基于拉曼频移的白宝石压腔无压标系统高温高压实验标定

在高压实验科学中, 各类宝石压腔是最为常见的高压设备之一, 其样品腔中压力的精确标定是实验的关键. 目前, 人们主要通过加入红宝石等压标物质来进行定压, 但压标物质的加入会增加实验的装样难度, 改变样品腔中的物理化学环境, 甚至直接与实验样品发生反应, 从而对实验结果产生影响. 在0–6.3 GPa和300–573 K下, 利用共聚焦拉曼显微镜, 根据白宝石压砧砧面的ν₁₂ 拉曼频移与温度和压力的变化关系, 建立了一套适用于高温高压水热体系的无压标白宝石压腔系统. 实验结果表明: 白宝石砧面的ν₁₂ 峰随着压力的升高发生线性蓝移, 而随着温度升高则发生线性红移, 且温度和压力对拉曼频移的影响存在耦合效应. 利用本实验结果, 可在高温高压下根据白宝石砧面的拉曼频移计算出样品腔的压力P=(Δλ-0.01913×ΔT)/(1.9158-0.00105×ΔT), 在物理学、材料学和地球科学等领域具有重要应用.     

1,3,4-oxadiazole crystals under high pressure-phase transitions and properties

Abstract

Two differently substituted crystalline 2,5-diphenyl-1,3,4-oxadiazole compounds are investigated in the pressure range up to 5GPa. Phase transitions are detected using Raman and X-ray investigations.     

高温高压下草酸脱羧反应中的拉曼光谱研究

应用水热金刚石压腔结合拉曼光谱技术研究了高温高压下草酸溶液的热稳定,使用拉曼光谱对其脱羧反应及产物进行监测。结果表明低温升温过程中,草酸的拉曼谱图中各个特征振动峰没有发生变化,随着温度的继续升高,其特征振动峰逐渐变弱。达到一定温度后,羧基的拉曼特征峰消失,草酸发生脱羧反应:C2H2O4—2CO2+H2生成CO2和H2。高温高压下草酸发生热分解的温度压力之间呈线性关系,其线性回归方程为P(MPa)=12.839 T(K)-5 953.7,R2=0.99。草酸脱羧反应的摩尔体积变化与温度压力的关系为ΔV(cm-3.mol-1)=16.69-0.002P(MPa)+0.005 2T(K),R2=0.99。     

Effect of solvation on polarization characteristics of fluorescence for 1,3,4-oxadiazole derivatives in binary solvents

We have studied the degree of fluorescence polarization for 1,3,4-oxadiazole derivatives in the binary solvents hexane-ethanol, toluene-ethanol, toluene-acetonitrile, 2-propanol-water, dioxane-water, 2-propanol-glycerol, glycerol-water vs. the composition of the solvent. We have shown that in systems with predominant universal interactions, the characteristic changes in the degree of polarization reflect the change in the viscosity of the mixture, while in systems with pronounced specific interactions they reflect the characteristic structural features of the fluorescent centers. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 677–683, September–October, 2008.     

Phase transitions in 1,3,4-oxadiazole crystals under high pressure

Crystalline 2,5-di(4-nitrophenyl)-1,3,4-oxadiazole (DNO) has been investigated at pressures up to 5 GPa using Raman and optical spectroscopy as well as energy dispersive X-ray techniques. At ambient pressure DNO shows an orthorhombic unit cell (a=0.5448 nm, b=1.2758 nm, c=1.9720 nm, density 1.513 g cm⁻³) with an appropriate space group Pbcn. From Raman spectroscopic investigations three phase transitions have been detected at 0.88, 1.28, and 2.2 GPa, respectively. These transitions have also been confirmed by absorption spectroscopy and X-ray measurements. Molecular modeling simulations have considerably contributed to the interpretation of the X-ray diffractograms. In general, the nearly flat structure of the oxadiazole molecule is preserved during the transitions. All subsequent structures are characterized by a stack-like arrangement of the DNO molecules. Only the mutual position of these molecular stacks changes due to the transformations so that this process may be described as a topotactical reaction. Phases II and III show a monoclinic symmetry with space group P2₁/c with cell parameters a=1.990 nm, b=0.500 nm, c=1.240 nm, β=91.7°, density 1.681 g cm⁻³ (phase II, determined at 1.1 GPa) and a=1.890 nm, b=0.510 nm, c=1.242 nm, β=89.0°, density 1.733 g cm⁻³ (phase III, determined at 2.0 GPa), respectively. The high-pressure phase IV stable at least up to 5 GPa shows again an orthorhombic structure with space group Pccn with corresponding cell parameters at 2.9 GPa: a=0.465 nm, b=1.920 nm, c=1.230 nm and density 1.857 g cm⁻³. For the first phase a blue pressure shift of the onset of absorption by about 0.032 eV GPa⁻¹ has been observed that may be explained by pressure influences on the electronic conjugation of the molecule. In the intermediate and high-pressure phases II–IV the onset of absorption shifts to increased wavelengths due to larger intermolecular interactions and enhanced excitation delocalization with decreasing intermolecular spacing.     

Raman spectra of forsterite and fayalite at high pressures and room temperature

Abstract

Raman spectra of the two pure end-members of olivine (forsterite and fayalite) were studied at high pressures and room temperature in a diamond-anvil cell using both single-crystals and polycrystalline samples in pressure mediums of either an ethanol-methanol mixture or H₂O. Variations with pressure were studied up to 170kbar for fayalite and to 300kbar for forsterite. Two intensive Raman bands of fayalite were definitely observable at high pressures, but only one of them can be reliably determined. Both have a linear variation within experimental uncertainty. Because of interference from the high spectral background, we found that nearly all the weak bands of forsterite could not be reliably determined at high pressures. However, the pressure variations of all bands of forsterite which can be reliably determined are non-linear. The rates of frequency change for the intense bands of forsterite determined in the present experiment are consistent with those of natural forsterites determined by Besson et al. ¹ and Gillet et al. ², but are in a slight discordance with those reported by Chopelas³. Furthermore, there is no evidence for the olivine ? spinel phase transition occurring at room temperature.     

Low temperature,high pressure Raman study of SbSBr

Abstract

We have measured the Raman spectra of the quasi-one-dimensional crystal SbSBr as a function of pressure at 295, 70, 37 and 25 K.

The pressure coefficients of the observed Raman modes have been determined and used to distinguish inter-from intrachain modes. Spectral features characteristic of the ferroelectric phase have been attributed to impurities or lattice imperfections and not to the presence of the ferroelectric phase, providing indication that the para-to ferroelectric phase transition occurs below 25 K.     

Raman phonon spectra and lattice dynamics of 9,10-dinitroanthracene under pressure

Abstract

Raman phonon spectra of 9, 10-dinitroanthracene have been recorded in the pressure range 0-6GPa. No phase transition is detected up to the maximum pressure studied. Quasi Harmonic Lattice Dynamics calculations, based on an atom-atom potential previously modeled on homologous 9,10-disubstituted anthracenes, have been performed. The optimized potential was used to calculate the equilibrium geometry and the lattice phonon frequencies as a function of pressure. The calculated structure at ambient conditions closely resembles the experimental one. The calculated phonon frequencies show a good agreement with the experimental values at all pressures measured.     

Raman study of datolite CaBSiO4(OH) at simultaneously high pressure and high temperature

Using an in situ method of Raman spectroscopy and resistance‐heated diamond anvil cell, the system datolite CaBSiO₄(OH) – water has been investigated at simultaneously high pressure and temperature (up to Р ~5 GPa and Т ~250 °С). Two polymorphic transitions have been observed: (1) pressure‐induced phase transition or the feature in pressure dependence of Raman band wavenumbers at P = 2 GPа and constant T = 22 °С and (2) heating‐induced phase transition at T ~90 °С and P ~5 GPа. The number of Raman bands is retained at the first transition but changed at the second transition. The first transition is mainly distinguished by the changes in the slopes of pressure dependence of Raman peaks at 2 GPa. The second transition is characterized by several strong changes: the wavenumber jumps of major bands, the merging of strong doublets at 378 and 391 cm⁻¹ (values for ambient conditions), the splitting of the intermediate‐intensity band at 292 cm⁻¹, and the transformation of some low‐wavenumber bands at 160–190 cm⁻¹. No spectral and visual signs of overhydration and amorphization have been observed. No noticeable dissolution of datolite in the water medium occurred at 5 GPa and 250 °С after 3 h, which corresponds to typical conditions of the ‘cold’ zones of slab subduction. Copyright © 2014 John Wiley & Sons, Ltd.     

Elemental sulfur under high hydrostatic pressure. An up-to-date Raman study

We report a high pressure Raman study of orthorhombic elemental sulfur from ambient pressure to ～ 25 GPa. Using a near infrared laser and low laser intensity on the scattering volume, we achieve off-resonant conditions up to larger pressures in comparison with previous studies. Raman spectra were recorded over the full spectral range including external (librational, translational) and internal (bond bending and bond stretching) modes. Drastic changes are observed as regards the peak frequencies, relative intensities and band splitting of degenerate modes. The main outcome of the present study is the observation of a “structural” transition at ～ 16 GPa manifested as slope changes of certain frequencies and sudden relative intensities changes. The present findings are discussed in the context of previous pressure Raman studies and comparison with existing X-ray diffraction as well as ab initio molecular dynamics results is attempted.     

Vibrational properties of Cs4W11O35 and Rb4W11O35 systems: high pressure and polarized Raman spectra

Cs₄ W₁₁O₃₅ (CW) and Rb₄ W₁₁O₃₅ (RW) belong to the class of hexagonal bronzes whose structure originates from the K_xWO₃ superconductor hexatungstate. Charge‐imbalanced tungsten bronzes are dielectric materials with rich polymorphism, ferroelectric properties and second‐harmonic generation. In this work, we report the polarized Raman spectra results for both CW and RW, as well as results of high‐pressure Raman scattering experiments (0.0–11.0 GPa) for the Cs₄ W₁₁O₃₅ system, in which we have observed two structural phase transitions at ∼4 and 7.5 GPa. We discuss these transformations and polarized Raman spectra on the basis of lattice dynamics calculation in the related system KNbW₂O₉. Polarized Raman spectra provide strong indication that the highest wavenumber modes observed in these systems originate from tungsten or oxygen vacancies. The observation of a soft‐like mode indicates that the observed phase transitions exhibit a displacive‐type behavior, thus further indicating that these transformations are likely related to reorientations of the octahedral units. The soft mode nature is discussed as well. PACS: 77.80.Bh; 78.30.Hv; 78.30.‐j. Copyright © 2010 John Wiley & Sons, Ltd.     

一种新的恶二唑衍生物在金刚石对顶砧中的电阻率随压力和温度的变化关系

利用金刚石对顶砧测量了恶二唑衍生物微晶, 1,4-bis[(4-methyloxyphenyl)-1,3,4-oxadiazolyl]- 2,5-bisheptyloxyphenylene (OXD-2), 电阻随压力和温度的变化关系，并利用有限元分析方法计算了样品的电阻率。实验中，测量压力和温度达到了16 GPa和150℃。样品的电阻率随着温度的升高而降低，说明样品表现出半导体传导特性。在90-100 ℃之间，样品的电阻率有一明显的下降，说明这时发生了温度诱导的相变。随着压力的增加，样品的电阻率在6GPa左右达到最大值，此后随着压力的增加而下降。结合原位x光数据，在6GPa左右的电阻突变应该是由于样品在压力的诱导下发生了无序化的相变。     

高压拉曼光谱方法研究联苯分子费米共振

测量了0—15 GPa压强下联苯分子的拉曼光谱. 结果表明,随压强增加,分子内和分子间π-π共轭和离域效应增强,谱线的绝对强度变大、蓝移. 联苯分子的两费米共振谱线强度比R_f/a减少,频率差Δ增加,当压强为8 GPa时,费米共振现象消失,利用Betran理论得出了固有频率差Δ₀和耦合系数ω随压强的变化关系,通过高压下相变进行了解释,并探讨了高压下费米共振耦合变弱的机理. 关键词： 联苯 费米共振 高压 拉曼光谱     

A shifted‐excitation Raman difference spectroscopy (SERDS) evaluation strategy for the efficient isolation of Raman spectra from extreme fluorescence interference

A biochemical characterization of pathologies in biological tissue can be provided by Raman spectroscopy. Often, the raw spectrum is severely affected by fluorescence interference. We report and compare various spectra‐processing approaches required for the purification of Raman spectra from heavily fluorescence‐interfered raw spectra according to the shifted‐excitation Raman difference spectroscopy method. These approaches cover the entire spectra‐processing chain from the raw spectra to the purified Raman spectra. In detail, we compared (1) area normalization versus z‐score normalization, (2) direct reconstruction of the difference spectra versus reconstruction of zero‐centered difference spectra and (3) collective baseline correction of the reconstructed spectra versus piecewise baseline correction of the reconstructed spectra and, finally, (4) analyzed the influence of the shift of the excitation wavelength on the quality of the reconstructed spectra. Statistical analysis of the spectra showed that – in our experiments – the best results were obtained for the z‐score normalization before subtraction of the normalized spectra, followed by zero‐centering of the difference spectra before reconstruction and a piecewise baseline correction of the pure Raman spectra. With our equipment, a wavelength shift from 784 to 785 nm provided reconstructed spectra of best quality. The analyzed specimens were different tissue types of pigs, tissue from the oral cavity of humans and a model solution of dye dissolved in ethanol. © 2015 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons Ltd.     

Probing structural stability of double-walled carbon nanotubes at high non-hydrostatic pressure by Raman spectroscopy

Theoretical calculations predict that the collapse pressure for double-walled carbon nanotubes (DWCNTs) is proportional to 1/R ³, where R is the effective or average radius of a DWCNT. In order to address the problem of CNT stability at high pressure and stress, we performed a resonance Raman study of DWCNTs dispersed in sodium cholate using 532 and 633 nm laser excitation. Raman spectra of the recovered samples show minor versus irreversible changes with increasing I _D/I _G ratio after exposure to high non-hydrostatic pressure of 23 and 35 GPa, respectively. The system exhibits nearly 70% pressure hysteresis in radial breathing vibrational mode signals recovery on pressure release which is twice that predicted by theory.     

Spectral properties of 1,3,4-oxadiazole derivatives in binary solvent mixtures

The influence of nonspecific intermolecular interactions on the fluorescence band shift of 1,3,4-oxadiazole derivatives has been investigated in the binary solvents hexane-ethanol, toluene-ethanol, toluene-acetonitrile, and carbontetrachloride-2-propanol. The dependences obtained have been analyzed in terms of preferential solvation. It has been shown that the maximum solvation energy for slightly polar compounds occurs in toluene-ethanol, toluene-acetonitrile, and carbontetrachloride-2-propanol. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 177–181, March–April, 2008.     

Effects of high pressure on unsaturated fatty acids

The objective of this study is to investigate the effects of high pressure processing on the molecular structure of some unsaturated fatty acids. Samples of elaidic acid, linoleic acid, ZE and EE conjugated linoleic acid are treated at 293 or 333?K at pressures up to 700?MPa. It is observed that the adiabatic heat generated from compression is able to bring the sample temperature above 373?K after 700?MPa. These relatively extreme conditions are of great interest for food sterilization, but they may induce undesirable change in fatty acid quality characteristics. To check for structural changes, Raman spectra of the samples are analysed after treatments. The comparison with Raman spectra of samples kept at atmospheric pressure shows that pressure induces some conformational changes at the hydrocarbon skeleton in solid samples, while the liquid ones remain unchanged. No cis/trans isomerization occurs, but gauche conformers are likely to be present.     

A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.