High-pressure Raman Spectroscopy Measurements Research Articles

Pressure-induced superconductivity in Bi4(I1−x Br x )4 crystals grown by chemical vapor transport and flux methods

Achieving superconductivity in topological materials is thought as a promising route for realizing topological superconductivity, which may provide potential applications to quantum computation. Previously, rich superconducting phases have been reported in the pressurized Bi4I4 and Bi4Br4 crystals which belong to an interesting quasi-one-dimensional topological system. In this work, we have performed a high-pressure study on some Bi4(I1−x Br x )4 crystals grown by two different methods. Remarkably, crystals grown by the chemical vapor transport (CVT) method and the self-flux method show clearly different pressure effects. In the CVT-grown crystals, only one superconducting transition is observed, while three superconducting transitions can be detected in crystals grown by the flux method. Through comparisons of the pressure-dependent phase diagrams and the upper critical field behaviors in the two kinds of crystals, the higher superconducting transition (>6 K) in the flux-grown crystals is suggested to come from the residual Bi. High-pressure Raman spectroscopy measurements on both kinds of crystals have confirmed the occurrence of a similar structural transition around 10 GPa in Br-doped samples. Overall, our data could be helpful for identifying the intrinsic pressure-induced superconductivity in various Bi-based materials.

Pressure-induced structural instability and amorphization in compressed α-V2O5

We report pressure-induced structural instability and amorphization in compressed orthorhombic α-V2O5 by in situ synchrotron x-ray diffraction observations and Raman spectroscopy measurements in diamond-anvil cells up to 20.2 GPa and 21.9 GPa, respectively. In situ x-ray lattice parameters (a, b, c) and d-spacings at high pressure show that the orthorhombic α-V2O5 exhibits an apparent discontinuity at ∼4.3 GPa, indicating the occurrence of structural distortion upon compression. Upon further pressurization, the distorted α-V2O5 starts to be amorphizing at ∼12.6 GPa and becomes fully amorphous at ∼20.2 GPa, which is again confirmed by our high-pressure Raman spectroscopy measurements. This obtained amorphous V2O5 polymorph can be recovered at ambient conditions, showing that the pressure-driven distorted orthorhombic-to-amorphous phase transformation is irreversible. Moreover, Grüneisen parameters of various Raman modes for α-V2O5 are derived, which are in agreement with the previously experiential study by Arora et al. Our results might be important to understand the high-pressure behaviors in M2O5 polymorphs for their uses at extreme conditions.

How does the flexibility of pyrrolidinium cations affect the phase behaviour of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide homologues under stressful conditions?

We conducted high-pressure Raman spectroscopy measurements on a series of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyr1n][TFSI], n = 3, 4, 6 and 8) homologues that have different alkyl chain lengths, n, at room temperature. The results showed that all [Pyr1n][TFSI] samples formed a glassy state in which the glass transition pressure (pg) slightly increased with an increase in n. This tendency is similar to prior results of high-pressure glass formation of [Cnmim][TFSI], although the pgs for [Pyr1n][TFSI] are larger than those for [Cnmim][TFSI] with corresponding n by ∼0.5 GPa. We discuss the local structural changes occurring in [Pyr1n][TFSI] in view of the conformational changes of the Pyr+1n cation and TFSI- anion.

High-pressure Raman spectroscopy of perovskite oxynitride SrTaO2N

Perovskite oxynitrides exhibit emergent physical properties driven by a substitution of nitrogen for oxygen. The nitrogen substitution distorts the octahedra and induces octahedral tilting which are responsible for a relaxor-like temperature-insensitive dielectric response. To study the local structural evolution of oxynitride SrTaO2N under pressure, we performed in-situ high-pressure Raman spectroscopy measurements with a diamond anvil cell up to a pressure of P = 19 GPa. We find that forbidden modes were active, indicating a lowering of local symmetry from non-polar I4/mcm due to the octahedral distortion. Under pressure, a mode associated with TaO4N2 exhibits a negative wavenumber shift and merges with a nearby mode at a pressure of P ≃ 4.2 GPa. This result indicates a suppression of octahedral tilting under pressure.

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.

High Pressure Raman Spectroscopic Study of the Effects of n‐Ethylamines and Water on the 2‐Nitropropane/Nitric Acid System

AbstractHigh pressure Raman spectroscopy measurements in a diamond anvil cell (0–10 GPa) on 2‐nitropropane/nitric acid/X (X=triethylamine, diethylamine, and water) ternary systems and 2‐nitropropane/nitric acid/water/Y (Y=triethylamine and diethylamine) quaternary systems are reported. The modifications of the chemical behavior of the 2‐nitropropane/nitric acid model system, induced by the presence of triethylamine, diethylamine, and/or water, were studied at ambient and high pressure. At ambient pressure, the ionization of the nitric acid has been observed with each of the additives. Moreover, in the case of ethylamines, new peaks have been observed and the hypothesis of a 2‐nitropropane/ethylamine complex is advanced. At high pressure, the decomposition of the 2‐nitropropane/nitric acid system, with an oxygen balance near zero, has been observed only in presence of triethylamine. The role of each additive to the 2‐nitropropane/nitric acid system in the modification of the respective reducing and oxidizing character of the components, and in the reactivity of the system, is discussed. Several hypotheses are advanced concerning the sensitizing effect of the additives on the 2‐nitropropane/nitric acid system.

High Pressure Raman Spectroscopy of Nitric Acid / 2-Nitropropane Mixtures

High pressure Raman spectroscopy measurements in a diamond anvil cell (0–20 GPa) on HNO3-2-nitropropane mixtures are reported. These mixtures have been chosen as a model propellant, where neither component is explosive by itself. The high pressure decomposition of the mixture with oxygen balance (O.B.=0) has been observed and the recovered products analyzed and identified. The initiation of the chemical decomposition is correlated with he weakening of the NO bonds in the mixture. The study of a mixture at O.B.=0 with H15NO3 permitted us to determine the origin of the nitrogen formation and to suggest a chemical pathway.

High Pressure Raman Spectroscopy of Nitric Acid

New high pressure Raman spectroscopy measurements on pure anhydrous HNO3 and HNO3−H2O mixtures up to 38 mol % in water (commercial grade concentration) are reported up to 50 GPa. The main feature is the reversible and progressive transformation of pure solid nitric acid at pressures between 10 and 17 GPa, evidenced by an enhancement of the 1057 cm-1 peak assigned to the ν1 vibrational stretching mode of the group and by the softening of the symmetric stretching NO2 mode with pressure. The formation of the H bonding by a charge transfer due to a strong interaction between the surrounding molecules is discussed. Two hypotheses are discussed: nitric acid might be pressure autoionized or cross-linked as a reversible polymer-like compound. These two hypotheses are not mutually exclusive. An increase of water concentration in nitric acid shifts the limit of the appearance of the vibrational mode to lower values of the pressure at constant temperature.