High-pressure Raman Study Research Articles

Iso-structural phase transition in pyrochlore iridates (Sm _{1-x}$Bix)2Ir2O7 (x = 0, 0.02, and 0.10): high-pressure Raman and x-ray diffraction studies

The pyrochlore iridates,A2Ir2O7, show a wide variety of structural, electronic, and magnetic properties controlled by the interplay of different exchange interactions, which can be tuned by external pressure. In this work, we report pressure-induced iso-structural phase transitions at ambient temperature using synchrotron-based x-ray diffraction (up to ∼20 GPa) and Raman-scattering measurements (up to ∼25 GPa) of the pyrochlore series (Sm_{1-x}Bix)2Ir2O7(x= 0, 0.02, and 0.10). Our Raman and x-ray data suggest an iso-structural transition in Sm2Ir2O7atPc∼ 11.2 GPa, associated with the rearrangement of IrO6octahedra in the pyrochlore lattice. The transition pressure decreases to ∼10.2 and 9 GPa forx= 0.02 and 0.10, respectively. For all the samples, the linewidth of three phonons associated with Ir-O-Ir (A1gandEg) and Ir-O (T2g4) vibrations show anomalous decrease up toPc, due to decrease in electron-phonon interaction.

Large Local Internal Stress in an Elastically Bent Molecular Crystal Revealed by Raman Shifts.

The structural dynamics involved in the mechanical flexibility of molecular crystals and the internal stress in such flexible materials remain obscure. Here, the study reports an elastically bending lipidated molecular crystal that shows systematic shifts in characteristic vibrational frequencies across the bent crystal region - revealing the nature of structural changes during bending and the local internal stress distribution. The blueshifts in the bond stretching modes (such as C═O and C-H modes) in the inner arc region and redshifts in the outer arc region of the bent crystals observed via micro-Raman mapping are counterintuitive to the bending models based on intermolecular hydrogen bonds. Correlating these shifts with the trends observed from high-pressure Raman studies on the crystal reveals the local stress difference between the inner arc and outer arc regions of the bent crystal to be ≈2GPa, more than an order of magnitude higher than the previously proposed value in elastically bending crystals. High local internal stress can have direct ramifications on the properties of molecular piezoelectric energy harvesters, actuators, semiconductors, and flexible optoelectronic materials.

High pressure Raman study of isobutyramide

Isobutyramide (IBA) has attracted considerable attention due to its expansive prospects for practical applications in the synthesis of drugs, dyes and other organic compounds. Herein we perform the high-pressure studies of IBA crystal by Raman spectral measurements at room temperature from ambient pressure to 30 GPa by using diamond anvil cells (DACs) to gain comprehensive insights into its structure and stability. Raman vibrational modes of IBA crystal at ambient pressure are resolved based on the experimental results and the first-principles theoretical calculations. High-pressure Raman scattering results show the Raman bands splitting, emergence/disappearance of the Raman bands and discontinuous wavenumber shifts at 1, 2 and 10 GPa, which indicate that IBA crystal undergoes three structural phase transitions at corresponding pressure. In addition, softening of the C = O and N–H stretching vibrational modes of IBA with increasing pressure can be interpreted by the reorganization of the hydrogen bond network of IBA molecules due to phase transition.

Covalent bonds formed in MoS2–C60/Ferrocene heterostructure under high pressure

Constructing mixed dimensional heterostructure can create the possibility for obtaining new structures and properties by combining the advantages of both materials. Further introducing external means to control intermolecular/atomic distance and configuration may affect the bonding form, band structure, and interfacial interaction, which brings more novel function materials. In this work, high pressure Raman studies and theoretical research have been carried out on monolayer MoS2–C60/Ferrocene (C60/Fc) heterostructures to explore the possible phase transitions and interfacial bonding changes. We found that the interaction between C60/Fc and MoS2 molecules gradually enhanced as pressure increased, accompanying by a large wavenumber splitting of Ag (2) mode and the appearance of new peaks around 700-800 cm−1. These phenomena indicated that not only the polymeric structure was formed between C60 molecules, but some C atoms connected with adjacent S atoms at the interface to form covalent bonds with the help of pressure. The theoretical studies further indicated the possible bonds formation between C and S atoms. Surprisingly, the covalent bonds can still be observed at lower pressure when pressure decompressed, while the long-range ordered polymer structure between C60s can be maintained at atmospheric pressure.

High pressure Raman study of Na4Ca(CO3)3 from the ambient pressure to 11 GPa

ABSTRACT Na-Ca double carbonates were found as the inclusions in ‘superdeep’ Juina diamonds and as the products of high pressure experiments and thus are regarded as the potential host of CO2 in the deep mantle. Recently, Na-rich carbonate (21.0% wt. of Na2O vs. 13.6% wt. of CaO) was reported as the product of experiment with carbonated eclogite treated at 21 GPa and 1200°C. Na/Ca ratio of this carbonate corresponds to Na4Ca(CO3)3 phase with cubic symmetry synthesized at 6 GPa. The stability of Na4Ca(CO3)3 at higher pressures has not been investigated yet. Herein, we report an in situ Raman study of Na4Ca(CO3)3 carbonate up to 11 GPa in order to reveal stability of its structure at mantle pressures. Obtained results strongly imply for reversible phase transition near 8 GPa associated with decrease of symmetry. The observed phase transition questions the stability of cubic Na4Ca(CO3)3 under conditions of mantle transition zone.

High-Pressure Behavior of δ-Phase of Formamidinium Lead Iodide by Optical Spectroscopies

The exceptional photovoltaic properties of hybrid organic–inorganic perovskites have attracted increasing interest in the past decades. Among these materials, FAPbI3 shows two structural phases: the high temperature perovskite α-phase, with direct bandgap close to the Shockley–Queisser limit, and the much less photoactive non-perovskite δ-phase, stable at ambient conditions. Although the presence of the δ-phase has been usually regarded as a limitation for FAPbI3 optoelectronic applications, recent studies have found that devices with increased stability and efficiency can be designed by mixing α- and δ-phases. This has brought out the need for a deeper understanding of the physical properties of δ-FAPbI3. In this paper, we present an original high-pressure Raman and photoluminescence study to address the effects of compression on the lattice and optoelectronic response of the sample. Also, based on the previous findings on different hybrid perovskites, our results for δ-FAPbI3 show that the cation configuration goes from a dynamically disordered regime at ambient conditions to a statically ordered phase at ∼1.5 GPa. On further increasing pressure, above 7 GPa, a statically disordered regime takes place, where the cations are locked at random orientations in the inorganic framework, giving rise to an amorphous-like state. Compared with α- FAPbI3, we found that the hexagonal δ-phase is less affected by external compression, as both the first detectable structural transition and the amorphous-like behavior occur at higher pressures.

Strain-induced structural and electronic phase transitions in ZrSe2: high pressure X-ray diffraction and Raman studies

Strain-induced structural and electronic phase transitions in ZrSe2: high pressure X-ray diffraction and Raman studies

High pressure Raman study of LiClO4

Lithium perchlorate (LiClO4), as one of the new high-energy oxidizers, is chosen for high pressure Raman research to gain a better understanding of the structure and stability, which is very important for the performance of an explosive. Raman spectra of LiClO4 crystal have been measured from ambient to 25.07 GPa with diamond anvil cells (DACs) at room temperature to investigate the structural stability of this system. Raman vibrational modes of LiClO4 crystal at ambient pressure were resolved comprehensively on the basis of our experimental and calculated results. Upon increase of pressure on LiClO4 crystal sample to 1.96 GPa, it was found that the LiClO4 crystal exhibited a pressure-induced first-order phase transformation behavior. The occurrence of a second phase transformation of LiClO4 crystal induced by pressure was observed at about 5.09 GPa. Both phase transformations were demonstrated based on the detailed spectroscopic analysis of the variations in the number of lattice modes, splitting of Raman bands and frequency jumps of the Raman vibrational modes of LiClO4 crystal.

Crystal structure solution of a high-pressure polymorph of scintillating MgMoO4 and its electronic structure

The structure of the potentially scintillating high-pressure phase of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ ($\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$) has been solved by means of high-pressure single-crystal x-ray diffraction. The phase transition occurs above 1.5 GPa and involves an increase of the Mo coordination from fourfold to sixfold accommodated by a rotation of the polyhedra and a concommitant bond stretching resulting in an enlargement of the $c$ axis. A previous high-pressure Raman study had proposed such changes with a symmetry change to space group $P2/c$. Here it has been found that the phase transition is isosymmetrical ($C2/m\phantom{\rule{4pt}{0ex}}\ensuremath{\longrightarrow}\phantom{\rule{4pt}{0ex}}C2/m$). The bulk moduli and the compressibilities of the crystal axes of both the low- and the high-pressure phase, have been obtained from equation of state fits to the pressure evolution of the unit-cell parameters which were obtained from powder x-ray diffraction up to 12 GPa. The compaction of the crystal structure at the phase transition involves a doubling of the bulk modulus ${B}_{0}$ changing from 60.3(1) to 123.7(8) GPa and a change of the most compressible crystal axis from the (0, $b$, 0) direction in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ to the ($0.9a$, 0, $0.5a$) direction in $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$. The lattice dynamical calculations performed here on $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ served to explain the Raman spectra observed for the high-pressure phase of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ in a previous work demonstrating that the use of internal modes arguments in which the ${\mathrm{MoO}}_{n}$ polyhedra are considered as separate vibrational units fails at least in this molybdate. The electronic structure of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ was also calculated and compared with the electronic structures of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ and ${\mathrm{MgWO}}_{4}$ shedding some light on why ${\mathrm{MgWO}}_{4}$ is a much better scintillator than any of the phases of ${\mathrm{MgMoO}}_{4}$. These calculations yielded for $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ a ${Y}_{2}\mathrm{\ensuremath{\Gamma}}\ensuremath{\longrightarrow}\mathrm{\ensuremath{\Gamma}}$ indirect band gap of 3.01 eV in contrast to the direct bandgaps of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ (3.58 eV at $\mathrm{\ensuremath{\Gamma}}$) and ${\mathrm{MgWO}}_{4}$ (3.32 eV at $Z$).

Are photo-induced crystalline sulfur and S-II equivalent under high pressure?

Sulfur is known to be a photo-sensitive material. Photo-induced crystalline sulfur (p-S) was found in high-pressure Raman studies more than two decades ago. However, its structureremains not well understood. In this work, a thin α-S8 crystal was pressurized to 8 GPa in a diamond anvil cell (DAC) and its surface was repeatedly scanned by a 532 nm laser to produce the largest possible amount of p-S. A high-quality XRD pattern of p-S was obtained for the first time. The refinement of experimental data indicated the structural consistency between p-S and high-pressure and high-temperature (HPHT) S-II phases. Moreover, HPHT experiments were conducted to study the effect of temperature on p-S formation. The current work provides a strategy for producing the high-purity photo-induced product. The combination of synchrotron XRD, Raman spectroscopy, and microscopical imaging technology shows an effective way for photochemical research.

High-pressure X-ray Raman Study of CO2-V and N2

High-pressure X-ray Raman Study of CO₂-V and N₂

Strain induced electronic transition in 1T′ MoTe2: high pressure Raman, x-ray diffraction, resistivity measurements and first principles theoretical studies

A detailed high pressure study is carried out on 1T′ MoTe2 using x-ray diffraction (XRD) and Raman spectroscopy measurements up to about 30.5 GPa along with a room temperature resistivity measurement up to 14.3(4) GPa and density functional theory calculations. Though high-pressure XRD measurements show no structural transition, all the lattice parameters exhibit anomalous changes in the pressure region 8.4 to 12.7 GPa. The compressibility of the sample is found to be reduced by almost four times above 12.7 GPa with respect to that below 8.4 GPa. The anomalies in the Raman mode corresponding to the out of plane vibrations of Mo atoms sitting in the unit cell surface indicate a strong electron–phonon coupling possibly mediated by differential strain inside the unit cell. A rapid decrease in resistivity value up to about 7.0(2) GPa of pressure agrees well with the increase in the density of states (DOS) at the Fermi energy with pressure. Pressure evolution of band structure, as well as DOS at the Fermi level, shows an enhancement of the metallic character of the sample. First principle calculations show increased stress in the x and y directions compared to the z-direction with the application of pressure.

A high-pressure Raman study of FeTiO3 ilmenite: Fermi resonance as a manifestation of Fe-Ti charge transfer

We investigated the 300 K high-pressure behavior of ilmenite using Raman spectroscopy to 54 GPa. Upon compression, we observe a Fermi resonance between the lowest frequency Ag symmetry peaks (ν4 and ν5) between ~ 10 and ~ 30 GPa: bands that involve major components of Ti–O and Fe–O-related displacements, respectively. The peaks’ relative intensities switch at ~ 18 GPa and they also reach their minimum separation at ~ 20 GPa, indicating that their maximum resonance occurs between 18 and 20 GPa. The negative shift of the Ti–O-associated ν4 vibration under compression is fully consistent with a shift in valence of Ti from 4 + to 3 + under compression. Anomalously small mode shifts of other, more localized vibrations are also consistent with a charge transfer from Fe to Ti under compression. At higher pressures, we have not found definitive evidence for a transition to the perovskite-structure at 300 K, which has been well characterized at high pressures and temperatures. At 40 GPa, we observe an apparent reversible disordering that persists up to our highest pressure. The 300 K mode shifts of the Raman active modes in FeTiO3 under pressure are notably different from those of other ABO3 compounds (where A = Mg, Mn and B = Ti, Si); in other ilmenite-structured compounds, the peaks shift at a faster rate and there has not been any observation of Fermi resonance. Thus, iron’s complex electronic structure, and its charge transfer with titanium, appears to play a primary role in the behavior of phonons in FeTiO3 ilmenite.

High-pressure Raman study of osmium and rhenium up to 200 GPa and pressure dependent elastic shear modulus C 44

High-pressure Raman scattering from hexagonal close-packed (HCP) metals Os and Re have been extended up to 200 GPa, and the pressure-dependent shear modulus C 44 has been deduced from the Raman-active mode E 2g , which is generated from the adjacent vibration of atoms in hexagonal planes, providing the valuable information about the elastic properties for HCP metals under high pressure. Combined with the available data of HCP metals from previous works, a further study indicates that the ratio would be close to a constant value, 0.01, with increasing atomic number of metals. The results obtained from high-pressure Raman scattering will allow us to probe the elastic anisotropy of the HCP metals at very high pressure.

High-Pressure Raman Study of Fe(IO3)3: Soft-Mode Behavior Driven by Coordination Changes of Iodine Atoms

We report high-pressure Raman spectroscopy studies of Fe(IO3)3 up to nearly 21 GPa that have been interpreted with the help of density-functional theory calculations, that include the calculation of phonon dispersion curves and elastic constants at different pressures. Zero-pressure Raman-active mode frequencies and their pressure dependences have been determined. Modes have been assigned and correlated to atomic movements with the help of calculations. Interestingly, in the high-frequency region there are several modes that soften under compression. These modes have been identified as internal vibrations of the IO3 coordination polyhedron. Their unusual behavior is a consequence of the changes induced by pressure in the coordination sphere of iodine, which gradually change from a three-fold to almost six-fold coordination under compression. The coordination change is favored by the decrease of the stereoactivity of the iodine lone electron pair, so that likely real six-fold coordination is attained after a first-order phase transition previously reported to occur above 21 GPa. The strong non-linear behavior found in Raman-active modes as well as in theoretically calculated elastic constants has been discovered to be related to the occurrence of two previously unreported isostructural phase transitions at 1.5-2.0 GPa and 5.7-6.0 GPa as shown by dynamic instabilities close to the Brillouin zone center.

High-pressure Raman study of the alkaline-earth metal fulleride, Ca2.75C60

The structural stability and electronic properties of [Formula: see text] are probed by means of high-pressure Raman spectroscopy using two different pressure transmitting media that lead to practically identical results. Although [Formula: see text] is isostructural with the rare-earth metal fulleride, [Formula: see text] of the same stoichiometry, the pressure coefficients of characteristic intramolecular [Formula: see text] modes are larger than those in the [Formula: see text] counterpart but similar to those in the pristine [Formula: see text] solid. Since the reduced pressure coefficients in the Sm fulleride have been attributed to strong [Formula: see text] coupling, our findings for the isostructural Ca fulleride are compatible with a reduced orbital mixing and [Formula: see text] coupling.

Effects of Composition, Pressure, and Temperature on the Elastic Properties of SiO2–TiO2 Glasses: An Integrated Ultrasonic and Brillouin Study

We have systematically investigated the elastic properties (ρ, VP, VS, K, μ and σ) of eight SiO2–TiO2 glasses, varying in composition from 1.3 to 14.7 wt% TiO2, as a function of pressure up to 0.5 GPa by the pulse superposition (PSP) ultrasonic technique, and two compositions (1.3 and 9.4 wt% TiO2) up to ~5.7 GPa by Brillouin scattering in a diamond anvil cell. The parameters were also measured after annealing to 1020 °C. Composition–elasticity relationships, except for K and σ, are more or less linear; the annealing simply makes the relationships more uniform (less scatter). There is excellent agreement between the ultrasonic and Brillouin measurements at ambient and high pressure. The pressure-induced anomalous elastic behavior (negative dVP/dP and dK/dP) becomes more negative (more compressible) with the increasing TiO2 content. Correspondingly, the acoustic Grüneisen parameters become more negative with increases in the TiO2 content, reaching a minimum near ~8–10 wt% TiO2. The comparison of the low- and high-pressure ultrasonic and Brillouin VP and VS in two glasses (1.3 and 9.4 wt% TiO2) shows excellent agreement, defining the reversible elastic behavior at low pressures and irreversible behavior at higher pressures (≥5.7 GPa) well. This result is consistent with our previous high-pressure Raman study showing an irreversible structural change in a similar pressure range.

In situ high pressure neutron diffraction and Raman spectroscopy of 20BaO-80TeO2 glass.

The short-range structure of 20BaO–80TeO2 glass was studied in situ by high pressure neutron diffraction and high pressure Raman spectroscopy. Neutron diffraction measurements were performed at the PEARL instrument of the ISIS spallation neutron source up to a maximum pressure of 9.0 ± 0.5 GPa. The diffraction data was analysed via reverse Monte Carlo simulations and the changes in the glass short-range structural properties, Ba–O, Te–O and O–O bond lengths and speciation were studied as a function of pressure. Te–O co-ordination increases from 3.51 ± 0.05 to 3.73 ± 0.05, Ba–O coordination from 6.24 ± 0.19 to 6.99 ± 0.34 and O–O coordination from 6.00 ± 0.05 to 6.69 ± 0.06 with an increase in pressure from ambient to 9.0 GPa. In situ high pressure Raman studies found that the ratio of intensities of the two bands at 668 cm−1 and 724 cm−1 increases from 0.99 to 1.18 on applying pressure up to 19.28 ± 0.01 GPa, and that these changes are due to the conversion of TeO3 into TeO4 structural units in the tellurite network. It is found that pressure causes densification of the tellurite network by the enhancement of co-ordination of cations, and an increase in distribution of Te–O and Ba–O bond lengths. The original glass structure is restored upon the release of pressure.

High Pressure Raman Study of Cs2CuCl4

High-pressure Raman spectra of frustrated antiferromagnet CS2CUCl4 is investigated in a diamond anvil cell. Four characteristic modes of CS2CUCl4 are observed at ambient conditions. As the pressure increases, the sample undergoes a structure phase transition at 5 GPa. After decompressing, the low-pressure Raman spectra are recovered, which indicates a reversible phase transition. We discussed the results in terms of the change of structure with pressure.

(Invited) Resonance Raman Spectroscopy Studies in 2D and 1D Nanocarbons

Raman spectroscopy is a nondestructive tool widely used to characterize a variety of nanomaterials including the novel 2D and 1D systems such as graphene and MoS2, linear carbon chains, among others. Since electrons and phonons are specially coupled to each other due to the reduced dimensionality, this technique provides detailed information about the structure and electronic properties of theses layered systems thus allowing one to probe in the phonon spectra the numbers of layers and their interactions with the environment. Most of these 2D materials are on substrate and the understanding on how the substrate affects the physical properties of these atomic thickness layers is a key point for fully characterize the materials. In this regard, strain is an important variable to take into account because the adhesion of the layered materials depends on the nature of the substrate and a key question is how the strain is transferred from a given substrate to the 2D material, for instance. The use of hydrostatic pressure has been an effective method modulate the interactions between the 2D materials with substrates and environment and Raman spectroscopy has been important for investigating these strained 2D nanomaterials because the Raman cross section of these low dimensional materials is very large. In this talk, we discuss recent results obtained for high-pressure Raman studies of graphene (mono- and bilayer) and MoS2 (mono-, bi-, tri- and many layers) sitting on substrate. Depending on the pressure transmitting medium, the pressure-induced changes in the Raman modes frequency at different rates, which has been attributed to different strain transferred from the substrate to the 2D materials. In the case of MoS2, the behavior of Raman modes suggested that a biaxial stress is transmitted from substrate to the MoS2 layers by means of the substrate deformation due to pressure and the magnitude of such stress transfer is found to be dependent of the number of layers. We also discuss the effects of strain on the vibrational properties of linear carbon chains encapsulated in carbon nanotubes.