₃)_n rods and zig-zag (Te₂)₂ chains].     

Structural properties of ZrTe5 and HfTe5 as seen by powder diffraction

Unit cell dimensions and positional parameters for the HfTe₅ type structures of ZrTe₅, as derived from powder X-ray and neutron diffraction data, are reported at 10 and 293 K. Structural data for ZrTe₅ and HfTe₅ collected between 10 K and the decomposition temperature (T_d = 828 ± 10 and 830 ± 10 K, respectively) show no indications of any phase transitions in this temperature interval.     

Lacking Raman spectroscopic evidence for a structural phase transition in ZrTe5 at 141 K

The Raman spectra of ZrTe₅ are recorded at various temperatures between 10 and 500 K. The data give no evidence for a structural phase transition, in spite of appreciable temperature dependent effects in some of the Raman modes. The lack of Raman spectroscopic evidence for a low temperature structural phase transition in ZrTe₅ is also confirmed by powder X-ray and neutron diffraction data collected over 100 to 295 and 10 to 295 K, respectively. The present findings strongly suggest that the reported anomaly in the electrical resistivity at 141 K must reflect change(s) in the electronic band structure of ZrTe₅, and the temperature dependences of the Raman spectra could be caused by variations in the first order susceptibility.     

Thermoelectric power of HfTe5 and ZrTe5

The thermoelectric power and the electrical resistance of the two low-dimensional conductors, HfTe₅ and ZrTe₅ have been measured over the temperature range of 7K to 380K. The thermoelectric power for both materials is large and positive at high temperatures and then drops precipitously and crosses zero at the temperature of the anomalous resistivity peak, T_p. At lower temperatures, the thermoelectric power reaches a large negative peak and then decreases to zero in a metallic fashion. The abrupt change in thermopower, which occurs at T_p for both materials, is indicative of a phase transition where the carrier type changes from hole-like to electron-like.     

Evidence for Ruelle's turbulent crystal?

The nuclear quadrupole interaction at¹⁸¹Ta on Hf/Zr-sites in ZrS₂, HfS₂, HfSe₂, and HfTe₅ exhibits anomalies as a function of temperature, indicative of lattice instabilities. We interpret our observations as being due to static or low frequency structural distortions perpendicular to the layers and take these as evidence for Ruelle's turbulent crystal.     

Anomalous electrical resistivity in HfTe5

The group IVB transition-metal pentachalcogenide, HfTe₅, exhibits a peak of the electrical resistivity in the vicinity of 76 K.     

Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials

Structural details of the amorphous binary GeTe and ternary Ge₂Sb₂Te₅ (GST) phase-change materials are investigated with the aid of Raman scattering. In the case of the a-GeTe, a plethora of Raman bands have been recorded and assigned on the basis of a network structure consisting of corner- and edge-sharing tetrahedra of the type GeTe_4−nGe_n (n=0, 1, 2, 3, 4). Significant temperature-induced structural changes take place in this material even at temperatures well below the crystallization temperature. These changes tend to organize the local structure, in particular the coordination number of Ge atoms, so as to facilitate the amorphous-to-crystal transformation. The much simpler Raman spectrum of GST, characterized by one vibrational band, is accounted for by the dominance of the Sb₂T₃ component in Raman scattering; reasons about this explanation, as well as for the lack of any Te–Te bonds are briefly described.     

Pressure and temperature dependence of Raman scattering and optical absorption in crystalline S4N4

Raman scattering and optical absorption in crystalline S₄N₄ have been measured both as a function of pressure at 295 K and low temperatures. Polarized single crystal Raman data were also obtained as an aid in the assignment of the Raman active phonons. The pressure coefficients of the Raman active external and S-S stretching modes show a discontinuity near 7 kbar indicative of a second order phase change. The optical absorption edge at about 2.5 eV of a sublimed film of S₄N₄ shows red shifts of 1.3 × 10^?5 eV bar^?1 and 6.3 × 10^?4 eV K^?1 with pressure and temperature respectively. In the light of these results, the electronic, vibrational and structural properties of the crystal are discussed.     

Nuclear quadrupole interaction in the low dimensional hf-trichalcogenides

We report on Time Differential Perturbed Angular Correlation measurements of the nuclear quadrupole interaction of¹⁸¹Hf(β⁻)¹⁸¹ and its temperature dependence in the low dimensional Hf- trichalcogenides HfS₃, HfSe₃ and HfTe₃. In HfS₃ the temperature dependence of the precession frequency is a slowly decreasing function with a negative curvature, the deviation from axial symmetry of the electric field gradient is practically temperature independent. In HfSe₃ the NQI shows a peculiar behaviour; between 10K and 300K the quadrupole frequency and the asymmetry parameter increase, between 300K and 400K the frequency decreases while the deviation from axial symmetry increases, indicating a structural phase transition near room temperature. In HfTe₃ the quadrupole frequency is practically temperature independent below 600K and increases linearely at higher temperatures, the asymmetry parameter increases steadily in the temperature range from 10K to 600K and then decreases. There is a strong correlation between the strength and the asymmetry of the electric field gradient so that the system depends on one control parameter only.     

Investigation of structural phase transition in polycrystalline SrTiO3 thin films by Raman spectroscopy

Polycrystalline SrTiO₃ thin films were prepared by pulsed laser deposition technique. The phonon properties and structural phase transition were studied by Raman spectroscopy. The first-order Raman scattering, which is forbidden in SrTiO₃ single crystal, has been observed in the films, due to the structural distortion caused by strain effect and oxygen vacancies. The Fano-type line shape of TO₂ phonon reveals the existence of polar microregions in the STO thin films. The evolution of TO₂ and TO₃ phonons with temperature shows the occurrence of a structural phase transition at 120 K related to the formation of polar macroregions in the films.     

Formation of amorphous sapphire by a femtosecond laser pulse induced micro-explosion

We report on structural characterization of void-structures created by a micro-explosion at the locus of a tightly focused femtosecond laser pulse inside the crystalline phase of Al₂O₃ (R3c space group). The transmission electron microscopy (TEM), micro-X-ray diffraction (XRD) analysis, and Raman scattering revealed a presence of strongly structurally modified amorphous regions around the void-structures. We discuss issues of achieving the required resolution for structural characterization and assignment of newly formed phases of nano-crystallites by TEM, XRD, and Raman scattering from micro-volumes of modified materials enclosed inside the bulk of the host phase.     

Electric field modulated Raman scattering in CdS

We have observed electric field modulated Raman scattering by A₁ LO phonons in CdS. The field induced scattering is observed with a geometry in which Raman scattering by A₁ LO phonons is normally allowed. The interference of the field induced and allowed terms in the transition susceptibility leads to a modulated Raman scattering intensity proportional to the applied field. This is contrasted with data previously reported on field induced Raman scattering by E₁ LO phonons in a configuration in which the Raman scattering is normally forbidden and in which there is no interference between linear wavevector dependent and field induced terms in the transition susceptibility. Electric field effects on Raman scattering by TO phonons and by 2 LO phonons is also discussed.     

Effects of rapid thermal annealing on the structural properties of TiO2 nanotubes

The structural properties of TiO₂ nanotubes with rapid thermal annealing (RTA) and traditional thermal annealing in O₂ were studied by X-ray diffraction (XRD) and Raman scattering measurements. From analyzing the line width of XRD and the correlation length of the Raman peak, we demonstrate that RTA can be an effective tool for amorphous-anatase transformation in TiO₂ nanotubes. The Raman peak redshifts and reduces its line width after thermal annealing and RTA, which may involves the reduction of oxygen-related defects.     

Magneto-optical conductivity study in three-dimensional Dirac semimetals of ZrTe<Subscript>5</Subscript>

The three-dimensional (3D) Dirac semimetal material of ZrTe₅ provides a possible platform for studying 3D Dirac fermions. It can realize both the point-node semimetal phase and line-node semimetal phase when the intrinsic Zeeman interaction acts along different crystalline directions. In this work we present a study of magneto-optical conductivity in ZrTe₅. We calculate the optical conductivities in different phases, which exhibit a series of resonant peaks lying on a growing background whose origins are explored. For the Weyl semimetal phase, two striking signatures are found, one is the existence of an additional n = 0 LL transition and another is the double-strong peak structure related to the LL transition in one dispersion branch. While in the line-node semimetal phase, the weak peak appear and the strong peaks have higher degeneracy. We discuss the implications of these results in experiment.     

Collective excitations in superconductors with structural transition

In this paper we have found new low-frequency collective excitations in superconductors with structural transformation. The connection between our results and the experiments on Raman scattering in 2H-NbSe₂ cDw- superconductor is discussed.     

ZnWO4晶体中W原子的配位数

本文依据X射线结构数据和Raman光谱测量,分析了闪烁体ZnWO₄单晶中W原子的配位情况。得出在属黑钨矿结构的ZnWO₄中,认为W的配位数Z为6较4要合适些,即把发光中心看作是WO₆原子基团比WO₄^-2离子更合适些。同时得到四条ZnWO₄晶体的Raman新谱线。 关键词：     

Molecular vibrations of the layered-perovskite cobalt oxides characterized by infrared and Raman spectroscopies coupled with crystal structure refinement

Molecular vibrations of the polycrystalline Nd_2−xSr_xCoO₄ are systematically characterized by powdered x-ray diffraction, Raman scattering and infrared spectra at different dopings as well as at different temperatures. Structural refinements reveal that all the specimens crystallize in the K₂NiF₄ structure with space group I4/mmm as is also confirmed by the phonon bands of Raman scattering and infrared transmittance. The frequency shifts of the phonon bands are found to be very well consistent with the elongation and/or contraction of the bond lengths. Moreover, for Nd_0.75Sr_1.25CoO₄, the phonon bands of the Raman active modes seem to retrace the evolutions of magnetic and structural temperature.     

The Raman spectrum of the γ′‐V2O5 polymorph: a combined experimental and DFT study

Structure and vibrational dynamics of γ′‐V₂O₅ synthesized from a pristine γ‐LiV₂O₅ sample via a chemical oxidation route was studied by means of Raman spectroscopy and quantum‐chemical calculations. The calculations based on density functional theory reliably reproduce the experimental structure of the γ′‐V₂O₅ lattice. The calculated Raman spectrum agrees remarkably well with the experimental one. Making use of the agreement, a complete assignment of Raman bands to vibrations of particular structural units is proposed. The comparison of Raman spectra and structural features of α‐V₂O₅ and γ′‐V₂O₅ polymorphs allowed establishing reliable ‘structure–spectrum’ correlations and identifying Raman peaks characteristic for different structural units. Copyright © 2015 John Wiley & Sons, Ltd.     

Lattice instabilities in HfTe5

We report on the determination of the nuclear quadrupole interaction of¹⁸¹Ta at Hf-sites in single crystals of HfTe₅ between 10K and 900K. Apart from the well-known low temperature anomaly at 75K — which can be easily suppressed by neutron irradiation induced dislocations — we discovered three new phase transitions.