High-pressure Raman Study Research Articles

Pressure-enhanced ferroelectric polarisation in a polar perovskite-like [C2H5NH3]Na0.5Cr0.5(HCOO)3metal–organic framework

We report high-pressure X-ray diffraction and Raman studies combined with DFT calculations.

Stability and flexibility of heterometallic formate perovskites with the dimethylammonium cation: pressure-induced phase transitions.

We report the high-pressure properties of two heterometallic perovskite-type metal-organic frameworks (MOFs) templated by dimethylammonium (NH2(CH3)2, DMA+) with the general formula [DMA]MI0.5CrIII0.5(HCOO)3, where MI = Na+ (DMANaCr) and K+ (DMAKCr). The high-pressure Raman scattering studies show crystal instabilities in the 4.0-4.4 GPa and 2.0-2.5 GPa ranges for DMANaCr and DMAKCr, respectively. The mechanism is similar in the two compounds and involves strong deformation of the metal-formate framework, especially pronounced for the subnetwork of CrO6 octahedra, accompanied by substantial compressibility of the DMA+ cations. Comparison with previous high-pressure Raman studies of sodium-chromium heterometallic MOFs show that the stability depends on the templated cation and increases as follows: ammonium < imidazolium < DMA+. Density functional theory (DFT) calculations are performed to get a better understanding of the structural properties leading to the existence of phase transitions. We calculate the energy of the hydrogen bonds (HBs) between the DMA+ cation and the metal formate cage, revealing a stronger interaction in the DMAKCr compound due to a HB arrangement that primarily involves the energetically preferred bonding to KO6 octahedra. This material however also has a smaller structural tolerance factor (TF) and a higher vibrational entropy than DMANaCr. This indicates a more flexible crystal structure, explaining the lower phase transition pressure, as well as the previously observed phase transition at 190 K, which is absent in the DMANaCr compound. The DFT high-pressure simulations show the largest contraction to be along the trigonal axis, leading to a minimal distortion of the HBs formed between the DMA+ cations and the metal-formate sublattice.

High Pressure Raman, Optical Absorption, and Resistivity Study of SrCrO4.

We studied the electronic and vibrational properties of monazite-type SrCrO4 under compression. The study extended the pressure range of previous studies from 26 to 58 GPa. The existence of two previously reported phase transitions was confirmed at 9 and 14 GPa, and two new phase transitions were found at 35 and 48 GPa. These transitions involve several changes in the vibrational and transport properties with the new high-pressure phases having a conductivity lower than that of the previously known phases. No evidence of chemical decomposition or metallization of SrCrO4 was detected. A tentative explanation for the reported observations is discussed.

High pressure Raman study of type-I collagen.

The high pressure response of type-I collagen from bovine Achilles tendon is investigated with micro-Raman spectroscopy. Fluorinert™ and methanol-ethanol mixtures were used as pressure transmitting media (PTM) in a diamond anvil cell. The Raman spectrum of collagen is dominated by three bands centred at approximately 1450, 1660 and 2930cm-1 , attributed to C-H deformation, C=O stretching of the peptide bond (amide-I band) and C-H stretching modes respectively. Upon pressure increase, using Fluorinert™ as PTM, a shift towards higher frequencies of the C-H stretching and deformation peaks is observed. Contrary, the amide-I band peaks are shifted to lower frequencies with moderate pressure slopes. On the other hand, when using the alcohol mixture as PTM, the amide-I band exhibits more pronounced C=O bond softening, deduced from the shift to lower frequencies, suggesting a strengthening of the hydrogen bonds between glycine and proline residues of different collagen chains due to the presence of the polar alcohol molecules. Furthermore, some of the peaks exhibit abrupt changes in their pressure slopes at approximately 2 GPa, implying a variation in the compressibility of the collagen fibres. This could be attributed to a pitch change from 10/3 to 7/2, sliding of the tropocollagen molecules, twisting variation at the molecular level and/or elimination of the D-gaps induced by kink compression. All spectral changes are reversible upon pressure release, which indicates that denaturation has not taken place. Finally, a minor lipid phase contamination was detected in some sample spots. Its pressure response is also monitored.

Pressure-induced Lifshitz and structural transitions in NbAs and TaAs: experiments and theory

High pressure Raman, resistivity and synchrotron x-ray diffraction studies on Weyl semimetals NbAs and TaAs have been carried out along with density functional theoretical (DFT) analysis to explain pressure induced structural and electronic topological phase transitions. The frequencies of first order Raman modes harden with increasing pressure, exhibiting a slope change at GPa for NbAs and GPa for TaAs. The resistivities of NbAs and TaAs exhibit a minimum at pressures close to these transition pressures and also a change in the bulk modulus is observed. Our first-principles calculations reveal that the transition is associated with an electronic Lifshitz transition at for NbAs while it is a structural phase transition from body centered tetragonal to hexagonal phase at for TaAs. Further, our DFT calculations show a structural phase transition at 24 GPa from body centered tetragonal phase to hexagonal phase.

Probing lattice dynamics and electron-phonon coupling in the topological nodal-line semimetal ZrSiS

Topological materials provide an exclusive platform to study the dynamics of relativistic particles in table-top experiments and offer the possibility of wide-scale technological applications. ZrSiS is a newly discovered topological nodal-line semimetal and has drawn enormous interests. In this report, we have investigated the lattice dynamics and electron-phonon interaction in single crystalline ZrSiS using Raman spectroscopy. Polarization and angle resolved measurements have been performed and the results have been analyzed using crystal symmetries and theoretically calculated atomic vibrational patterns along with phonon dispersion spectra. Wavelength and temperature dependent measurements show the complex interplay of electron and phonon degrees of freedom, resulting in resonant phonon and quasielastic electron scatterings through inter-band transitions. Our high-pressure Raman studies reveal vibrational anomalies, which were further investigated from the high-pressure synchrotron x-ray diffraction (HPXRD) spectra. From HPXRD, we have clearly identified pressure-induced structural transitions and coexistence of multiple phases, which also indicate possible electronic topological transitions in ZrSiS. The present study not only provides the fundamental information on the phonon subsystem, but also sheds some light in understanding the topological nodal-line phase in ZrSiS and other iso-structural systems.

A high pressure Raman study on confined individual iodine molecules as molecular probes of structural collapse in the AlPO4-5 framework.

The mechanical stability of porous zeolitic materials has long been an important issue due to their advanced applications in many fields. Here, we choose to study the pressure induced structural modifications on the AlPO4-5 (AFI) framework. We find that the Raman characteristics of the confined iodine molecules in the AFI channels, with a low filling density, show discontinuities at around 3 and 10 GPa, which can be attributed to the implications of framework changes. Subsequent theoretical simulations on the AFI framework demonstrate that both a tilting mechanism along the c axis and a rotating mechanism in the ab plane of the tetrahedrons contribute to the structural deformation, and the AFI framework is collapsible at 4 and 10 GPa, which confirms those values found in the Raman studies. In this nanoconfinement system of I@AFI, the host and guest depend on and interact with each other mutually. No supporting effect on the AFI framework is found for the confined individual iodine molecules with such a low filling density, but they can be regarded as molecular probes to reflect the structural collapse of AFI. Thus, we provide a novel way to detect the structural deformation of porous materials under high pressure.

Исследование методом спектроскопии комбинационного рассеяния света сверхгидратации цеолита ломонтита при высоком давлении в различных водосодержащих средах

Исследование методом спектроскопии комбинационного рассеяния света сверхгидратации цеолита ломонтита при высоком давлении в различных водосодержащих средах

Nanofluid lubrication and high pressure Raman studies of oxygen functionalized graphene nanosheets

Ultralow friction coefficient in reduced graphene oxide (rGO) nanofluid was observed at high pressure lubrication conditions. High pressure Raman spectroscopic studies of graphene oxide (GO) and rGO in a hydrostatic pressure medium in diamond anvil cell (DAC) showed an increase in G-band linewidth in GO but this value was decreased in rGO at same pressure range due to the defect relaxation in sp2 networks. Moreover, loss of recovery of G band linewidth in decompression cycles was clearly noticed in both the samples but it was significantly higher in GO due to the irreversible sp3 into sp2 planar phase.

Pressure-induced electronic and structural phase transitions in Dirac semimetal Cd3As2: Raman study

We report on the high-pressure Raman study of , a three-dimensional Dirac semimetal, up to 19 GPa at room temperature. Our study shows that light scattering by intervalley and intravalley density fluctuations give rise to electronic Raman scattering (ERS), with Lorentzian-like lineshape at low frequency. The strength and linewidth of the ERS are pressure dependent and exhibit a significant drop at , signifying a breakdown of the Dirac semimetal to a semiconducting phase. The first phase transition at Pc1 is also clearly identified by the significant changes in the pressure derivatives of phonon frequencies and linewidths. Pressure dependence of phonon parameters also reveal a second phase transition at , being reported for the first time. This semiconductor-to-semiconductor transition coincides with the abrupt changes seen in the activation energy (band gap) of the semiconductor phase.

Pressure-induced polymorphism in hypervalent CsI3

We report the results of ambient temperature high-pressure synchrotron-based x-ray diffraction, Raman, and electrical resistance study of $\mathrm{Cs}{\mathrm{I}}_{3}$ up to 29, 25, and 8 GPa, respectively, and confirm three-phase transitions under quasihydrostatic conditions. The ambient orthorhombic (space group (SG): Pnma) phase of $\mathrm{Cs}{\mathrm{I}}_{3}$ is stable up to a pressure of $\ensuremath{\sim}1.3\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, above which a phase transition to a trigonal ($\mathrm{SG}:P\text{\ensuremath{-}}3c1$) phase is observed. The stability region of the trigonal phase has been found to be up to 22.6 GPa, above which the trigonal phase transforms to a cubic ($\mathrm{SG}:Pm\text{\ensuremath{-}}3n$) phase which remains stable until the maximum pressure of 29 GPa achieved in this study. A third-order Birch-Murnaghan equation of state fit to the pressure volume ($P\text{\ensuremath{-}}V$) data yields a bulk modulus of 17.7(9) GPa for the trigonal phase. Raman spectroscopic measurements however indicate three-phase transitions at $\ensuremath{\sim}1.3,\phantom{\rule{0.16em}{0ex}}4.0$, and 22.6 GPa, respectively. The electrical resistance measured in the low-pressure region up to 8 GPa indicates an electronic transition at around 4 GPa confirming the Raman result observed at 4.0 GPa. The $P\text{\ensuremath{-}}V$ data when transformed to the universal equation of state (UEOS) show a deviation from linearity around 4.0 GPa confirming the electronic transition. The present study has thus revealed a three-phase structural sequence in alkali trihalides, viz., orthorhombic (SG: Pnma) to trigonal ($\mathrm{SG}:P\text{\ensuremath{-}}3c1$) to a cubic ($\mathrm{SG}:Pm\text{\ensuremath{-}}3n$) phase.

High-pressure Raman study on the superconducting pyrochlore oxide Cd2Re2O7

The superconducting pyrochlore oxide Cd2Re2O7 (Tc=1K), which is now considered as a candidate of the spin-orbit-coupled metal, shows an inversion-symmetry-breaking structural transition at Ts1=200K. Ts1 decreases with increasing pressure and disappears at around Pc=4.2GPa, where at least four high-pressure phases with tiny structural distortions are suggested by means of powder X-ray diffraction [Yamaura PRB 2017]. We have carried out Raman scattering experiments to investigate changes in the crystal symmetry under high pressures up to 4.8GPa. A structural transition at 1.9–3.0GPa and the recovery of inversion symmetry above Pc are observed at 12K.

High pressure Raman studies of single- and bi-layer graphene on copper

High pressure Raman studies of single- and bi-layer graphene on copper

High-pressure and high-temperature Raman study of cinnabar

ABSTRACTThe effects of temperature (300–600 K) and pressure (0–31.8 GPa) on the Raman spectra of natural cinnabar powder were investigated in a heatable diamond anvil cell. Raman spectral changes caused by phase transitions were observed at high pressures, which suggests a phase transformation from pure hexagonal to the coexistence of a hexagonal and rock salt structure, and finally to the rock salt structure. With increasing pressure, A1 and modes exhibited a blueshift (0.87 cm−1/GPa) and redshift (−3.64 cm−1/GPa), respectively, while both modes underwent a redshift (−2.22 and −2.09 × 10−2 cm−1/K, respectively) with increasing temperature. The mode Grüneisen parameters of A1 and at room temperature were calculated by the pressure dependences of the vibrational frequencies and the X-ray diffraction data from previous research. Simultaneous high-pressure and high-temperature results were globally fitted, and the coupling coefficients for temperature and pressure dependence of the Raman shifts were determined to be 7.28 × 10−4 and 5.12 × 10−4 cm−1/K · GPa for A1 and , respectively.

Pressure induced metastable polymerization in doped C60 materials

High pressure Raman studies have been carried out on C60/AgNO3 and C60/Ni(OEP) up to 30 GPa. In both these doped C60 materials, pressure-induced metastable ordered polymers can be observed after pressure release. The results show that both the quenched materials contain chainlike polymers and dimers. We also find that the degree of polymerization is higher in these doped C60 materials than in bulk C60 materials after similar high pressure treatment and that C60/AgNO3 contains a higher fraction of chainlike polymers than C60/Ni(OEP) after decompression from same pressure. The results can be understood by considering the different initial lattice structures of these materials and the confinement effects of the dopants.

Structural transitions of ordered kesterite-type Cu2ZnSnS4 under pressure

We have investigated the high-pressure structural and vibrational behavior of the ordered kesterite-type Cu2ZnSnS4 compound. Our investigations have revealed two structural transitions: a kesterite-to-disordered kesterite transition was observed between 7 and 9 GPa, which involves a Zn/Cu disorder within the respective cationic sublattice, whereas a rocksalt-type structure was realized at ∼15 GPa. The latter transition is accompanied by a cationic coordination increase from fourfold-to-sixfold with respect to the sulfur anions. The predicted kesterite-to-stannite transition was not detected. Furthermore, our high-pressure Raman studies have shown that the aforementioned Zn/Cu cationic disorder will always be present in Cu2ZnSnS4 under relatively moderate compression.

水热法合成Pb&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;8&lt;/sub&gt;及其物性研究

利用水热合成法制备了Pb2Te3O8单晶样品，通过晶体结构精修软件GSAS对X射线衍射图谱的精修确定了样品相及其结构参数。随后对Pb2Te3O8进行了红外吸收光谱以及高压拉曼光谱测试。红外光谱实验中确定了Pb2Te3O8对应的红外吸收峰位置，高压拉曼研究是利用金刚石压砧在室温下进行，在0.3~21.8 GPa压强范围内测定了拉曼光谱，通过分析拉曼峰的频移和劈裂变化情况，发现随着压力的增加所有振动模的频率是单调线性变化的并且没有新峰出现，拉曼谱图的这种变化表明在Pb2Te3O8在压力高达21.8 GPa高压拉曼实验中并没有压力引起的相变发生。 Pb2Te3O8 samples were prepared by hydrothermal synthesis. The phase and structural parameters of the synthesized Pb2Te3O8 sample were determined by Rietveld refinement of the observed powder XRD data. The infrared absorption spectra, ambient pressure and high pressure Raman spectra of Pb2Te3O8 were measured. The infrared absorption peak of Pb2Te3O8 was determined by infrared spectroscopy. The high-pressure Raman study was carried out at room temperature by using a diamond anvil, and the Raman spectra were measured in the range of 0.3 - 21.8 GPa. By analyzing the frequency shift and splitting of the Raman peak, it is found that the frequency of all the vibrational modes varies monotonically and without the presence of new peaks as the pressure increase, this change in the Raman spectrum indicates that the structure of Pb2Te3O8 can be stabilized to 21.8 GPa without phase transition.

Pressure dependent Raman studies in the K2Mo2O7·H2O crystal

The pressure dependent Raman scattering in the potassium molybdenum oxide hydrate crystal, K2Mo2O7·H2O, was measured. The high pressure Raman study showed, that the compound remains in the triclinic structure within the 0.0–3.81GPa range and undergoes a structural phase transition between 3.81 and 4.13GPa. This particular phase transition is most likely connected with changes in the Raman spectrum, in which the number of modes associated originally with the stretching vibrations in the MoO5 and MoO6 units is increased. However, the phase at atmospheric pressure shows bands due to the presence of only one equivalent site, while in the high-pressure phase, two bands are associated with the stretching modes. Continuing the pressure evolution up to 17.04GPa, two further phase transitions occurred in this crystal in the 6.3–8.1GPa and the 12.3–14.0GPa range, respectively. The Raman spectra measured at about 17.04GPa presented a crystal structure, which experienced a pre-amorphization with a total loss of all lattice modes. This particular result is indicative that this material may have undergone a complete amorphization at pressures larger than 17.04GPa. Then, the reversible character in the triclinic P-1 (Ci1) structure was recovered after releasing the pressure.

High-pressure behavior of bromine confined in the one-dimensional channels of zeolite AlPO4-5 single crystals.

We present a joint experimental and theoretical study on the high-pressure behavior of bromine confined in the one-dimensional (1D) nanochannels of zeolite AlPO4-5 (AFI) single crystals. Raman scattering experiments indicate that loading bromine into AFI single crystals can lead to the formation of bromine molecular chains inside the nanochannels of the crystals. High-pressure Raman and X-ray diffraction studies demonstrate that high pressure can increase the length of the confined bromine molecular chains and modify the inter- and intramolecular interactions of the molecules. The confined bromine shows a considerably different high-pressure behavior to that of bulk bromine. The pressure-elongated bromine molecular chains can be preserved when the pressure is reduced to ambient pressure. Theoretical simulations explain the experimental results obtained from the Raman spectroscopy and X-ray diffraction studies. Furthermore, we find that the intermolecular distance between confined bromine molecules gradually becomes comparable to the intramolecular bond length in bromine molecules upon compression. This may result in the dissociation of the bromine molecules and the formation of 1D bromine atomic chains at pressures above 24 GPa. Our study suggests that the unique nanoconfinement has a considerable effect on the high-pressure behavior of bromine, and the confined bromine species concomitantly enhance the structural stability of the host AFI single crystals.

High-pressure Raman study of Terephthalonitrile

The in situ high-pressure Raman spectra of Terephthalonitrile (TPN) have been investigated from ambient to 12.6GPa at room temperature. All the fundamental vibrational modes of TPN at ambient were assigned based on the first-principle calculations. A detailed Raman spectroscopy analysis revealed that TPN underwent a phase transition at ~5.3GPa. The frequencies of the TPN Raman peaks increase with increasing the pressure which can be attributed to the reduction in the interatomic distances and the escalation of effective force constants. The intensity of the C-C-C ring-out-plane deformation mode increases gradually as the frequency remains almost constant during the compression which can be explained by the existence of π-π interactions in TPN molecules. Additionally, the pressure-induced structural changes of TPN on the Fermi resonance between the C≡N out-of-plane vibration mode and the C−CN out-of-plane vibration mode have been analyzed.