High-pressure Raman-scattering Research Articles

Anisotropic Phonon Behavior and Phase Transition in Monolayer ReSe2 Discovered by High Pressure Raman Scattering.

Re-based transition metal dichalcogenides have attracted extensive attention owing to their anisotropic structure and excellent properties in applications such as optoelectronic devices and electrocatalysis. The present study methodically investigated the evolution of specific Raman phonon mode behaviors and phase transitions in monolayer and bulk ReSe2 under high pressure. Considering the distinctive anisotropic characteristics and the vibration vectors of Re and Se atoms exhibited by monolayer ReSe2, we perform phonon dispersion calculations and propose a methodology utilizing pressure-dependent polarized Raman measurements to explore the precise structural evolution of monolayer ReSe2 under the stress fields. Varied behaviors of the Eg-like and Ag-like modes, along with their specific vector transformations, have been identified in the pressure range 0-14.59 GPa. The present study aims to offer original perspectives on the physical evolution of Re-based transition metal dichalcogenides, elucidating their fundamental anisotropic properties and exploring potential applicability in diverse devices.

Coupling of organic cation and inorganic lattice in methylammonium lead halide perovskites: Insights into a pressure-induced isostructural phase transition

Organic-inorganic hybrid metal-halide perovskite (OIHP) materials provide a tunable platform for engineering their optoelectronic properties. Although several high-pressure studies have been conducted from the OIHP family of single crystals and films, the exact nature of the dynamic coupling of the $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}$ (MA) cation with the octahedral lattice framework and the mechanisms responsible for the structural phase transformation under pressure are not well captured. By combined photoluminescence (PL), synchrotron-based x-ray diffraction, and Raman-scattering studies as a function of pressure from methylammonium lead bromide ($\mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{Br}}_{3}$), we shed light on an isostructural phase transition due to the coupling of the MA cation and the $\mathrm{Pb}{\mathrm{Br}}_{6}$ lattice through hydrogen bonding. The sharp discontinuities at $\ensuremath{\sim}$ 1 GPa and $\ensuremath{\sim}$ 3 GPa in the PL peak positions correlate with the structural changes observed in high-pressure XRD and Raman-scattering studies. The electronic band edge as a function of pressure is calculated within density-functional theory. The PL peak position, intensity and width of the excitonic peak show significant changes at 2 GPa, which corroborate the changes observed in high-pressure Raman-scattering studies. The frequencies of the lattice modes and the C--H/N--H bending and stretching modes of the MA cation show anomalous changes and other nuances at 2 GPa. The suppression of rotational and orientational disorder of the organic moiety is initiated at 2 GPa and the ordering is completed by 3.0 GPa, leading to an order-disorder type cubic II ($Im\overline{3}$) to orthorhombic ($Pnma$) phase transition. Along with the revelation of an isostructural transformation at 2 GPa, this paper highlights the impact of molecular vibrations on the electronic properties of $\mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{Br}}_{3}$ under pressure.

The optical and high-pressure raman scattering study of La(Fe,Mo)0.5O3 ceramics

This study was focused on the optical-vibrational analysis and high-pressure Raman-scattering spectroscopy of La(Fe,Mo)0.5O3 ceramics at room temperature. This material was fabricated by using the sol-gel method. The element composition was checked by X-ray Fluorescence confirming the atomic ratio of La, Fe and Mo. Fourier Transform Infrared spectroscopy confirmed the (Fe/Mo −O) bonding vibration which is typical perovskite structure. The band gap energy investigated by UV-Vis spectra was found to be 2.03 eV. At room temperature, the Raman scattering spectra under high pressure was characterized by the Raman-scattering spectrum in the range of 0.90 to 16.7 GPa. The high-pressure Raman scattering confirmed the structural transition after the pressure surpasses 7.0 GPa. From this phenomena, it well indicates the lattice structure stability is still preserved under 7.0 GPa.

Study of LaFeO3 perovskite material by high-pressure Raman scattering

We use high-pressure Raman-scattering spectroscopy to study LaFeO3 synthesized by using the sol-gel method. The structural evolution of the samples under high pressure and at room temperature is monitored via the Raman-scattering spectrum in the pressure range of 0.50 to 22.1 GPa. When the pressure surpasses 12 GPa, the Raman spectra change significantly around 180 cm−1, which indicates a structural phase transition. However, the Raman spectrum remains constant under 5.0 GPa, which indicates that the LaFeO3 structure is preserved at these pressures.

High-pressure Raman scattering in bulk HfS2: comparison of density functional theory methods in layered MS2 compounds (M\u2009=\u2009Hf, Mo) under compression

We report high-pressure Raman-scattering measurements on the transition-metal dichalcogenide (TMDC) compound HfS2. The aim of this work is twofold: (i) to investigate the high-pressure behavior of the zone-center optical phonon modes of HfS2 and experimentally determine the linear pressure coefficients and mode Grüneisen parameters of this material; (ii) to test the validity of different density functional theory (DFT) approaches in order to predict the lattice-dynamical properties of HfS2 under pressure. For this purpose, the experimental results are compared with the results of DFT calculations performed with different functionals, with and without Van der Waals (vdW) interaction. We find that DFT calculations within the generalized gradient approximation (GGA) properly describe the high-pressure lattice dynamics of HfS2 when vdW interactions are taken into account. In contrast, we show that DFT within the local density approximation (LDA), which is widely used to predict structural and vibrational properties at ambient conditions in 2D compounds, fails to reproduce the behavior of HfS2 under compression. Similar conclusions are reached in the case of MoS2. This suggests that large errors may be introduced if the compressibility and Grüneisen parameters of bulk TMDCs are calculated with bare DFT-LDA. Therefore, the validity of different approaches to calculate the structural and vibrational properties of bulk and few-layered vdW materials under compression should be carefully assessed.

High-pressure lattice-dynamics of NdVO4

High-pressure Raman-scattering measurements and ab initio calculations on NdVO4 have been carried out up to 30GPa. Our combined experimental and theoretical study confirms that beyond 5.9GPa NdVO4 undergoes an irreversible zircon to monazite transition. The coexistence of zircon and monazite phases is experimentally observed up to ~8GPa (which agrees with the theoretical transition pressure), stabilizing the monazite phase as a single phase around 10GPa. Calculations additionally predict the existence of a second high-pressure phase transition at 12.4GPa. This reversible phase transition has been experimentally observed beyond 18.1GPa and remains stable up to 30GPa. The post-monazite phase is predicted to have a monoclinic structure isomorphic to the BaWO4-II type structure. The calculated structure for the three polymorphs of NdVO4 is reported and the pressure dependence of their Raman modes is discussed.

Pressure-induced structural phase transitions and phonon anomalies inReO3: Raman and first-principles study

We report high-pressure Raman-scattering studies on single-crystal $\mathrm{Re}{\mathrm{O}}_{3}$ up to 26.9 GPa at room temperature, complemented by first-principles density functional calculations to assign the modes and to develop understanding of the subtle features of the low-pressure phase transition. The pressure $(P)$ dependence of phonon frequencies $(\ensuremath{\omega})$ reveals three phase transitions at 0.6, 3, and 12.5 GPa with characteristic splitting and changes in the slope of $\ensuremath{\omega}(P)$. Our first-principles theoretical analysis confirms the role of the rotational modes of $\mathrm{Re}{\mathrm{O}}_{6},\phantom{\rule{0.16em}{0ex}}{M}_{3}$, to the lowest pressure structural transition, and shows that the transition from the $Pm3m$ to the $Im3$ structure is a weak first-order transition, originating from the strong anharmonic coupling of the ${M}_{3}$ modes with the acoustic modes (strain).

High-pressure Raman scattering of CaWO₄ up to 46.3 GPa: evidence of a new high-pressure phase.

The high-pressure behavior of CaWO4 was analyzed at room temperature by Raman spectroscopy. Pressure was generated using a diamond-anvil cell and Ne as pressure-transmitting medium. The pressure range of previous studies has been extended from 23.4 to 46.3 GPa. The experiments reveal the existence of two reversible phase transitions. The first one occurs from the tetragonal scheelite structure to the monoclinic fergusonite structure and is observed at 10 GPa. The onset of a previously unknown second transition is found at 33.4 GPa. The two high-pressure phases coexist up to 39.4 GPa. The Raman spectra measured for the low-pressure phase and the first high-pressure phase are consistent with previous studies in the pressure range where comparison is possible. The pressure dependence of all the Raman-active modes is reported for different phases. We also report total-energy and lattice-dynamics calculations, which determine the occurrence of two phase transitions in the pressure range covered by the experiments. The first transition is in full agreement with experiments (scheelite-to-fergusonite). According to calculations, the second-highest pressure phase has an orthorhombic structure (space group Cmca). Details of this structure, its Raman modes, and its electronic band structure are given. The reliability of the reported results is supported by the consistency between the theoretical and experimental values obtained for transition pressures, phonon frequencies, and phonon pressure coefficients.

High-pressure Raman scattering in InGaN heteroepitaxial layers: Effect of the substrate on the phonon pressure coefficients

We perform high-pressure Raman-scattering measurements on different InxGa1-xN/Si(111) epilayers (0.19 < x < 0.45). We find that the experimental pressure coefficient of the A1(LO) mode measured in these samples is larger than that expected from the linear interpolation between the corresponding values of GaN and InN. Similar measurements in InGaN epilayers grown on GaN/sapphire templates yield much lower values, below the linearly interpolated pressure coefficients. We conclude that the phonon pressure coefficients measured in InGaN are mainly determined by the different compressibility of the substrate and epilayer material. Neglecting substrate effects may yield highly inaccurate phonon pressure coefficients and mode Grüneisen parameters.

Vibrational Properties and Phase Transitions of Silicon Clathrates Studied by High-Pressure Raman Scattering(Current Topics)

Vibrational Properties and Phase Transitions of Silicon Clathrates Studied by High-Pressure Raman Scattering(Current Topics)

Evidence for high-pressure polymorphism in resorcinol

Results of high-pressure Raman-scattering measurements carried out on resorcinol up to pressures of 14.5 GPa are reported. Changes observed in the Raman spectra suggest transition to a new phase $\ensuremath{\gamma}$ around $5.6\ifmmode\pm\else\textpm\fi{}0.2 \mathrm{GPa}$ besides the well-known isosymmetric $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\alpha}}\ensuremath{\beta}$ transition at 0.5 GPa. The $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\beta}}\ensuremath{\gamma}$ transition is found to be reversible with hysteresis when released from peak pressures of about 11 GPa. Raman line shapes at high-pressures suggest that the new phase $\ensuremath{\gamma}$ is disordered. Rapid pressurization of $\ensuremath{\alpha}$-resorcinol, with the suppression of $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\alpha}}\ensuremath{\beta}$ transition, leads to a new phase $\ensuremath{\delta}$ above 3 GPa which turns out to be more ordered, unlike the $\ensuremath{\gamma}$ phase, at comparable pressures. A luminescence peak centered around 2.25 eV is observed in the high pressure phase $\ensuremath{\gamma}$ as well as $\ensuremath{\delta}$ above 6 GPa. Above 11 GPa, intensity of the Raman bands weaken drastically and submerge in the luminescence background. The ambient pressure Raman spectrum of resorcinol is not recovered when released from pressures above 11 GPa.

High Pressure Raman Scattering Study of CuAlS2

The Raman active modes of CuAlS2 have been investigated at ambient temperature under high pressure using nitrogen gas as pressure transmitting medium up to 16.3 GPa. An assignment of the symmetry of the zone-centered modes is proposed on the basis of their pressure dependence as compared to that of the corresponding modes in the zincblende structure (ZnS). In this sense, a schematic representation of the cubic Brillouin zone showing its high symmetry points and the correlation with the CuAlS2 chalcopyrite lattice modes is presented. The high pressure effect on the phonon frequencies is discussed in terms of the Grïneisen parameters. A structural first-order phase transition was observed at about 16.3 GPa. After a pressure cycle was completed, the Raman scattering measurements performed on the retrieved samples gave some information about the size of the crystallites created by the phase transition.

High Pressure Raman Scattering Study of CuAlS2

The Raman active modes of CuAlS2 have been investigated at ambient temperature under high pressure using nitrogen gas as pressure transmitting medium up to 16.3 GPa. An assignment of the symmetry of the zone-centered modes is proposed on the basis of their pressure dependence as compared to that of the corresponding modes in the zincblende structure (ZnS). In this sense, a schematic representation of the cubic Brillouin zone showing its high symmetry points and the correlation with the CuAlS2 chalcopyrite lattice modes is presented. The high pressure effect on the phonon frequencies is discussed in terms of the Grïneisen parameters. A structural first-order phase transition was observed at about 16.3 GPa. After a pressure cycle was completed, the Raman scattering measurements performed on the retrieved samples gave some information about the size of the crystallites created by the phase transition.

High-pressure Raman-scattering study of germanium diselenide

We have measured the Raman-scattering spectra of the \ensuremath{\beta} modification of ${\mathrm{GeSe}}_{2}$ at low temperatures and under hydrostatic pressures up to 8 GPa at room temperature. A total of 28 modes of symmetry ${A}_{g}$ and 29 of symmetry ${B}_{g}$ were resolved by polarized light-scattering measurements at 10 K. The mode frequencies harden with pressure up to 2.5 GPa. At 6.2 GPa the changes of the spectra reveal a transition to an amorphous phase. Further increase of the pressure up to 7 GPa yields strong indication of a phase transition from the disordered phase to the low-temperature modification of ${\mathrm{GeSe}}_{2}.$ At 8 GPa, this modification transforms back to the disordered phase. All these phase transformations are reversible.

High-Pressure Raman and X-Ray Scattering Studies on Vermiculite Layered-Silicate Intercalation Compounds.

Single crystals of vermiculite were studied by high-pressure Raman scattering and x-ray diffraction methods using diamond-anvil cells with distilled water and mineral oil as pressure-transmitting media. When water was used, intercalation of water molecules to vermiculite was observed (pressure-induced intercalation). High-pressure Raman spectra from vermiculite samples in various intercalation states revealed the roles of interlamellar intercalant molecules and atoms in the lattice dynamics of vermiculite.

High-Pressure Raman Scattering of Biaxially Strained GaN on GaAs

ABSTRACTWe present results of high-pressure Raman-scattering experiments on bulk GaN and GaN grown on GaAs. We determined the Grüneisen parameters of both the cubic TO and LO phonon modes and the hexagonal A1, E1 and E2 modes. Our measurements reveal that the Grüneisen parameters for the GaAs substrate are about 30% smaller than those of bulk GaAs. This is a consequence of the lower compressibility of GaN compared to GaAs, which results in a pressure-induced biaxial strain on the substrate. From the pressure behavior of the GaAs modes and by comparing with our results for bulk GaN we obtained information about the biaxial strain in the GaN epitaxial layer.

Raman-scattering study on pressure amorphization of LiNbO3 crystal

A high-pressure Raman-scattering study reveals that crystalline LiNbO2 becomes amorphous at about 31 GPa, and this transition is irreversible above 35.8 GPa. The spectrum of pressure- amorphized LiNbO3 is similar to that of glassy LiNbO3.

Effect of pressure on the Raman vibrational modes of layered aluminosilicate compounds.

We have conducted extensive high-pressure Raman-scattering studies in order to determine the effects of structure on the vibrational properties of layered aluminosilicate materials. The basal oxygen torsional mode is found to shift up with pressure at a rate that agrees reasonably with that expected from a simple van der Waals model. We identify a mode at 50 cm -1 in margarite as the interlayer shear mode. The Raman band near 700 cm -1 in all clays is linked to an oxygen-layer breathing mode. The pressure coefficient of the hydroxyl stretching mode near 3600 cm -1 is found to have a structure-dependent sign; it is positive for trioctahedral structures and negative for dioctahedral silicates

High Pressure Raman Scattering from the Forbidden Bending Mode v 2 of CO2

High Pressure Raman Scattering from the Forbidden Bending Mode <i>v</i> ₂ of CO₂