High pressure investigations of melamine

We have performed mid- and far-infrared (IR), Raman, and angular dispersive X-ray diffraction studies on melamine at high pressure up to 36 GPa. We have confirmed the presence of three phase transitions; the first between 1 and 2 GPa, the second between 7 and 9 GPa, and the third near 16 GPa. We observed a softening of the N–H symmetric and antisymmetric vibrations with pressure, suggesting that intermolecular hydrogen bonding increases as the intermolecular distance decreases similarly to what was observed in triamino-trinitrobenzene. The molecular decompression data from core intramolecular peaks of mid-IR and Raman indicate that melamine did not chemically decompose up to the highest investigated pressures but the sample suffered some irreversible amorphization. We have further clarified the lack of observation of any phase transitions in prior Raman and IR studies by examining the pressure dependence of other uninvestigated modes of vibration.     

Pressure-induced structural transitions in PbI2: A high-pressure Raman and optical absorption study

The effect of pressure on the 2H and 4H polytype of PbI₂ has been investigated by Raman and optical absorption spectroscopy, using the diamond anvil cell. The 2H-polytype undergoes pressure-induced phase transitions at 5 kbar and near 30 kbar. The 4H-polytype exhibits phase transitions near 8 kbar and above 30 kbar. The Raman modes abruptly change at these pressures. The optical absorption edge shifts red at the rate of 15±1 MeV/kbar in the 2H-PbI₂ and at the rate of 7 MeV/kbar in phase II. The latter phase is most likely to possess a 3d-structure and not a layer type. The possible structures for the high pressure phases are discussed.     

High pressure study of the 2D polymeric phase of C60 by means of raman spectroscopy

Abstract

The effect of high hydrostatic pressure, up to 12GPa, on the intramolecular phonon frequencies and the material stability of the two-dimensional tetragonal Cm polymer has been studied by means of Raman spectroscopy in the spectral range of the radial intramolecular modes (200-800cm^?1). A number of new Raman modes appear in the spectrum for pressures ～ 1.4 and ～ 5.0 GPa. The pressure coefficients for the majority of the phonon modes exhibit changes to lower values at P=4.0 GPa, which may be related to a structural modification of the 2D polymer to a more isotropic phase. The peculiarities observed in the Raman spectra are reversible and the material is stable in the pressure region investigated.     

Optical measurement of the pH value of intransparent samples under high pressure

We present a method to optically study the pH value of intransparent media under pressure. To this end, we prepare and use immobilized pH-value indicators. The indicators are brought into contact to the medium and their optical signal is read out in reflection with a spectrometer. The pH value and pressure response of the indicator were calibrated with imidazole buffers. As an example for its applicability to foodstuff, we study the pressure-induced pH-value change of fat and meat pastes.     

Micro‐Raman study of PET single fibres under high hydrostatic pressure: phase/conformation transition and amorphization

The micro/nano structural evolution of a PET single fibre under hydrostatic pressure has been studied by Raman micro spectroscopy in a diamond anvil cell (DAC). Different bands in the Raman spectra were used as probes: the low wavenumber collective modes (<250 cm⁻¹) representative of the long‐range chain organization, as well as the stretching and bending amide and aromatic ring modes representative of the local chain behaviour. The in situ analysis at different pressures shows an evolution from an axial oriented trans‐conformation to an amorphous, isotropic material, i.e. the reverse transformation observed during the process of drawing the fibre from an isotropic amorphous precursor. Copyright © 2007 John Wiley & Sons, Ltd.     

Raman G‐mode of single‐wall carbon nanotube bundles under pressure

Raman studies of nanotubes under pressure have been a lively area of research. However, the results are not always as expected and at times have not been adequately explained. One example of the diversity of the results is the higher energy Raman mode (the graphitic mode, GM) shift to higher wavenumber under pressure. Here we report a new high‐pressure Raman study showing that the effects of the variation in the tube diameters and the pressure transmitting medium are both crucial for understanding the outcomes of such high‐pressure experiments. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Phase transitions of sulfur at high pressure: Influence of temperature and pressure environment

Abstract

Phase transitions of orthorhombic sulfur were investigated above 10 GPa by Raman spectroscopy using red light excitation. Transitions into several phases that have been reported in previous studies using green light excitation, are confirmed. The phase behaviour is observed to depend strongly on the preparation method. In the presence of a pressure transmitting medium (methanol/ethanol, 4:1), a sequence of phases α-S₈ → [intermediate phase (“ip”) + S₆] → [S₆ + high pressure-low temperature phase (“hplt”)] is described and characterized. Without the use of a pressure transmitting medium, the phase sequence α-S₈ → [“ip” + “hplt”] + “hplt” is observed. In addition, contributions of amorphous sulfur are detected around 10 GPa, i.e. at pressures below the transformation of α-S₈ into the above-mentioned phases. Characteristic Raman spectra of the different phases are extracted and documented over a wide pressure range.     

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.     

压强对苯分子费米共振的影响

测量了在0—13GPa压强下苯分子的拉曼光谱,结果表明:所有拉曼谱带随压强的增加发生蓝移,并伴随某些谱带的劈裂和相对强度的变化;而对于苯分子的ν1+ν6—ν8两费米共振谱带,根据Betran理论,计算出它们的频率间隔Δ0随压强的增加而增大,这是由于合频ν1+ν6频移随压强增加速度大于相应于基频ν8速度的结果,进而导致了费米共振耦合系数ω和强度比Rf/a减小.当压强增加至11GPa时,费米共振现象消失,该现象用高压下相变给予解释.本结果丰富了环境变化对分子费米共振影响的研究内容,对谱线认证、归属有参考价值.     

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     

显微拉曼光谱研究导电高分子复合微米管

利用氧化铝模板在三氟化硼乙醚溶液中电化学聚合苯和噻吩制得聚苯/聚噻吩复合微米管。通过扫描电子显微镜和显微拉曼光谱研究了复合微米管的形态与结构。结果表明:复合微米管具有三个明显的区域,它们分别是聚苯区,聚苯/聚噻吩复合区和聚噻吩区。     

Raman spectroscopic characterization of CH4 density over a wide range of temperature and pressure

The positions of the CH₄ Raman ν₁ symmetric stretching bands were measured in a wide range of temperature (from −180 °C to 350 °C) and density (up to 0.45 g/cm³) using high‐pressure optical cell and fused silica capillary capsule. The results show that the Raman band shift is a function of both methane density and temperature; the band shifts to lower wavenumbers as the density increases and the temperature decreases. An equation representing the observed relationship among the CH₄ ν₁ band position, temperature, and density can be used to calculate the density in natural or synthetic CH₄‐bearing inclusions. Copyright © 2014 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.     

原位高压测试技术在高压结构及性质研究中的应用

高压科学是研究不同压力条件下物质的结构、状态、理化性质及变化规律的学科。在高压科学研究中,多以凝聚态物质为研究对象,涉及的领域也非常广泛,包括物理学、化学、材料学、地质学、生物学、航天学等等,是一门以实验为基础的学科。高压科学之所以能成为一门独立的学科,还因为高压研究需要使用特殊且精巧的技术和方法来实现,是以技术创新为牵引的科学研究领域。而今,各种实验测试手段已经可以成熟地运用在该学科中,比较常见的有：高压拉曼散射、高压红外光谱、高压布里渊散射、高压同步辐射XRD、高压电学测量以及高压磁学测量等诸多技术。文章系统介绍了以上高压原位实验测试方法的原理、发展、作用及应用,有助于读者对原位高压测试技术有更深刻的认识和理解,为更高压力下的原位高压探测技术的发展提供重要的基础和借鉴。     

高压下碱式聚苯胺的理论吸收谱

利用改进的Ginder-Epstein模型计算了碱式聚苯胺在参数V2,0不同取值下的自洽变分基态，结果表明芳环扭角和能隙随此参数的减小而缩减。据此，用这个参数的减小来表示垂直于主链方向加压，估算聚合物压强与芳环扭角的对应关系并对自洽变分基态作拟合，给出了V2,0与压强之间的经验公式。最后作出高压吸收谱：加压至饱和压强7.69 GPa时，π — π*吸收峰由2.17eV蓝移至0.74eV。此理论谱供压力定标参考。     

高压下碱式聚苯胺的理论吸收谱

利用改进的Ginder-Epstein模型计算了碱式聚苯胺在参数V2,0不同取值下的自洽变分基态,结果表明芳环扭角和能隙随此参数的减小而缩减.据此,用这个参数的减小来表示垂直于主链方向加压,估算聚合物压强与芳环扭角的对应关系并对自洽变分基态作拟合,给出了V2.0与压强之间的经验公式.最后作出高压吸收谱:加压至饱和压强7.69 GPa时,π-π*吸收峰由2.17 eV蓝移至0.74 eV.此理论谱供压力定标参考.     

高压下某些导电高分子色散关系的研究

利用晶格动力学方法,研究在高压下几种导电高分子具有不同晶格链时的色散关系及其曲线的变化.链间耦合作用的减弱使横波与纵波的ω差值相应增大,且在BZ边界处拉开一个间隙,这是维度作用的结果.