High-pressure Raman Spectroscopy Research Articles

Structural distortions in the high-pressure polar phases of ammonium metal formates

The high-pressure behaviour of ammonium metal formates has been investigated using high-pressure single-crystal X-ray diffraction on ammonium iron and nickel formates, and neutron powder diffraction on ammonium zinc formate in the pressure range of 0–2.3 GPa. A structural phase transition in the pressure range of 0.4–1.4 GPa, depending on the metal cation, is observed for all three ammonium metal formates. The hexagonal-to-monoclinic high-pressure transition gives rise to characteristic sixfold twinning based on the single-crystal diffraction data. Structure solution of the single-crystal data and refinement of the neutron powder diffraction characterise the pressure-induced distortions of the metal formate frameworks. The pressure dependence of the principal axes shows significantly larger anisotropic compressibilities in the high-pressure monoclinic phase (K1 = 48 TPa−1, K3 = −7 TPa−1) compared to the ambient hexagonal phase (K1 = 16 TPa−1, K3 = −2 TPa−1), and can be related to the symmetry-breaking distortions that cause deformation of the honeycomb motifs in the metal formate framework. While high-pressure Raman spectroscopy suggests that the ammonium cations remain dynamically disordered upon the phase transition, the pressure-induced distortions in the metal formate framework cause polar displacements in the ammonium cations. The magnitude of polarisation in the high-pressure phase of ammonium zinc formate was calculated based upon the offset of the ammonium cation relative to the anionic zinc formate framework, showing an enhanced polarisation of Ps ∼ 4 μC cm−2 at the transition, which then decreases with increasing pressure.

Morphology- and lattice stability-dependent performance of nanostructured Li4Ti5O12probed by in situ high-pressure Raman spectroscopy and synchrotron X-ray diffraction

In situhigh-pressure measurements of two different nanostructured Li4Ti5O12materials revealed important structural origins that influence their electrochemical performance.

Study of Phase Transformation in BaTe2O6 by in Situ High-Pressure X-ray Diffraction, Raman Spectroscopy, and First-Principles Calculations.

Structural and vibrational properties of orthorhombic BaTe2O6, a mixed valence tellurium compound, have been investigated by in situ synchrotron X-ray diffraction (XRD) studies up to 16 GPa and Raman spectroscopy up to 37 GPa using a diamond-anvil cell. The structure of orthorhombic BaTe2O6 has layers of [Te2O6]2–, formed by TeO6 octahedra and TeO5 square pyramids and Ba2+ ions stacked alternately along the ⟨010⟩ direction. A reversible pressure-induced structural transformation from the ambient orthorhombic (Cmcm) to a monoclinic (P21/m) structure is observed in both XRD and Raman spectroscopic investigations around 10 GPa. Ab initio calculations using density functional theory (DFT) corroborate this phase transition as well as the transition pressure. Both XRD and DFT calculations reveal that the high-pressure monoclinic structure is closely related to the ambient pressure orthorhombic structure, and the transformation is accompanied by a slight rearrangement of the structural units. Pressure evolution ...

Pressure-induced electronic topological transition in Sb2S3

We report the high-pressure vibrational properties and a pressure-induced electronic topological transition in the wide bandgap semiconductor Sb2S3 (Eg = 1.7–1.8 eV) using Raman spectroscopy, resistivity and x-ray diffraction (XRD) studies. In this report, high-pressure Raman spectroscopy and resistivity studies of Sb2S3 have been carried out to 22 GPa and 11 GPa, respectively. We observed the softening of phonon modes , and B2g and a sharp anomaly in their line widths at 4 GPa. The resistivity studies corroborate this anomaly around similar pressures. The changes in resistivity as well as Raman line widths can be ascribed to the strong phonon–phonon coupling, indicating clear evidence of isostructural electronic topological transition in Sb2S3. The previously reported pressure dependence of a/c ratio plot obtained also showed a minimum at ~5 GPa consistent with our high-pressure Raman and resistivity results.

Successive disorder to disorder phase transitions in ionic liquid [HMIM][BF4] under high pressure

In situ high-pressure Raman spectroscopy and synchrotron X-ray diffraction have been employed to investigate the phase behavior of ionic liquid, 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM][BF4]) under high pressure up to 20 GPa at room temperature. With increasing pressure, some characteristic bands of [HMIM][BF4] disappear, and some characteristic bands of [HMIM][BF4] display non-monotonic pressure-induced frequency shift and non-monotonic variation of full width at half-maximum. Two successive phase transitions at ∼1.7 GPa and 7.3 GPa have been corroborated by the results above. The glass transition pressure (Pg) of [HMIM][BF4] at ∼7.3 GPa has been obtained by ruby R1 line broadening measurements and analysis of synchrotron X-ray diffraction patterns, and its glass transition mechanism is also analyzed in detail. These facts are suggestive of two successive disorder to disorder phase transitions induced by compression, that is, [HMIM][BF4] serves as a superpressurized glass under the pressure above 7.3 GPa, which is similar to the glassy state at low temperature, and a compression-induced liquid to liquid phase transition in [HMIM][BF4] occurs at ∼1.7 GPa. Besides, the conformational equilibrium of the GAAA conformer and AAAA conformer was converted easily in liquid [HMIM][BF4], while it was difficult to be influenced in glassy state.

Soft modes and anharmonicity inH3[Co(CN)6]: Raman spectroscopy and first-principles calculations

In situ high-pressure Raman spectroscopy and ab initio calculations are carried out to investigate the phase stability and the thermal expansion behavior of ${\mathrm{H}}_{3}[\mathrm{Co}(\mathrm{CN}{)}_{6}]$. Raman studies at high pressures in a diamond anvil cell identify soft phonons and phase instability at 2.3 GPa. Evidence of pressure-induced amorphization is found at 11 GPa. The phonon frequencies and eigenvectors obtained from ab initio calculation are used to complement the observed phonon spectra and for assignment of Raman modes. The computed eigenvector displacement patterns indicate that the soft modes correspond to the CN-librational vibrations (${E}_{\mathrm{g}}$ mode) of the Co-CN-H-NC-Co linkages and their Gr\"uneisen parameters are found to be negative, in agreement with our measured values. The thermal expansion coefficient $(15.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$ calculated using our computed mode Gr\"uneisen parameters is found to be in good agreement with the reported value $(20\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$. Temperature-dependent phonon spectra down to 77 K are used to obtain the anharmonicities of different modes.

Cobalt ferrite nanoparticles under high pressure

We report by the first time a high pressure X-ray diffraction and Raman spectroscopy study of cobalt ferrite (CoFe2O4) nanoparticles carried out at room temperature up to 17 GPa. In contrast with previous studies of nanoparticles, which proposed the transition pressure to be reduced from 20–27 GPa to 7.5–12.5 GPa (depending on particle size), we found that cobalt ferrite nanoparticles remain in the spinel structure up to the highest pressure covered by our experiments. In addition, we report the pressure dependence of the unit-cell parameter and Raman modes of the studied sample. We found that under quasi-hydrostatic conditions, the bulk modulus of the nanoparticles (B0 = 204 GPa) is considerably larger than the value previously reported for bulk CoFe2O4 (B0 = 172 GPa). In addition, when the pressure medium becomes non-hydrostatic and deviatoric stresses affect the experiments, there is a noticeable decrease of the compressibility of the studied sample (B0 = 284 GPa). After decompression, the cobalt ferrite lattice parameter does not revert to its initial value, evidencing a unit cell contraction after pressure was removed. Finally, Raman spectroscopy provides information on the pressure dependence of all Raman-active modes and evidences that cation inversion is enhanced by pressure under non-hydrostatic conditions, being this effect not fully reversible.

Structural insights and elasticity of single-crystal antigorite from high-pressure Raman and Brillouin spectroscopy measured in the (010) plane

We report high-pressure Raman and Brillouin spectroscopy results measured in the (010) plane of a natural antigorite single crystal. We find that structural changes at >6 GPa lead to (1) an intensity crossover between Raman modes of the Si-O-Si bending vibrations, (2) changes of the compression behavior of Raman modes related to the SiO4 tetrahedra, (3) changes of the pressure derivative of the Raman shifts associated with OH stretching vibrations, (4) the emergence of a new Raman band in the OH spectral region, (5) a softening of the elastic constants c33 and c11, and (6) a directional change of the slowest compressional wave velocity in the a-c plane. In addition to the structural insights at high-pressure, the unique characteristics of our single-crystal sample allows for first direct measurements of the acoustic velocity anisotropy in a plane perpendicular to the basal a-b plane. Comparison to previously published data indicates that the elastic anisotropy of antigorite strongly depends on the FeO and/or Al2O3 content. In contrast, it seems not to be affected by increasing temperature as inferred from an additional high-temperature experiment performed in our study. These constraints are important for the interpretation of seismic anisotropy observations in subduction zone environments.

Ferroelectric soft mode of polarZnTiO3investigated by Raman spectroscopy at high pressure

Polar zinc titanate (ZnTiO${}_{3}$) is a promising lead-free ferroelectric compound with high spontaneous polarization of 75 $\ensuremath{\mu}\phantom{\rule{0}{0ex}}C/c\phantom{\rule{0}{0ex}}{m}^{2}$, which is comparable with that of PbTiO${}_{3}$. In this work a collaboration of researchers from Germany, UK, and USA present a detailed investigation of vibrational properties of ZnTiO${}_{3}$ via the high-pressure Raman spectroscopy, second-harmonic generation, and $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations.

Selected Reactive Sites Tuned by High Pressure: Oligomerization of Solid-State Cyanamide

We studied high-pressure reaction of solid cyanamide in a diamond anvil cell up to a pressure of 21.7 GPa. The results of in situ high-pressure Raman spectroscopy and angle-dispersive X-ray diffraction experiments indicated the occurrence of pressure-induced oligomerization in cyanamide over 13 GPa. The Raman, NMR, and UV absorption spectra of recovered samples confirmed that the chemical reaction was irreversible and that the high-pressure products contained C═N bonds. Subsequently, results of kinetic studies showed the relation between pressure and the reaction rate. Meanwhile dependence of activation volume with pressure indicated that the steric factors affected the reaction. Additionally, the increase in the amount of C–C bonds of recovered samples was attributed to the change of reactive sites of adjacent C≡N groups. These results suggest that the application of pressure can tune both the reactive sites and the reaction rate to affect oligomerization and even other chemical reactions.

Linear Carbon Chains under High-Pressure Conditions

A high-pressure resonance Raman spectroscopy study of linear carbon chains encapsulated inside multiwalled carbon nanotubes (MWCNTs) is reported. While the frequencies of the tangential modes of carbon nanotubes (G band) harden as the pressure increases, the vibrational frequencies of the chain modes (around 1850 cm–1) decrease, thus indicating a softening of the carbon–carbon bonds in this 1D solid. Pressure-induced irreversible structural changes in the linear carbon chains are unveiled by the red shift in the vibrational modes when pressure is released. These results have been interpreted as being due to a coalescence of carbon chains, and this hypothesis is supported by state-of-the-art atomistic reactive molecular dynamics simulations.

Pressure induced structural transitions in CuSbS2 and CuSbSe2 thermoelectric compounds

We have investigated the structural behavior of CuSbS2 and CuSbSe2 thermoelectric materials under high pressure conditions up to 80GPa using angle dispersive X-ray diffraction in a diamond anvil cell (DAC). We have also performed high pressure Raman spectroscopy measurements up to 16GPa. We observed a pressure-induced structural transformation from the ambient orthorhombic structure with space group Pnma to a triclinic type structure with space group P1 beginning around 8GPa in both samples and completing at 13GPa and 10GPa in CuSbS2 and CuSbSe2, respectively. High pressure Raman experiments complement the transitions observed by high pressure X-ray diffraction (HPXRD). The transitions were found to be reversible on releasing the pressure to ambient in the DAC. The bulk modulus and compressibility of these materials are further discussed.

Single-crystal diffraction and Raman spectroscopy of hedenbergite up to 33 GPa

Pyroxenes are important minerals in Earth’s upper mantle and subducting plate. Here, we report results of high-pressure single-crystal X-ray diffraction and Raman spectroscopy experiments conducted on natural Ca, Fe pyroxene hedenbergite up to ~33 GPa in diamond anvil cell. Unit cell parameters a, b, c, β and V, as well as bond lengths of hedenbergite are reported within the studied pressure range. Cell parameters exhibit continuous decrease on compression. Axial compressibilities of a, b and c are calculated to be 1.7(2), 4.9(5) and 2.13(9) × 10−3 GPa−1, respectively. Bulk modulus and its pressure derivative are determined to be 131(4) GPa and 3.8(3) by fitting third-order Birch–Murnaghan equation of state. Compression mechanism is dominated by polyhedral and bond compression trends typical of clinopyroxenes. In general, shorter bonds show lower compressibility, and SiO4, the smallest polyhedron, shows the lowest compressibility. Angle and elongation distortions are reported for the three types of polyhedra at high pressure. Thirteen vibrational modes are observed with Raman spectroscopy up to ~33 GPa. All observed mode frequencies increase as pressure increases.

Synthesis and high (pressure, temperature) stability of ZnTiO3 polymorphs studied by Raman spectroscopy

The phase-purity of ilmenite-type ZnTiO3 prepared by the ceramic method was investigated in dependence of the conditions during ball milling. The previously proposed addition of 2 ml ethanol to the starting materials led to a significant contamination of the product phase after a subsequent sintering process at 1073 K. However, by omitting ethanol this synthesis route led to a phase-pure sample of ZnTiO3 as confirmed by X-ray powder diffraction and Raman spectroscopy. High-temperature high-pressure experiments gave an ilmenite-type to perovskite-type phase boundary with a slope of dT/dP∼−135 K GPa−1 crossing ambient temperature conditions at ∼ 24 GPa in good agreement with previous calculations. Room-temperature high-pressure Raman spectroscopy experiments have shown the stability of the ilmenite-type phase up to a pressure of at least 38.5 GPa, the highest pressure applied in this study, indicating the presence of a kinetic barrier in this phase transition. The synthesis of ferroelectric LiNbO3-type ZnTiO3 was confirmed by second harmonic generation.

Room-temperature vibrational properties of potassium gadolinium double tungstate under compression up to 32 GPa

KGd(WO4)2 has been studied by high-pressure Raman spectroscopy at room temperature up to 32.2GPa. Evidences of two pressure-driven phase transitions have been detected. The low-pressure monoclinic phase undergoes a phase transition at 7.2(±0.9)GPa. The second transition is found at 14.2(±1.6)GPa. Both transitions are reversible. No evidence of pressure-induced amorphization is found up to 32GPa. The pressure dependence of the Raman active modes of the low- and high-pressure phases is reported. A Raman mode is detected to exhibit a weak softening in the low-pressure phase. Three Raman modes are also observed to have a similar behavior in the second high-pressure phase. The reported results are discussed comparing with the studies available in related tungstates. Emphasis is placed on the physical implications of the results.

High pressure Raman spectroscopy of H2O-CH3OH mixtures.

Complex intra-molecular interactions and the hydrogen-bonding network in H2O-volatile mixtures play critical roles in many dynamics processes in physical chemistry, biology, and Earth and planetary sciences. We used high pressure Raman spectroscopy to study the pressure evolution of vibrational frequencies and bonding behavior in H2O-CH3OH mixtures. We found that the presence of low CH3OH content in H2O increases the transition pressure where water crystallizes to ice VI, but does not significantly change the pressure where ice VI transforms to ice VII. Furthermore, the stiffening rates of C-H stretching frequencies dω/dP in CH3OH significantly decrease upon the crystallization of water, and the softening rates of the O-H stretching frequencies of ice VII are suppressed over a narrow pressure range, after which the frequencies of these modes shift with pressure in ways similar to pure CH3OH and ice VII, respectively. Such complex pressure evolution of Raman frequencies along with pronounced variations in Raman intensities of CH3OH within the sample, and the hysteresis of the water-ice VI phase transition suggest pressure-induced segregation of low content CH3OH from ice VII. These findings indicate the significant influence of volatiles on the crystallization of sub-surface ocean and thermal evolution within large icy planets and satellites.

Pressure driven ferroelectric to paraelectric transition in Sr doped BaTiO3

High pressure Raman spectroscopy, X-ray diffraction, and dielectric measurements have been carried out in Ba1−xSrxTiO3 (x = 0.05 and 0.1). Detailed structural analysis revealed a single phase transition from tetragonal P4mm to cubic Pm3m symmetry. Increase in Sr ion concentration resulted in decrease in the phase transition pressure. The dielectric measurements showed considerable lowering of transition pressure which has been attributed to bulk behaviour of the material.

Pressure Modulation of Backbone Conformation and Intermolecular Distance of Conjugated Polymers Toward Understanding the Dynamism of π-Figuration of their Conjugated System.

Continuous tuning of the backbone conformation and interchain distance of a π-conjugated polymer is an essential prerequisite to unveil the inherent electrical and optical features of organic electronics. To this end, applying pressure in a hydrostatic medium or diamond anvil cell is a facile approach without the need for side-chain synthetic engineering. We report the development of high-pressure, time-resolved microwave conductivity (HP-TRMC) and evaluation of transient photoconductivity in the regioregular poly(3-hexylthiophene) (P3HT) film and its bulk heterojunction blend with methanofullerene (PCBM). X-ray diffraction experiments under high pressure were performed to detail the pressure dependence of π-stacking and interlamellar distances in P3HT crystallites and PCBM aggregates. The HP-TRMC results were further correlated with high-pressure Raman spectroscopy and density functional theory calculation. The increased HP-TRMC conductivity of P3HT under pressure was found to be relevant to the planarity of the backbone conformation and intramolecular hole mobility. The effects of pressure on the backbone planarity are estimated to be ∼0.3 kJ mol(-1) based on the compressibility derived from the X-ray diffraction under high pressure, suggesting the high enough energy to cause modulation of the planarity in terms of the Landau-de Gennes free energy of isolated P3HT chains as 0.23 kJ mol(-1). In contrast, the P3HT:PCBM blend showed a simple decrease in photoconductivity irrespective of the identical compressive behavior of P3HT. A mechanistic insight into the interplay of intra- and intermolecular mobilities is a key to tailoring the dynamic π-figuration associated with electrical properties, which may lead to the use of HP-TRMC for exploring divergent π-conjugated materials at the desired molecular arrangement and conformation.

High pressure Raman spectroscopy investigation on acetonitrile and acetonitrile–water mixture

High-pressure Raman scattering studies on pure acetonitrile and an acetonitrile–water mixture at a molar ratio of (nCH3CN : nH2O) 1 : 7.25 were performed in a diamond anvil cell at room temperature.

High-pressure Raman spectroscopy of Re3N crystals

Well-crystallized plate-like Re3N crystals were synthesized through a high-pressure solid-state metathesis (HPSSM) reaction in large volume press (LVP) at 14GPa and 1973K. The vibrational properties of Re3N crystals were studied by high-pressure Raman spectroscopy in diamond anvil cell (DAC) up to 20GPa. The mode-Grüneisen parameters for Raman modes of Re3N were determined experimentally, which were found substantially being larger than previously calculated values.