Raman Spectra Of Crystals Research Articles

Raman study of the conformational instability of a ferrocene molecule at high pressure: Influence of a crystal field

AbstractThe Raman spectra of ferrocene crystals were measured at pressures up to 20 GPa, and an abnormally large bandwidth of intermolecular phonons at ambient pressure was found. With an increase in the pressure, the bandwidth increased to a maximum at ~2 GPa and then decreased to a minimum at ~4 GPa, which was equal to the pressure‐independent bandwidth of intramolecular phonons. The unusual behavior of the bandwidth was related to the instability of a ferrocene molecule caused by jumps between its D5d and D5h conformations. A decrease in the time of jumps between the conformations to the period of crystal lattice vibrations led to a loss of coherence and broadening of intermolecular phonon bands. The energy barrier between the conformations was determined to be ~17.6 meV/molecule under ambient conditions and 80 meV/molecule at 4.9 GPa. An increase in the barrier with pressure was due to the enhancement of the crystal field, which resulted in the inhibition of the jumps and the stabilization of the molecule in the D5d conformation.

Raman scattering of omphacite at high pressure: towards its possible application to elastic geothermobarometry

Abstract Due to their widespread occurrence in several geological settings, omphacite inclusions could be used for elastic Raman geothermobarometry. However, the Raman scattering of complex silicate minerals entrapped in a host depends on both the chemical composition and elastic strain developed during the metamorphic pathway, which makes the task very challenging. Here, as a very first step to probe the potential of omphacite to be used as a mineral inclusion in elastic geothermobarometry, we report the pressure dependence of the Raman spectra of omphacite crystals with the same composition, approximately Jd45Di55, but having different symmetry because of the existence (P2/n) or absence (C2/c) of chemical order at the six- and eight-coordinated cation sites. The experimental results are complemented by ab initio quantum mechanical simulations on fully ordered omphacite (Jd50Di50). We demonstrate that the position of the well-resolved Raman peak near 688 cm-1, arising from Si-O-Si bond bending, is very sensitive to pressure but independent of the state of chemical order, which makes it promising to be utilized in Raman geobarometry. The width of this peak varies with chemical order but not with pressure and therefore can be used to constrain the temperature of inclusion entrapment, because the chemical order is indicative of the closure temperature of the cation-exchange reaction. However, further detailed analyses on the compositional variation of the Raman spectra of omphacite is required before considering omphacite-in-garnet systems as suitable for Raman elastic geothermobarometry.

Effect of Thickness and Thermal Treatment on the Electrical Performance of 2D MoS2 Monolayer and Multilayer Field-Effect Transistors

We deposited high-quality molybdenum disulfide (MoS2) monolayer and multilayer crystals on SiO2/Si substrates, by means of a chemical vapor deposition (CVD) process at atmospheric pressure. Notably, NaCl salt was used as component of the precursors to assist the growth of MoS2 crystals, which were intended for use as the active channel layer in the fabrication of field-effect transistors (FETs). The resulting MoS2 crystals from this CVD process were analyzed by optical, scanning electron, and atomic force microscopies, and by Raman and photoluminescence spectroscopies. The optical images and the micrographs obtained by SEM revealed the formation of dispersed MoS2 crystals with a triangular shape all over the SiO2 surface. The thickness of the MoS2 crystals, analyzed by atomic force microscopy, showed minimum values of around 0.7 nm, confirming the formation of monolayers. Additionally, multilayers with larger thickness were also identified. The Raman and photoluminescence spectra of the MoS2 crystals corroborated the formation of single and multiple layers. The fabrication of the FET back-SiO2 -gate configuration was made by depositing patterned source and drain Ti contacts on the dispersed MoS2 crystals to achieve the Ti/MoS2/SiO2/Si layer stacks. MoS2-based FETs with one and three layers were assembled and their electrical response analyzed by I–V output and transfer curves showing the typical characteristics of an n-type semiconductor channel operating in depletion mode. The electrical performance parameters of the devices, such as mobility and threshold voltage, were also determined from this analysis. Finally, to enhance their electrical response, the MoS2-based devices were thermally annealed at 200 °C for 30 min in Ar atmosphere. The increase in the mobility of the device was 176% compared to the device before the treatment.

New assignments of Raman spectra of tetrahedra and the effects of electronegativity. II. Silicate glasses

Our density functional theory calculations (by GAUSSIAN) on the Si4O104− moiety and Na4Si4O10 molecule reproduce the experimental Raman frequencies and relative intensities for the three A1 modes in the experimental Raman spectra of Na2Si2O5 and BaSi2O5 crystals and glasses. Taking the center of mass into consideration, accompanied by visualization using GaussView, these calculations provide a firm assignment of these three peaks: the 1060 cm−1 peak to the Si-NBO stretch (NBO = Si–O or Si–O–Na), the ∼600 cm−1 peak to the Si-BO stretch (BO = Si–O–Si), and the ∼500 cm−1 peak to the Si-BO-Si bend. Our GAUSSIAN calculations on modified “non-equilibrium” SiO44− tetrahedra, constrained to have the same Si–O bond lengths and O–Si–O bond angles as in the Si4O104− moiety, confirm the above-mentioned assignment and show reasonable agreement with the experimental Raman frequencies and relative intensities. We then examine the regular trends in the Si-NBO and Si-BO stretch frequencies for all Qn (n = 0–4) species in silicate glasses and crystals. The large systematic change in the differences between these frequencies (Δ1) is explained by electronic changes within the SiO4 tetrahedron. For example, the larger Δ1 for the Q3 tetrahedra than the Q2 tetrahedra is due to the changes in Si, BO, and NBO electron densities on the tetrahedral Si and O atoms, as measured by Si 2p and O 1s x-ray photoelectron spectra. This trend strongly suggests that the Si-BO stretch frequency for the Q4 species in v-SiO2 should be ∼500 cm−1, close to the A1 breathing mode frequency of 495 cm−1 for four membered rings previously assigned.

Calculation of the Phonon Spectrum of PbMnBO4 Crystal Using Density Functional Theory

The phonon dispersion and Raman spectrum of the PbMnBO4 ferromagnetic crystal have been calculated within the density functional theory. Imaginary phonon branches have been observed at the points Y, Z, and Г and along the X–S direction of the Brillouin zone, which indicates structural instability and a possible phase transition with variation in external factors (temperature and pressure). The shapes of vibrations and symmetry types of the normal modes of the crystal at the center of the Brillouin zone have been determined. The calculation results are compared with the experimental and theoretical spectra from other studies. It is shown that the vibrational mode of highest intensity at 692.5 cm–1 in the spectrum and the mode at 272.3 cm–1, corresponding to the experimental modes at 690.5 and 224.7 cm–1, are bending vibrations of oxygen atoms in distorted MnO6 octahedra.

A comprehensive study on intrinsic alpha corpuscular radiation damage in monazite crystals using picometer-scale imaging, coupled with SCXRD and Raman spectroscopy

We have investigated monazite crystals, known to experience lattice distortions due to alpha particle emission from their atomic nuclei. Our aim in this investigation is to study the complex relationship between the lattice parameters and Raman spectra of monazite crystals. Through this study, we seek to understand the interdependency between these physical parameters and their influence on one another for an improved understanding of radiation-induced damage in these crystals. In addition to this, we also attempted to establish a correlation between these properties and their picometer-scale images to gain a deeper understanding of the structural changes that occur at the atomic scale. We conducted our study on four monazite samples withstanding intrinsic alpha decay radiation, denoted as M1, M2, M3, and M4, with crystal unit cell volume ranging from 299.961 Å3 to 301.96 Å3. Our Pearson statistical analysis revealed a correlation R2 (0.96) between the SCXRD-derived Ce–P Distance of monazite and the FWHM of PO4 band active mode in Raman spectra. This indicated lattice distortions due to alpha decay radiation impacting the Raman spectra. While Raman PO4 band broadening can be a consequence of impurities, dopants, and radiation damage, HRTEM scrutiny of the samples at picometer scales revealed the presence of point defects, plane rotation, and lattice distortions within the samples, suggesting the impact of alpha decay on the crystal lattice. HRTEM analysis has added additional confirmation that the major contribution to PO4 band broadening is due to alpha radiation damage in these samples. Through our study, we conclude a multiple-approach correlation is necessary to accurately study radiation damage dynamics at atomic scales.

Crystal Structure, Raman, FTIR, UV-Vis Absorption, Photoluminescence Spectroscopy, TG-DSC and Dielectric Properties of New Semiorganic Crystals of 2-Methylbenzimidazolium Perchlorate.

Single crystals of 2-methylbenzimidazolium perchlorate were prepared for the first time with a slow evaporation method from an aqueous solution of a mixture of 2-methylbenzimidazole (MBI) crystals and perchloric acid HClO4. The crystal structure was determined by single crystal X-ray diffraction (XRD) and confirmed by XRD of powder. Angle-resolved polarized Raman and Fourier-transform infrared (FTIR) absorption spectra of crystals consist of lines caused by molecular vibrations in MBI molecule and ClO4- tetrahedron in the region ν = 200-3500 cm-1 and lattice vibrations in the region of 0-200 cm-1. Both XRD and Raman spectroscopy show a protonation of MBI molecule in the crystal. An analysis of ultraviolet-visible (UV-Vis) absorption spectra gives an estimation of an optical gap Eg~3.9 eV in the crystals studied. Photoluminescence spectra of MBI-perchlorate crystals consist of a number of overlapping bands with the main maximum at Ephoton ≅ 2.0 eV. Thermogravimetry-differential scanning calorimetry (TG-DSC) revealed the presence of two first-order phase transitions with different temperature hysteresis at temperatures above room temperature. The higher temperature transition corresponds to the melting temperature. Both phase transitions are accompanied by a strong increase in the permittivity and conductivity, especially during melting, which is similar to the effect of an ionic liquid.

Structure particularities and first, second order Raman spectra of LiNbO3:Tb near the concentration threshold

Defective structure particularities have been studied in pure non-stoichiometric (Li/Nb≠1) lithium niobate (NSLN) crystals and LiNbO3:Tb (2.24 wt%) crystal. NSLN Raman spectra contain low-intensity extra bands in the area 100-1000 cm−1 at different scattering geometries. The bands are not provided by the selection rules for LN. The bands are caused by static disorder in cation sublattice of crystals. Besides, we have established additional bands in the area 1300-2000 cm−1 of a LiNbO3:Tb (2.24 wt%) crystal Raman spectrum. The bands correspond to overtone states of crystal lattice polar fundamental modes. Second-order Raman spectrum of LiNbO3:Tb contains bands with frequencies higher than that of overtones of the highest-frequency fundamental polar modes with А1(Z) and Е(X,Y) symmetry type. The particularities are manifestation of bonded states of polar modes – bipolaritons; they convert in two entangled photons or in a bonded state – biphoton at an exit from the crystal.

Oxygen out, sulfur in: The alkali metal sulfidotungstates K2[WS4] and novel Na2[WS4] ⋅ 4 H2O

AbstractAn attempt to synthesize a holmium sulfidotungstate using potassium polysulfides as reactive flux resulted in orange single crystals of K2[WS4], isotypic to K2[MoS4] (orthorhombic; Pnma; a=935.10(3), b=694.22(2), c=1221.85(4) pm; Z=4). A similar attempt to obtain a neodymium sulfidotungstate with sodium polysulfide yielded a violet substance, which underwent hydrolysis yielding orange‐yellow single crystals of Na2[WS4] ⋅ 4 H2O (monoclinic, C2/c, a=1098.08(4), b=904.46(3), c=1144.27(4) pm, β=101.577(3)°, Z=4). Further attempts to synthesize these products deliberately as single phase materials only proved to be successful for K2[WS4] but not for Na2[WS4] ⋅ 4 H2O. Both compounds contain non‐condensed [WS4]2− tetrahedra, which surround the alkali metal cations with coordination numbers of nine and eight in K2[WS4]. In Na2[WS4] ⋅ 4 H2O, the sodium cations are coordinated by two sulfide anions plus four water molecules, resulting in a coordination number of six. Besides crystal structure determination, infrared and single crystal Raman spectra were collected and the optical band gap of K2[WS4] was determined.

Temperature-dependent Raman spectra of KTaO3 crystal and melt along with its phonon and electronic properties

In situ high temperature Raman spectra of KTaO3 crystal and melt were measured over the wavenumber range of 100–1600 cm−1 from ambient temperature to 1723 K by intensified charge coupled device (ICCD) coupled with high temperature observational furnace. The band structure, density of states (DOS), phonon density of states (PDOS) and phonon dispersion of KTaO3 crystal were calculated by density functional theory (DFT). According to the theoretical calculation, strong covalence of Ta-O bonds and band gap were confirmed, phonon branch assignments for the second order Raman spectrum of KTaO3 crystal were made. A series of model clusters were constructed and their Raman spectra were simulated by DFT to determine the microstructures of KTaO3 melt. The relation between TaO bond lengths and characteristic Raman-active vibration wavenumbers of the molten KTaO3 has been investigated by which Raman spectrum of KTaO3 melt was deconvoluted by a sum of Gaussian peaks based on DFT results. Results showed that the lattice framework collapsed and the second order Raman peaks disappeared while the crystal being melt. Different from previous conclusion that there was only one type of TaO complex, a mixture of TaO anion complexes was proved to coexist in the molten KTaO3 while disproportionation equilibrium being reached, and those complexes were identified in this work.

Characterization of crystallographic orientation and lattice disorder in hydroxyapatite thin films by Raman scattering

This report presents the way of evaluating the crystallographic orientation and crystallinity of hydroxyapatite (HAp) thin films by Raman scattering spectroscopy. According to the Raman selection rule, the symmetric stretching mode (ν1) of P−O bonds in an isolated PO43− units is Raman active, whereas the asymmetric stretching mode (ν3) is not. Nevertheless, a weak ν3 signal is usually observed in the Raman spectra of HAp powder crystals. We will show that the ν3 signal is entirely absent for uniaxially oriented HAp films, whereas it appears in the case of randomly-oriented ones. This principle is feasible for discriminating the uniaxial orientation from a random one in HAp crystals. Another simple approach for evaluating crystallinity is exploiting the line width of the ν1 PO43− Raman signal. If the full-width at half maximum (FWHM) of the ν1 signal is smaller than 7.4 cm−1, the HAp crystal is uniaxially oriented in one direction, whereas if the FWHM value is larger than 7.4 cm−1, the film is composed of randomly-oriented crystallites.

Conversion of 2-dimensional GaSe to 2-dimensional β-Ga2O3 by thermal oxidation

We demonstrate the conversion to quasi two-dimensional (2D) β-Ga2O3 by thermally oxidizing layered GaSe of different thicknesses (from bilayer to 100 nm). GaSe flakes were prepared by mechanical exfoliation onto Si with a 300 nm SiO2 layer, highly oriented pyrolytic graphite, and mica substrates. The flakes were then annealed in ambient atmosphere at different temperatures ranging from 600 °C to 1000 °C for 30 min. Raman spectroscopy confirmed the formation of β-Ga2O3 in the annealed samples by comparison with the Raman spectrum of a β-Ga2O3 reference crystal. Atomic force microscopy was employed to study the morphology and the thickness of the β-Ga2O3 flakes. In addition, we used energy dispersive x-ray spectroscopy together with scanning electron microscopy to investigate the evolution of the composition, especially Se residuals, and the sample topography with annealing temperature. β-Ga2O3 appears at temperatures above 600 °C and Se is completely evaporated at temperatures higher than 700 °C. The thicknesses of the resulting β-Ga2O3 flakes are half of that of the initial GaSe flake. Here we therefore present a straightforward way to prepare 2D β-Ga2O3 by annealing 2D GaSe.

Polar modes and overtone states in Raman spectra of lithium tantalate crystals

The conditions for the excitation of transverse and longitudinal polar modes in the processes of Raman scattering of light in mono- and polydomain lithium tantalate single crystals are established using a backscattering geometry. The relative intensity of the overtone Raman spectrum on the longitudinal and transverse fundamental polar excitations is much higher in the polydomain sample. Unfortunately, the exact reason for the difference in the Raman spectra of the mono- and polydomain lithium tantalate samples is unknown. The frequencies of the two overtone bands exceeded the exact value of the overtone frequency corresponding to the fundamental longitudinal polar mode $4{A}_{1}(\text{LO})$, which clearly indicates the existence of bound phonon pairs (biphonons). This is consistent with theory and has been shown for both mono- and polydomain samples. On the basis of the general theory of bound states of quasiparticles, the possibility of the existence of bound states of quadrupole excitations of the antipolar type in the vibrational spectrum of a polydomain crystal of lithium tantalate is predicted.

Morphological and structural study of crystals in black-to-brown glazes of Yaozhou ware (Song dynasty) using imaging and spectroscopic techniques

The Yaozhou kiln complex (Shaanxi, China) is a famous black-to-brown ceramic production center due to its significant volume of production, long manufacturing history (from Tang Dynasty to Yuan Dynasty) and variety of decorations. In this work, four representative types of black-to-brown ware from the Yaozhou kilns were selected to study the morphology, structure and distribution of the precipitated crystals in the glazes using a series of imaging and analytical techniques (optical microscopy, XRF, SEM-EDS, TEM-EDS, μRS and UV–vis-IR). The results show that the rare metastable ε-Fe2O3 phase, already observed in typical sauce glaze, was also detected in other types of brown glaze decorations, such as light brown stripes on a darker background or irregular brown spots on a black background. Cross-section analyses also showed a three well-separated layered crystalline distribution in the glazes of lighter brown colors as it has been noted in glazes from Qilizhen kilns. Raman analyses revealed that the crystals contained in third layer are of varying nature: ε-Fe2O3 in the sauce glaze and magnetite in the light brown stripes. The peak shifts and line broadenings observed in the Raman spectra of ε-Fe2O3 crystals were also investigated. They are the result of Al, Ti and Mg substitutions, which were identified using TEM-EDS. Such ionic substitutions would stabilize the metastable ε-Fe2O3 crystals by increasing the cationic disorder. In addition, Mg-, Ca- and Fe-rich spinel crystals were observed for the first time in a black glaze of Yaozhou wares.

Structural Phase Transition in (NH4)3GeF7–Raman Spectroscopy Data

We obtained Raman spectra of (NH4)3GeF7 crystals with a wide range of frequencies (10–3400 cm−1) and temperatures (8–300 K), including phase transition, which was accompanied by a symmetry increase with a temperature decrease. The internal vibrations of the GeF62− group were classified by the positional symmetry method. Considerable transformations of the Raman spectra were observed at the lower frequency range of lattice vibrations and the ranges of the internal vibrations of the ammonium ions. In contrast, the internal modes of the GeF62− groups changed only slightly due to their resonance splitting in the multiplied unit cell, which agreed well with the proposed phase transition mechanism induced by ammonium group ordering.

Effect of isotope disorder on the Raman spectra of cubic boron arsenide

Boron arsenide (c-BAs) is at the forefront of research on ultrahigh thermal conductivity materials. We present a Raman scattering study of isotopically tailored cubic boron arsenide single crystals for 11 isotopic compositions spanning the range from nearly pure c-$^{10}$BAs to nearly pure c-$^{11}$BAs. Our results provide insights on the effects of strong mass disorder on optical phonons and the appearance of two-mode behavior in the Raman spectra of mixed crystals. Strong isotope disorder also relaxes the one-phonon Raman selection rules, resulting in disorder-activated Raman scattering by acoustic phonons.

Hardness, Raman spectrum, thermal properties, and laser damage threshold of Y3Sc2Ga3O12 single crystal

Y3Sc2Ga3O12 (YSGG) single crystal has multifunctional applications in the areas of substrate, scintillator, and laser matrix owing to its unique features of structure, thermal, and optical properties. However, the some basic physical and optical properties of YSGG crystal remain unexplored. In this work, the Vickers hardness, Raman spectrum, thermal properties, as well as laser damage threshold of YSGG single crystal were determined for the first time, to the best of our knowledge. The thermal conductivity of YSGG crystal was decreases with the increase of temperature from 298 to 773 K, and the room-temperature thermal conductivity was determined to be 5.76 W m− 1 K− 1. With 355 nm pulse laser illumination, the laser damage threshold of YSGG crystal was determined to be 5.18 J cm−2. Besides, based on the measured Raman spectrum of YSGG crystal, ten Raman vibrations were assigned. The results provide some valuable insight into YSGG crystal, which are important for the further investigation and application of this crystal.

Structure, electrical conductivity, and Raman spectra of (Cu1–xAgx)7GeS5I and (Cu1–xAgx)7GeSe5I mixed crystals

The structure of (Cu1–xAgx)7GeSe5I mixed crystals with Cu+↔Ag+ substitution is described and compared to (Cu1–xAgx)7GeS5I employing the results of X-ray diffraction (XRD) and the atomic coordinates by Rietveld refinement. Based on the electrical measurements, the compositional behaviour of ionic and electronic conductivity as well as the ratio of ionic and electronic components of conductivity for (Cu1–xAgx)7GeS5I and (Cu1-xAgx)7GeSe5I is discussed. Raman spectra of (Cu1-xAgx)7GeSe5I and (Cu1-xAgx)7GeSe5I are consistent with the XRD data regarding their cubic structure. A one-mode compositional behaviour of the most prominent peak (corresponding to the vibrations of GeS4 or GeSe4 tetrahedra) under Cu→Ag cation substitution is observed, an unexpectedly strong shift of the peak frequency with x reveals its marked effect on the dynamics of GeS4 or GeSe4 tetrahedra. Compounds with high Ag content undergo photochemical surface transformations under laser irradiation during Raman measurements.

Raman Tensor of Layered SnS2.

Recently, tin disulfide (SnS2) has become a hot research focus in various fields due to its advantages of a high transistor switching ratio, an adjustable band gap in visible light range, excellent Li storage performance, sensitive gas recognition, and efficient photocatalytic capability. However, at present, studies of its basic structure mostly stay on the regulation related to the number of layers. To maximize the value of SnS2 in the application design, this paper analyzes the angle-resolved polarized Raman spectra of SnS2 crystals grown under high-temperature sealing systems. Under the parallel scattering configuration test of both the sample basal plane and the cross plane, we observed that how the Raman scattering intensity of the two test planes varies with the polarization angle is different. Combining this experimental result with theory support allows us to reach a conclusion that the differential polarizability of the phonon vibration mode along the z-axis of the cross plane of SnS2 is proven to be the strongest. This finding is expected to provide favorable support for the application of structural regulation of SnS2 and work as a reference for studying other van der Waals layered materials with greater potential.

Structural, optical, and mechanical properties of gamma beam-irradiated pure and CeCl3-doped potassium hydrogen phthalate (KHP) crystals for scintillating applications

Potassium hydrogen phthalate (KHP) is a high-energy scintillating material having superior properties. The solution crystal growth technique at room temperature is employed for growing undoped and CeCl3-doped KHP crystals. The 60Co γ-ray-irradiated crystals were analyzed through powder X-ray diffraction analysis to evaluate the cell parameters of the pure and doped KHP crystals. The functional groups and vibration modes were identified by Fourier transform infrared spectral analysis. The optical transparency of the grown crystals was analyzed by UV–vis spectroscopy which shows less absorption in the CeCl3-doped KHP crystals when compared to pure KHP. The vibration properties were discussed with the use of Raman spectra of pure and doped crystals. The photoluminescence analysis reveals broad peaks from green to violet ranges for undoped and CeCl3-doped KHP crystals. The microhardness test was carried out at different applied loads varying in the range 25–100 g. It is revealed that the Vicker’s hardness values (Hv) increase with increasing applied load. The Meyer’s index number (n) is determined and it confirms that the gamma-irradiated pure, CeCl3-doped KHP crystals belong to the soft material category. The mechanical properties such as the elastic stiffness constant (C11), yield strength (σv), brittleness index (Bi), and the fracture toughness (Kc) are measured for both pure and 1–4 mol% CeCl3-doped KHP crystals.