Raman Spectra Of Single Crystals Research Articles

Defect studies in transition metal dichalcogenide MoSe1.8S0.2 using resonant Raman spectroscopy

Using resonant Raman spectroscopy with 632.8 nm (1.96 eV) laser, we describe the Raman spectra of single crystals of transition metal dichalcogenides with nominal composition MoSe(2−x)Sx for x = 0.2. Changes in Raman spectra at some regions of the sample indicated non-stoichiometry and, in particular, chalcogenide vacancies. At low temperatures around 77 K, we observed unusual temperature dependent enhancement in the intensity of non-zone center modes as well as overtones and combination modes in Raman spectra. This enhancement in the intensity is correlated to the resonance achieved in the non-stoichiometric regions of the crystal at low temperatures. Observed resonance is attributed to modification in the electronic structure due to defects. Energy dispersive x-ray spectroscopy measurements confirmed chalcogenide vacancies in the crystals. The change in the electronic structure due to defects is also corroborated by photoluminescence spectroscopy measurements.

Synthesis under high pressure: crystal structure and properties of cubic Dy36O11F50[AsO3]12 ∙ H2O.

The multi-anionic compound with the composition Dy36O11F50[AsO3]12 ∙ H2O, which can be described in the non-centrosymmetric cubic space group F c, already shows an unusually large unit cell with an axis of a = 2587.59(14) pm. Its crystal structure exhibits isolated ψ1-tetrahedral [AsO3]3- anions, but both the coordination numbers and the linking schemes of the Dy3+-centered polyhedra differ significantly from the mostly layered structures described so far in literature. (Dy1)3+ is sevenfold coordinated by oxygen atoms and F- anions, forming a capped trigonal prism [(Dy1)O4.333F2.667]8.333-, and the remaining two cations (Dy2)3+ and (Dy3)3+ both reside in an eightfold coordination of anions. In both cases they form slightly distorted square antiprisms, which have the compositions of [(Dy2)O3.667F4.333]8.667- and [(Dy3)O4.667F3.333]9.667-, respectively. Some of the oxygen atoms are not part of ψ1-[AsO3]3- tetrahedra, but occur as O2- anions and one of these even shares a common crystallographic position with fluoride (F-). It is also worth mentioning that the single crystals were obtained as comparatively large cubes with an edge length of several 100µm providing very good data with regard to single-crystal X-ray diffraction. To verify the simultaneous presence of oxygen and fluorine, electron-beam microprobe analysis was carried out, and a single-crystal Raman spectrum ruled out the presence of hydroxide anions or protonated [AsO3]3- groups, but proved the interstitial crystal-water molecules, which could not be determined precisely by the crystal-structure refinement.

The Evident of Coupled Spin and Electron-phonon Interactions in Cd2Os2O7 Pyrochlore

We present the results of the temperature dependence of the Raman spectra of pyrochlore Cd2Os2O7 single crystal in a temperature range of 100-270 K. We found that, among the six observed Raman active modes, the 762 cm-1 phonon has a significant peak shift and asymmetric Fano lineshape. The temperature-dependent of the frequency and full width at half maximum (FWHM) of the phonon shows the kinks near the Néel temperature. The complex temperature-dependent asymmetry Fano parameter and the anomalies of the FWHM below Néel temperature indicate coupling of the 762 cm-1 phonon to both electron and spin degrees of freedom. In addition, we found the Fano asymmetry rapidly drops below the Néel temperature towards its minimum value of ~ 200 K. Thus, at temperature ranging from 200 K to TN, the pyrochlore is suggested to be in an all-in -all-out ordered semimetallic state. We also confirm that Raman scattering temperature dependence provides a simple and powerful method for investigating temperature transitions and coupling in pyrochlore materials.
 
 

Structure, heat capacity and Raman spectra of Ba2MgWO6 single crystals grown in BaCl2-MgCl2 flux

We present a new method of Ba2MgWO6 single crystal synthesis that allows to grow larger crystals using a BaCl2 and MgCl2 flux. Difficulties to grow single crystals of a size suitable for macroscopic material property measurements caused the majority of characterisation being published on polycrystalline samples. Our single crystal diffraction and energy dispersive X-ray analysis confirmed high quality of synthesised samples. Heat capacity measurements from 300 K to 2 K do not show any phase transitions. However, Raman spectra measured down to 77 K contain additional weak peaks at all temperatures probed, which is in a contrast with only 4 Raman active modes expected from the crystal structure with the space group Fm3¯m. This calls for a more detailed study of potential symmetry breaking that could also influence the electronic properties of the material.

PCMW2D and 2D Raman correlation spectroscopy evidence for presence of spin-phonon coupling in hexagonal LuMnO3

Two-dimensional correlation spectroscopy (2D-COS) and perturbation-correlation moving window two-dimensional correlation spectroscopy (PCMW2D) analysis are performed on the temperature-dependent Raman spectra of hexagonal LuMnO3 single crystal. Under the resonance with the on-site Mn d-d transitions, the correlation between the phonons which are relate to the vibration of Mn ions’ bonds and spin-excitation peaks suggest a strong spin-phonon coupling in LuMnO3. The PCMW2D results clearly show that the significant change in phonons and spin-excitation peaks occurs around the Néel temperature and the spin reorientation transition. The multiple components in the broad spin-excitation peaks also suggest variations in spin symmetries in the ground state. Furthermore, we propose that the 2D-COS and PCMW2D Raman correlation spectroscopies provide a simple and powerful method for investigating the couplings and the transitions, which would be very important for understanding systematically the magnetoelectric properties of multiferroic materials.

Optical properties of extreme tellurium nanowires formed in subnanometer-diameter channels.

Single tellurium (Te) chains attract much attention as extreme nanowires with unique electronic and spintronic properties. Here, we encapsulate Te from a melt into channels of zeolites AFI (∼0.73 nm-channel diameter) and mordenite (MOR, ∼0.67 × 0.7 nm2 channel cross-section) via high-pressure injection. Using polarized Raman and optical absorption spectra (RS and OAS) of zeolite single crystals with Te (AFI-Te and MOR-Te), we discriminate between features of Te chains and rings formed in the zeolites. We demonstrate good agreement of AFI-Te-chain RS and OAS with the calculated single Te-helix phonon and electron spectra. This suggests a very weak interaction of the AFI-Te-chain with the zeolite and its nearly perfect helix structure lacking inversion/mirror symmetry. An AFI-Te OAS feature, attributed to the electron transitions between Te-helix-Rashba-split valence and conduction bands confirms its 1D-electron-band origin with predicted possibilities of identifying Majorana fermions, manipulating spin transport and realizing topological superconductivity.

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.

Raman activity of the longitudinal optical phonons of the LiNbO3 crystal: Experimental determination and quantum mechanical simulation

AbstractIn this study, the and the two equivalent / polarized Raman spectra of a LiNbO3 single crystal have been recorded and used as a benchmark test for the density functional theory (DFT) calculation of the longitudinal modes and of their Raman activity. The theoretical approach, based on periodic boundary conditions and a linear combination of atomic orbitals (LCAO), provides excellent predictions of phonon wavenumbers and relative bands intensities for both A1 and E Longitudinal Optical (LO) modes and complements a previous paper limited to the study of Transverse Optical (TO) modes. Overall, the present investigation demonstrates that the LCAO approach, as implemented in the CRYSTAL software, gives results of similar accuracy for the TO and the LO phonons features. By means of a band deconvolution scheme applied to the experimental spectra, we present, for the first time, a quantitative comparison between experimental and theoretically predicted polarized Raman band intensities of LiNbO3 LO modes. This analysis highlights the role of the suitable determination of the static and high‐frequency dielectric matrices that are needed for the prediction of the TO/LO frequency split but also the first nonlinear electric susceptibility tensor for an accurate description of the Raman intensity pattern.

Raman and optical absorption spectra of oriented Se8 and Se12 rings formed in zeolites: Dependence on the Se loading density

We encapsulated selenium (Se) into AFI and LTA zeolites via Se vapour adsorption at ∼450 °C and studied Raman and optical absorption spectra (RS and OAS, respectively) of zeolite single crystals with incorporated Se. At high Se loading density ( D ), Se 8 rings are formed in the AFI channels along with Se chains. The rings are oriented by their 4-fold axis parallel to channels. Se 8 is also formed in the LTA large cavities with its 4-fold axis oriented parallel to the 4-fold axis of LTA at any D up to ∼8 Se atoms per cavity (at/c). However, at D > 0.5 at/c, nearly unexplored Se 12 rings oriented by their 3-fold axis parallel to the LTA 3-fold axis are formed along with Se 8 in neighboring cavities. We discriminate between Se 8 and Se 12 rings via 1) RS frequencies; 2) RS and OAS band intensities vs. D ; 3) RS band intensities vs. excitation wavelength and light polarization configuration. With an increase in D , the number of Se 12 rings grows faster than that of Se 8 . We extract OAS of Se 8 and Se 12 from the experimental AFI-Se and LTA-Se OAS and show that the 3.0–3.3 eV absorption of Se 12 is enhanced compared to that of Se 8 . This absorption is responsible for the observed resonant Raman effect of Se 12 at the 514.5 nm wavelength excitation. Both LTA-confined Se 8 and Se 12 display rather high thermal stability. • Se 8 rings co-exist with Se chains and dominate over Se 6 rings in high- D AFI. • Se 8 is formed in LTA at any D while Se 12 appear at D > 0.5 Se atoms per cavity. • Se 12 optical absorption spectrum is obtained for the first time. • Se 12 3.0–3.3 eV absorption causes resonant Raman effect at 514.5 nm excitation. • Se 8 and Se 12 orientations are determined and their thermal stability is shown.

On the issue of textured crystallization of Ba(NO3)2 in mesoporous SiO2: Raman spectroscopy and lattice dynamics analysis

The lattice dynamics of preferentially aligned nanocrystals formed upon drying of aqueous Ba(NO3)2 solutions in a mesoporous silica glass traversed by tubular pores of approximately 12 nm are explored by Raman scattering. To interpret the experiments on the confined nanocrystals polarized Raman spectra of bulk single crystals and X-ray diffraction experiments are also performed. Since a cubic symmetry is inherent to Ba(NO3)2, a special Raman scattering geometry was utilized to separate the phonon modes of Ag and Eg species. Combining group-theory analysis and ab initio lattice dynamics calculations a full interpretation of all Raman lines of the bulk single crystal is achieved. Apart from a small confinement-induced line broadening, the peak positions and normalized peak intensities of the Raman spectra of the nanoconfined and macroscopic crystals are identical. Interestingly, the Raman scattering experiment indicates the existence of comparatively large,∼10–20 μm, single-crystalline regions of Ba(NO3)2 embedded in the porous host, near three orders of magnitude larger than the average size of single nanopores. This is contrast to the initial assumption of non-interconnected pores. It rather indicates an inter-pore propagation of the crystallization front, presumably via microporosity in the pore walls.

La4Te7O20: A new ternary rare-earth metal(III) oxotellurate(IV) with isolated [TeO3]2– and [Te3O8]4– anions

The new rare-earth metal(III) oxotellurate(IV) La4Te7O20 was obtained as a single-crystalline by-product of the La2Te3O9 synthesis through a solid-state reaction of the corresponding binary oxides La2O3 and TeO2. The colorless, lath-shaped crystals of La4Te7O20 crystallize in the monoclinic space group C2/c with the lattice parameters a ​= ​2740.63(16) pm, b ​= ​580.92(3) pm, c ​= ​1326.71(8) pm and β ​= ​106.849(3)° for Z ​= ​4. The structure consists of two crystallographically distinct lanthanum-oxygen polyhedra with the coordination numbers of seven plus one and eight (d(La3+–O2–) ​= ​241 – 262 pm plus 310 pm). These lanthanum-oxygen polyhedra are attached to one ψeq1-trigonal [TeO4]4– bipyramid (d(Te4+–O2–) ​= ​186 – 214 pm, ​2× each) and three different ψ1-[TeO3]2– tetrahedra (d(Te4+–O2–) ​= ​184 – 189 pm) with twice the frequency of occurrence. So the central [TeO4]4– core is linked with two [TeO3]2– groups via common oxygen vertices to form an isolated [Te3O8]4– anion, leading to the structural formula La4[Te3O8][TeO3]4. The presence of all these different oxotellurate(IV) units is confirmed by a single-crystal Raman spectrum.

Study of the influence of electronic radiation on the surface, structure and Raman spectrum of a TlInS2 single crystal

The effect of electrons on the surface, Raman spectra, structure and phase transition in a TlInS2 single crystal was studied. Irradiation of TlInS2 with electrons with an energy of 2 MeV and a fluency of 2 × 1017 electron/cm2 leads to an increase in the height of the bump and its width on the sample surface. Radiation treatment of TlInS2 with electrons indicates the appearance of new peaks at 204 cm−1, 224 cm−1 in the Raman spectra and stimulates the formation of a single-phase state with a hexagonal structure (space group P63/mmc).

The complete series of sodium rare-earth metal(III) chloride oxotellurates(IV) Na2 RE 3Cl3[TeO3]4 (RE = Y, La–Nd, Sm–Lu)

Abstract The complete series of sodium rare-earth metal(III) chloride oxotellurates(IV) with the composition Na2 RE 3Cl3[TeO3]4 (RE = Y, La–Nd, Sm–Lu) has been synthesized via solid-state reactions. For these conversions mixtures of the respective rare-earth metal(III) oxides, tellurium dioxide and sodium chloride as flux and reactant were prepared, intimately ground and heated for 5 days at 1225 K. The almost colorless single crystals were characterized via single-crystal X-ray diffractometry. In the monoclinic crystal structure of these compounds two crystallographically different rare-earth metal(III), but only one sodium cation sites occur. [REO8]13− polyhedra around both RE 3+ positions as well as sodium-centered polyhedra [NaO4Cl4]8− form layers via different connectivity modes. These layers spread out parallel to the (001) plane and arrange alternatingly resulting in the three-dimensional network of the Na2 RE 3Cl3[TeO3]4 structure, where the Te4+ lone-pair cations at two different sites work as linkers by forming isolated ψ 1-tetrahedra [TeO3]2−. Some of these compounds were represented before in different settings of space group C2/c. Now the complete series of the Na2 RE 3Cl3[TeO3]4 representatives with RE = Y, La–Nd, Sm–Lu is described consistently for a better comparison and understanding. Additionally, a single crystal of Na2Pr3Cl3[TeO3]4 was measured via energy dispersive X-ray analysis to verify the included elements, powder samples of Na2Nd3Cl3[TeO3]4 were characterized by X-ray diffractometer data for a phase-purity check and a single-crystal Raman spectrum of Na2Yb3Cl3[TeO3]4 served for proving the signature of discrete [TeO3]2− anions.

Raman scattering during a magnetic phase transition in a LiNiPO4 single crystal

The temperature evolution of the LiNiPO4 single crystal Raman spectra in a region of 2–25 K, which includes a phase transition to a magnetically ordered state, is studied. For the first time, magnetic contribution in the Raman spectra was divided into single-magnon and two-magnon excitations at 2 K. The energies of the detected single-magnon excitations are in good agreement with the data obtained earlier from transmission spectra in the near-IR region. The shape of the two-magnon scattering band corresponds to the published data for the density of two-magnon states. A shift in some phonon lines was detected in the transition to a magnetically ordered state, which indicates a significant magnon-phonon interaction. A change in the scattering spectrum intensities below TN was observed, which depends on the polarization of the incident light. It is suggested that this effect is associated with a shift of the Ni2+-ion absorption bands during magnetic ordering in the region of the 532 nm excitation line. This indicates a significant interaction between the electronic and magnetic subsystems in the crystal under study.

Achieving Macroscopic V4C3Tx MXene by Selectively Etching Al from V4AlC3 Single Crystals.

Hexagon-like MAX-phase V4AlC3 single crystals grown by a high-temperature flux method were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDX). We report, for the first time, the first-order Raman spectra (RS) of V4AlC3 single crystals experimentally and theoretically. Via the combination of the results of thermogravimetric analysis, differential scanning calorimetry, XRD, FE-SEM, and EDX, the oxidation performance and mechanism of V4AlC3 single crystals between 300 and 1473 K in air were clarified. Importantly, we carefully investigated the room-temperature corrosion behaviors of V4AlC3 single crystals in concentrated acids [HCl, H2SO4, hydrofluoric acid (HF), and HNO3] and alkalis (NaOH and KOH). V4AlC3 single crystals are stable in concentrated HCl, H2SO4, and NaOH but unstable and even dissolved completely in concentrated KOH and HNO3. In particular, our XRD, RS, FE-SEM, and EDX results have confirmed that HF can dissolve the Al layers of V4AlC3 single crystals but cannot corrode V4C3 layers at room temperature, which eventually led to the formation of macroscopic V4C3Tx MXene. This reported approach of macro-sized V4C3Tx MXene can be adapted for obtaining other macroscopic MXenes and will inspire plenty of theoretical and experimental investigations to explore their intrinsic nature and applications, especially for electronic and photonic applications.

Growth and characterizations of tin telluride (SnTe) single crystals

In recent time, materials in two-dimensional (2D) forms have gained importance along with nano-forms both from the fundamental and the technological point of view. Amongst the vast compound semiconductors, tin telluride (SnTe) is a potential candidate to find interest in the 2D form. The single crystals in the 2D flake forms of SnTe are grown by vapour transport technique in direct mode within closed quartz ampoule geometry. The stoichiometry of as-grown crystals is confirmed by the energy-dispersive X-ray analysis, which showed that the as-grown SnTe single crystals to be near stoichiometric but slightly Te rich. The analysis by X-ray diffraction of as-grown SnTe single crystals confirmed the cubic structure having lattice parameters of a = b = c = 6.31 A and α = β = γ = 90°. The surface study is done by electron microscopy in scanning mode which shows bunching of layers one over other, near flat surface, and flower-like pattern on the edges of the crystals. The spot pattern observed in electron diffraction from selected area confirms the single-crystalline nature of crystals. The Raman spectrum of SnTe single crystal showed sharp peak at 132 cm−1, attributed to transverse optical phonon vibration mode. The variation of d.c. electrical resistivity with temperature showed the SnTe single crystal to be metallic in nature and the value of bandgap is 0.19 eV. The thermal analysis of the as-grown SnTe single crystals is performed by recording the thermogravimetric, differential thermogravimetric, and differential thermal analysis curves. The kinetic parameters are derived by Kissinger method from the thermocurves data. The single crystals growth and varied characterization results on SnTe single crystals are thoroughly discussed.

Raman Scattering in Non-Stoichiometric Lithium Niobate Crystals with a Low Photorefractive Effect

Raman spectra of lithium niobate single crystals strongly doped by zinc and magnesium, it has been established, contain low-intense bands with frequencies 209, 230, 298, 694, and 880 cm−1. Ab ignition calculations fail to attribute these bands to fundamental vibrations of A2 symmetry type unambiguously. Such vibrations are prohibited by the selection rules in the space group C3V6 (R3c). Ab initio calculations also proved that low-intense “extra” bands with frequencies 104 and 119 cm−1 definitely do not correspond to vibrations of A2 symmetry type. We have paid special attention to these extra bands that appear in LiNbO3 single crystals Raman spectra despite the fact that they are prohibited by the selection rules. In order to do so, we have studied a number of lithium niobate single crystals, both nominally pure and doped, by Raman spectroscopy. We have assumed that some “extra” bands correspond to two-particle states of acoustic phonons with a total wave vector equal to zero. We have also detected a Zn concentration area (0.05–0.94 mol.% ZnO in a crystal) where doped crystal structure is more ordered: The order of alternation of the main, doping cations, and vacancies along the polar axis is increased, and oxygen octahedra are less distorted.

Press to Success: Gd5FW3O16—The First Gadolinium(III) Fluoride Oxidotungstate(VI)

The gadolinium(III) fluoride oxidotungstate(VI), with the formula Gd5FW3O16, represents the first published fluoride-derivative of a rare-earth metal oxidotungstate. It is synthesized by a mixture of GdF3, Gd2O3, and WO3 at 800 °C and a pressure of 2 GPa with the help of a belt press. The title compound crystallizes in the monoclinic space group P21/c (no. 14) with four formula units per unit cell and the following lattice parameters: a = 539.29 (4), b = 1556.41 (12), c = 1522.66 (11) pm, and β = 93.452 (4). The crystal structure comprises five crystallographically distinguishable Gd3+ cations, which are surrounded by either oxide and fluoride anions (Gd1–3) or by oxide anions only (Gd4, Gd5), with coordination numbers ranging between seven and nine. The fluoride anions are trigonal non-planar coordinated by three Gd3+ cations (Gd1–3). The distorted [WO6]6− octahedra in this structure form isolates edge- and vertex-connected entities of the compositions [W2O10]8− and [W2O11]10−, respectively. According to the presented units, a structured formula can be written as Gd4[FGd3]2[W2O10][W2O11]2. The single-crystal Raman spectrum reveals the typical symmetric stretching vibration mode of octahedral oxidotungstate(VI) units at about 871 cm−1.

Jahn–Teller Distortions of Metal Phthalocyanine Anions [MIVOPc]n– (M = Ti, V)

Based on X-ray diffraction data, a comparative analysis of the molecular geometry of eight phthalocyanine anions [TiIVOPc]n–, [VIVOPc]n– (n = 1, 2) and neutral phthalocyanines [TiIVOPc] and [VIVOPc] is carried out to study Jahn–Teller distortions of their molecular structure. It is experimentally shown that the occupation of a doubly degenerate lowest unoccupied molecular orbital of the [MIVOPc] molecule level by additional electrons leads to core tetragonal symmetry lowering from $${{C}_{{4{v}}}}$$ to $${{C}_{{2{v}}}}$$ with a significant change in the bond lengths distribution in the molecule. The effect manifests itself to a greater extent in imine bonds of the C–Nim–C fragment, in which a systemic alternation of bonds into short and long is observed. The degree of distortion is proportional to the charge of the metal phthalocyanine macrocycle of the organic part of the molecule, C32H16N8. The Jahn–Teller distortions of the macrocycle noticeably affect the position of frequencies characteristic of [MIVOPc] in the Raman spectra of single crystals of anionic phthalocyanine complexes in the range 1100–1700 cm–1. The experimental patterns agree fairly well with the results of DFT calculations.

Raman Scattering in Lithium Niobate and Lithium Tantalate Single Crystals and Ceramics

We have found conditions for high-speed measurements of Raman spectra of lithium niobate and lithium tantalate single crystals and ceramics. To measure Raman spectra, we used 180° scattering geometry and a minispectrometer equipped with an array detector. The Raman spectra obtained at two distinct orientations of the single crystals relative to the excitation beam direction have been shown to have different frequencies of transverse and longitudinal optical modes. The Raman spectra of the ceramics are mainly contributed by transverse optical modes.