Temperature Dependent Raman Spectroscopic Studies Research Articles

Tuning the spin-reorientation towards room temperature and the evidence for spin-phonon coupling in modified SmFeO3

The spin-reorientation phenomenon exhibited by rare-earth based ferrite systems generate lot of interest due to its application potential in related fields like spintronic devices. In this work, it was demonstrated that the spin-reorientation temperature (TSR) of SmFeO3 was tuned towards near room temperature by Cr doping. The SmFe1-xCrxO3 (x = 0.0, 0.1, 0.3 and 0.4) samples were prepared by solid state reaction method and the structural refinements carried out on the x-ray diffraction patterns revealed that the samples were crystallized in orthorhombic structure with Pbnm space group. The magnetic measurements displayed that the samples were all ordered antiferromagnetically and the ordering temperature (TN) shows decreasing trend with increase in Cr doping. Furthermore, TSR was tuned from 472 K to 317 K when the composition was changed from x = 0.0 to x = 0.4. The temperature-dependent Raman spectroscopic studies on x = 0.4 sample revealed the existence of the spin-phonon coupling. In addition, this sample also displayed an interesting negative magnetization and compensation temperature. The details are discussed in this work.

Nonlinear Temperature-Dependent Phonon Decay in Heavily Doped Silicon: Predominant Interferon-Mediated Cold Phonon Annihilation.

A nonlinear Fano interaction has been reported here which is manifest in terms of a parabolic temperature-dependent phonon decay process observable in terms of a Raman spectral parameter. Temperature-dependent Raman spectroscopic studies have been carried out on heavily and moderately doped crystalline silicon to investigate the behavior of anharmonic phonon decay in semiconductor systems where Fano interactions are present inherently. Systematic study reveals that in heavily doped systems an interferon-mediated decay route exists for cold phonons present at lower temperatures (<475 K) where Fano coupling is stronger and dominates over the typical multiple-phonon decay process. On the other hand, the anharmonic phonon decay remains the predominant process at higher temperatures irrespective of the doping level. Temperature-dependent phonon self-energy has been calculated using experimentally observed Raman line-shape parameters to validate the fact that the nonlinear decay of phonons through interferon mediation is a thermodynamically favorable process at low temperatures.

Characterization of V2O3 Nanoscale Thin Films Prepared by DC Magnetron Sputtering Technique

Vanadium sesquioxide V2O3, a transition metal oxide, is an important metal transition insulator due to its potential applications in novel electronic and memory devices. V2O3 thin films of thickness around 230 nm were grown on Si/SiO2/Ti/Pt substrates at deposition temperature of 723 K in a controlled Ar:O2 atmosphere of 35:2.5 sccm employing Direct Current (DC) magnetron sputtering. X-ray diffraction studies confirmed single phase of the material stabilized in corundum rhombohedral R3¯C phase. X-ray photoelectron spectroscopic results revealed chemical oxidation states are of V3+ and O2− and have nearly stochiometric elemental compositions in the films. Magnetization studies down to 10 K predicts a canted antiferromagnetic transition around 55 K. Out of 7 expected Raman active modes (2A1g + 5Eg), two A1g Raman active modes at 242 and 500 cm−1 were observed at ambient R3¯C phase. Temperature dependent Raman spectroscopic studies carried out from 80 to 300 K identified a monoclinic to rhombohedral phase transition at ~143 K.

Studies of Optical, Dielectric, Ferroelectric, and Structural Phase Transitions in 0.9[KNbO3]-0.1 [BaNi1/2Nb1/2O3−δ

The compound 0.9[KNbO3]-0.1[(BaNi1/2Nb1/2O3−δ] (KBNNO), a robust eco-friendly (lead-free) ferroelectric perovskite, has diverse applications in electronic and photonic devices. In this work, we report the dielectric, ferroelectric, and structural phase transitions behavior in the KBNNO compound using dielectric, X-ray diffraction, and Raman studies at ambient and as a function of temperature. Analyses of X-ray diffraction (XRD) data at room temperature (rtp) revealed the orthorhombic phase (sp. Gr. Amm2) of the compound with a minor secondary NiO cubic phase (sp. Gr. Fm3m). A direct optical band gap Eg of 1.66 eV was estimated at rtp from the UV–Vis reflectance spectrum analysis. Observation of non-saturated electric polarization loops were attributed to leakage current effects pertaining to oxygen vacancies in the compound. Magnetization studies showed ferromagnetism at room temperature (300 K) in this material. XRD studies on KBNNO at elevated temperatures revealed orthorhombic-to-tetragonal and tetragonal-to-cubic phase transitions at 523 and 713 K, respectively. Temperature-dependent dielectric response, being leaky, did not reveal any phase transition. Electrical conductivity data as a function of temperature obeyed Jonscher power law and satisfied the correlated barrier-hopping model, indicating dominance of the hopping conduction mechanism. Temperature-dependent Raman spectroscopic studies over a wide range of temperature (82–673 K) inferred the rhombohedral-to-orthorhombic and orthorhombic-to-tetragonal phase transitions at ~260, and 533 K, respectively. Several Raman bands were found to disappear, while a few Raman modes such as at 225, 270, 289, and 831 cm−1 exhibited discontinuity across the phase transitions at ~260 and 533 K.

Spin-phonon coupling and magnon scattering in few-layer antiferromagnetic FePS3

Temperature-dependent Raman spectroscopic studies of spin-phonon (SP) coupling and magnon scattering in bulk and few-layer (FL) antiferromagnet (AFM) ${\text{FePS}}_{3}$ with N\'eel temperature (${T}_{N}$) $\ensuremath{\approx}$ 120 K were performed. Bulk and FL (2--5 atomic layers) samples show four distinct modes at room temperature between 150 and $400 {\text{cm}}^{\ensuremath{-}1}$ and a broad peak at $\ensuremath{\approx}105 {\text{cm}}^{\ensuremath{-}1}$. On lowering the temperature, three distinct phenomena are observed. First, we see the SP coupling, identified by the deviation from the usual two- or three-phonon anharmonic behavior of the higher wave number peaks ($\ensuremath{\ge}150 {\text{cm}}^{\ensuremath{-}1}$) at or below ${T}_{N}$. The strength of SP coupling can be calculated for bulk and FL flakes considering mean-field approximations. Secondly, we see the spin ordering marked by the evolution of three peaks at lower wave numbers (around $105 {\text{cm}}^{\ensuremath{-}1}$) below ${T}_{N}$ due to incommensurate magnetic cells at low temperature. Thirdly, magnon excitation in FL pristine ${\text{FePS}}_{3}$ is detected by the emergence of a distinct peak at $120 {\text{cm}}^{\ensuremath{-}1}$ ($\ensuremath{\sim}3.6$ THz) at a temperature much lower than ${T}_{N}$ ($\ensuremath{\approx}60$ K). Tracking the magnon mode in the designed van der Waals heterostructures with ${\text{Bi}}_{2}{\text{Te}}_{3}$ and ${\text{Cr}}_{2}{\text{Ge}}_{2}{\text{Te}}_{6}$ reveal interfacial electron and hole transfer from ${\text{FePS}}_{3}$, respectively. Raman spectroscopy can thus predict the magnetic transition temperature of FL magnetic insulators via SP coupling, zone-boundary phonons, and magnons. Quasi-two-dimensional AFMs and their heterostructures involving different electronic and magnetic orders may be promising candidates for ultrafast magnon transport involving magnetoelastic waves.

Anomalous vibrational behavior of two dimensional tellurium: Layer thickness and temperature dependent Raman spectroscopic study

A few layer/multilayer two-dimensional (2D) Tellurium (Te) nanoflakes were mechanically exfoliated and their structural and vibrational properties were probed. Layer thickness reduction was confirmed using optical microscopy, atomic force microscopy and kelvin probe force microscopy. Raman spectroscopy measurements revealed anomalous shift in the frequencies of their intrachain vibrational modes, where, after an initial increasing trend (blue shift) with thickness reduction from bulk, they changed to decreasing trend (red shift) on further layer thinning. Such unusual vibrational behaviour of 2D Te, exhibiting reversal from blue shift to red shift, is attributed mainly to the deformations occurring in the Te chains on layer thinning, followed by modifications in the local structures and bondlengths of the corresponding Te atoms. The deformation under relaxed conditions are also confirmed from first principle calculations. Red shift can also be correlated to lesser hole concentration for thinner flakes, as revealed by work function changes measured on the nanoflakes surfaces. Interestingly, in spite of such anomalous behavior, the difference in frequencies of both modes showed initial increase with thickness and saturation beyond a certain thickness, as generally observed in other 2D materials. Temperature dependent Raman spectroscopic measurements further confirmed anomalous behaviour for thinner flakes.

Role of bonding nature on the temperature dependent erosion behavior of solid materials: A detailed high temperature Raman spectroscopic analysis

Solid particle erosion is an unavoidable problem for gas turbine compressor blades, wind turbines, crude oil pipes, rovers in outer space, and other machine components operated in harsh conditions. Gas turbine components are usually operated in the temperature range of 100–1400 °C. However, the influence of temperature on the solid particle erosion mechanism has not been studied adequately. In this paper, the influence of temperature on the solid particle erosion properties of metals (Ti6Al4V, SS 304, Al-8011, and Ti/TiN multilayers) and non-metals (Al2O3, fused quartz, and Si) is studied. The erosion tests were conducted in the temperature range of 30–800 °C. The erosion rate of metallic and ionic solids (viz., Ti6Al4V, SS 304, Al-8011, Al2O3, and Ti/TiN multilayers with the thermal expansion coefficient α ≥ 8 μm/°C) initially decreased with increasing temperature and then increased at higher temperatures, whereas for covalently bonded materials such as fused quartz (α = 0.55 μm/°C) and Si (α = 2.6 μm/°C), the erosion rate increased with increasing temperature. As the temperature increases, it causes lattice vibrations and bond flexibility to increase the high thermal expansion coefficient materials due to their non-directional ionic or metallic bonds. It helps these materials to absorb the erodent energy without much damage, whereas the same does not happen in the case of low thermal expansion coefficient materials due to their strong directional covalent bonding. Temperature dependent Raman spectroscopic studies were carried out to understand the lattice vibrations and phase changes during heating.

Temperature dependence Raman spectroscopy and DFT calculations of Bi2(MoO4)3.

This work reports a theoretical and experimental study on the electronic and vibrational properties of Bi2(MoO4)3. First-principle calculations were applied to increase the understanding on the properties of the chemical composition through the energy bands. The conduction band minimum (CBM) is found at the high symmetric Γ-point, while the valence-band maximum (VBM) is located between the Z and the Γ-points. Therefore, these facts confirm that the Bi2(MoO4)3 crystal is a semiconductor compound with an indirect band-gap of about 2.1 eV. Moreover, lattice dynamic properties were calculated using density functional perturbation theory (DFPT) in order to assign the experimental Raman bands. In addition, we performed temperature-dependent Raman spectroscopic studies in the Bi2(MoO4)3 crystals to obtain information on structural changes induced by effects of the temperature change. From the changes observed in the Raman spectra phase transitions at ∼ 668 and 833 K were inferred, with the last one possibly related to the disorder due to the heating process.

Modulation of thermal conductivity and thermoelectric figure of merit by anharmonic lattice vibration in Sb2Te3 thermoelectrics

Anharmonic lattice vibration in metallic and semiconducting Sb2Te3 is investigated using temperature dependent Raman spectroscopic studies, synchrotron powder diffraction, and heat-capacity measurements. Thermal variation of structural parameters divulges temperature dependence of structural anisotropy in both the samples. It is revealed that semiconducting Sb2Te3, having higher defect density and larger phonon anharmonicity, possesses lower thermal conductivity. As compared to its metallic counterpart, significant increase in thermoelectric figure of merit, ZT is reported for the semiconducting Sb2Te3. Correlation among structural anisotropy, phonon anharmonicity and ZT is established.

Temperature-dependent Raman spectroscopic study of ferroelastic K2Sr(MoO4)☆☆Project supported by the Natural Science Foundation of Anhui Province, China (Grant Nos. KJ2018A0588 and KJ2017A625).

Raman scattering measurements of K2Sr(MoO4)2 were performed in the temperature range of 25–750 °C. The Raman spectrum of the low-temperature phase α-K2Sr(MoO4)2 that was obtained by first-principle calculations indicated that the Raman bands in the wavenumber region of 250–500 cm−1 are related to Mo–O bending vibrations in MoO4 tetrahedra, while the Raman bands in the wavenumber region of 650–950 cm−1 are attributed to stretching vibrations of Mo–O bonds. The temperature-dependent Raman spectra reveal that K2Sr(MoO4)2 exhibits two sets of modifications in the Raman spectra at ∼150 °C and ∼475 °C, attributed to structural phase transitions. The large change of the Raman spectra in the temperature range of 150 °C to 475 °C suggests structural instability of the medium-temperature phase β-K2Sr(MoO4)2.

Temperature-induced isostructural phase transition on NaCe(MoO4)2 system: A Raman scattering study.

Temperature-dependent Raman spectroscopic study has been performed on scheelite-type sodium‑cerium molybdate - NaCe(MoO4)2 - in the temperature range 113-873 K. This study provides phonon properties of NaCe(MoO4)2, which are very important to understand the mechanism governing eventual phase transitions undergone by the structure, since phonons are very sensitive to structural changes. The ambient scheelite phase remains stable in a low-temperature range (113-293 K), and no relevant modification is observed in the Raman spectra. However, the experiments reveal the existence of one reversible phase transition at high-temperature. The vibrational spectra of NaCe(MoO4)2 system showed anomalies above 748 K, where overlaps of some bands and the appearance of a band at 458 cm-1 are observed. These modifications were attributed to an isostructural phase transition and a discussion about the possible mechanism of this transformation is furnished.

Spectroscopic study of native defects in the semiconductor to metal phase transition in V2O5 nanostructure

Vanadium is a transition metal with multiple oxidation states and V2O5 is the most stable form among them. Besides catalysis, chemical sensing, and photo-chromatic applications, V2O5 is also reported to exhibit a semiconductor to metal transition (SMT) at a temperature range of 530–560 K. Even though there are debates in using the term “SMT” for V2O5, the metallic behavior above the transition temperature and its origin are of great interest in the scientific community. In this study, V2O5 nanostructures were deposited on a SiO2/Si substrate by the vapour transport method using Au as a catalyst. Temperature dependent electrical measurement confirms the SMT in V2O5 without any structural change. Temperature dependent photoluminescence analysis proves the appearance of oxygen vacancy related peaks due to reduction of V2O5 above the transition temperature, as also inferred from temperature dependent Raman spectroscopic studies. The newly evolved defect levels in the V2O5 electronic structure with increasing temperature are also understood from the downward shift of the bottom most split-off conduction bands due to breakdown of pdπ bonds leading to metallic behavior in V2O5 above the transition temperature.

Temperature dependent structural, vibrational and magnetic properties of K3Gd5(PO4)6.

Herein we report the evolution of the crystal structure of K3Gd5(PO4)6 in the temperature range from 20 K to 1073 K, as observed from variable temperature X-ray diffraction and Raman spectroscopic studies. K3Gd5(PO4)6 has an open tunnel containing a three dimensional structure built by [Gd5(PO4)6]3- ions which in turn are formed of PO4 tetrahedra and GdOn (n = 8 and 9) polyhedra. The empty tunnels in the structure are occupied by K+ ions and maintain charge neutrality in the lattice. Evolution of unit cell parameters with temperature shows a systematic increase with temperature. The average axial thermal expansion coefficients between 20 K and 1073 K are: αa = 10.6 × 10-6 K-1, αb = 5.5 × 10-6 K-1 and αc = 16.4 × 10-6 K-1. The evolution of distortion indices of the various coordination polyhedra with temperature indicates a gradual decrease with increasing temperature, while those of Gd2O9 and K2O8 polyhedra show opposite trends. The overall anisotropy of the lattice thermal expansion is found to be controlled largely by the effect of temperature on GdOn polyhedra and their linkages. Temperature dependent Raman spectroscopic studies indicated that the intensities and wavenumbers of most of the Raman modes decrease continuously with increasing temperature. Anharmonic analyses of Raman modes indicated that the lattice, rigid translation and librational modes have larger contributions towards thermal expansion of K3Gd5(PO4)6 compared to high frequency internal modes. The temperature and field dependent magnetic measurements indicated no long range ordering down to 2 K and the observed effective magnetic moment per Gd3+ ion and the Weiss constant are 7.91 μB and 0.38 K, respectively.

Role of Vanadyl Oxygen in Understanding Metallic Behavior of V2O5(001) Nanorods

Vanadium pentoxide (V2O5), the most stable member of vanadium oxide family, exhibits interesting semiconductor to metal transition in the temperature range of 530-560 K. The metallic behavior originates because of the reduction of V2O5 through oxygen vacancies. In the present report, V2O5 nanorods in the orthorhombic phase with crystal orientation of (001) are grown using vapor transport process. Among three nonequivalent oxygen atoms in a VO5 pyramidal formula unit in V2O5 structure, the role of terminal vanadyl oxygen (OI) in the formation of metallic phase above the transition temperature is established from the temperature-dependent Raman spectroscopic studies. The origin of the metallic behavior of V2O5 is also understood due to the breakdown of pdpi bond between OI and nearest V atom instigated by the formation of vanadyl OI vacancy, confirmed from the downward shift of the bottom most split-off conduction bands in the material with increasing temperature.

Temperature-dependent Raman spectroscopic studies of microstructure present in dipotassium molybdate crystals and their melts

Temperature-dependent Raman spectra of K2MonO3n+1 (n = 1, 2, 3) crystals up to and above their melting points were recorded, and their vibration modes in solid and molten states were assigned. Basic structural units and the corresponding cluster forms in molten dipotassium monomolybdates, dimolybdates, and trimolybdates were studied by in situ high-temperature Raman spectroscopic studies together with theoretical calculations, including density functional theory and quantum chemistry ab initio calculation. Anion units of [MoO4]2−, [Mo2O7]2−, and [Mo3O10]2− were shown to principally exist in molten K2MoO4, K2Mo2O7, and K2Mo3O10, respectively. The mechanisms of the microstructural evolution of K2MonO3n+1 (n = 1, 2, 3) crystals while being melted are schematically illustrated. Copyright © 2016 John Wiley & Sons, Ltd.

Temperature and Pressure Dependent Phase Transitions of β'-LiZr2(PO4)3 Studied by Raman Spectroscopy.

LiZr2(PO4)3 (LZP) belongs to the NASICON family of compounds whose ionic conductivity can be tuned by substitution of different cations or by increasing the temperature or pressure. Besides its conductivity, thermal and electrochemical stability makes it useful as a cathode material for lithium-ion energy storage devices. Temperature dependent Raman spectroscopic studies were carried out on the monoclinic (β') phase of LZP in the temperature range 298-853 K. A reversible structural phase transition driven by disorder in lithium sites is observed at 603 K. The spectral data enable an understanding of dynamics of the mobile Li ion and PO4 internal modes across the orthorhombic structural phase transition. On the basis of these studies, a reported change in the conductivity around 600 K is explained. High pressure Raman spectroscopic measurements on β'-LiZr2(PO4)3 reveal the onset of a structural phase transformation at 3.8 GPa and amorphization above 10 GPa. On decompression from 26 GPa, the amorphous phase remains unchanged, indicating irreversible nature of pressure-induced amorphization. Three low frequency Raman modes at 100, 124, and 144 cm(-1), which soften with an increase in pressure could be the driving force for the phase transition at 3.8 GPa. Pressure-induced phase transition prior to amorphization in β'-LiZr2(PO4)3 could be due to collapse of Zr-O-P bond angles. Pressure-induced amorphization in this compound might be due to kinetic hindrance of equilibrium decomposition.

Deuterium ordering found in new ferroelectric compound Co2(OD)3Cl

A detailed temperature-dependent Raman spectroscopic study revealed a new type of deuterium-order ferroelectrics in a geometrically frustrated magnet Co2(OD)3Cl at Tϵ = 229 K. Significant changes in the parameters of the Raman vibration modes were observed near Tϵ, suggesting a strong phonon-charge coupling. Additional asymmetric phonon bands appeared below around Tϵ, which are consistently interpreted by phonon folding processes due to a small local structural change resulting from the ordering of deuterium. The wavenumber and intensity changes of the Raman-active modes, as well as the normalized intensities of the additional bands, all follow a power-law fit Δω, ΔI, I ∝ (1 − T/TC)2β, wherein TC = 230 K ∼ Tϵ and β = 0.35(2), clearly demonstrating an ordering process below Tϵ. The critical exponent is reminiscent of a second order transition. Our study presents a rare and new type of multiferroic material with ferroelectricity arising from the deuterium ordering in geometrically frustrated magnets.

Temperature-dependent Raman spectroscopic study of bismuth borate Bi2ZnOB2O6**Project supported by the National Natural Science Foundation of China (Grant Nos. 50932005 and 51102239).

A temperature-dependent Raman spectroscopic study on Bi2ZnOB2O6 crystal was carried out to investigate the structure change of the crystal with the increase of temperature. Raman spectra of crystal Bi2ZnOB2O6 were recorded in the spectral range 10–1600 cm−1 at room temperature first. Compared with the vibrational spectra of the referred compounds, satisfactory assignment of most of the high-energy modes to vibrations of Bi–O, B–O, and Zn–O bonds was achieved. In particular, the Raman high-frequency peak located at 1344 cm–1 was attributed to the B–O vibration in the BO3 triangle. This temperature-dependent Raman spectroscopic study was carried out up to 600 °C. It was found that all the Raman lines exhibit decreases in frequency and the widths of the Raman peaks increase with increasing temperature. No phase transition was observed under 600 °C.

Effect of pressure and temperature on Raman scattering and an anharmonicity study of tin dichalcogenide single crystals

Raman spectra of Tin dichalcogenide (SnS2 and SnSe2) single crystals have been measured at high pressures up to 20GPa and low temperatures up to 80K. The present study indicates a good stability of these compounds at high pressure and low temperature. Here, we report a detailed temperature-dependent Raman spectroscopic study of A1g phonon mode of Tin dichalcogenide by considering the contributions from thermal expansion and lattice anharmonicity and analyze the data based on known anharmonicity model.

Phase transition, crystal water and low thermal expansion behavior of Al2−2x(ZrMg)xW3O12·n(H2O)

Al2−2x(ZrMg)xW3O12 for 0≤x≤1.0 are synthesized to reduce the phase transition temperature of Al2W3O12. It is found that the incorporation of (ZrMg)6+ into the lattice of Al2W3O12 not only reduces its orthorhombic-to-monoclinic phase transition temperature but also elevates its softening temperature, broadening its applicable temperature range considerably. Al2−2x(ZrMg)xW3O12 with x<0.5 exhibit low coefficients of thermal expansion (CTEs) and non-hygroscopicity, while those for x≥0.7 are obviously hygroscopic and the CETs decrease with increasing the content of (ZrMg)6+ so that Al0.2(ZrMg)0.9W3O12 and ZrMgW3O12 exhibit negative thermal expansion. Temperature-dependent Raman spectroscopic study shows the hardening of W–O bonds above 373K which is attributed to the release of crystal water. The effect of crystal water on the thermal expansion property is discussed based on the hydrogen bond between H in crystal water and electronegative O in Al(ZrMg)–O–W linkages.