Phase transformations in amorphous fullerite C60 under high pressure and high temperature

First phase transformations of amorphous fullerite C₆₀ at high temperatures (up to 1800 K) and high pressures (up to 8 GPa) have been investigated and compared with the previous studies on the crystalline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С₆₀ molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature–pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous-like or crystalline graphite were formed at lower temperatures than in crystalline fullerite.     

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.     

Na2CO3的高压拉曼光谱研究

碳酸盐是碳在地球内部的重要载体之一,其在地幔高温高压条件下的晶体化学是理解地球深部碳的赋存状态和循环过程的关键,而结构稳定性和相变是晶体化学最基本的研究内容。碳酸钠（Na₂CO₃）是一种常见的碱性碳酸盐矿物,在产自地幔过渡带-下地幔的金刚石中已发现含钠的碳酸盐矿物包裹体,这成为碳酸钠能够俯冲进入地幔深部的直接矿物学证据。前人利用拉曼光谱技术研究了Na₂CO₃在常温常压下的晶格振动模式,但其在高压下的稳定性和结构变化却鲜有报道。利用金刚石压腔装置结合先进的共聚焦拉曼光谱技术,以硅油作为传压介质,在准静水压力条件下,在0.001～27.53 GPa压力区间对Na₂CO₃粉末在600~1 200 cm-1波段的振动特征进行了细致地分析。本次实验重点分析了[CO₃]2-基团振动模式在升压和卸压过程中的行为。结果表明,在0.001～11.88 GPa压力范围内,[CO₃]2-基团对称伸缩振动γ₁（1 088.06和1 070.76 cm-1）、反对称伸缩振动γ₃（865.10和797.50 cm-1）和面内弯曲振动γ₄（720.10和696.71 cm-1）都出现了振动峰的分裂。随着压力增加,所有振动峰都向高频率漂移,半高宽也逐渐增加。在13.40 GPa时,Na₂CO₃发生结构相变,具体表现为690.08 cm-1处出现1条新的拉曼峰,并且随着压力升高该峰的强度逐渐增大。同时反对称伸缩振动峰γ₃以及面内弯曲振动峰γ₄的强度持续减弱,半高宽也继续变大。这些现象表明Na₂CO₃结构相变源于[CO₃]2-内部晶格变化。当压力卸载到4.18 GPa时,[CO₃]2-的振动模式与常温常压下的完全吻合,相变出现的新峰也已经消失,表明该相变是由[CO₃]2-基团畸变引起的并且具有可逆性。继续升压至27.53 GPa,拉曼光谱继续蓝移,Na₂CO₃的拉曼谱线再没有变化,说明高压相在这一压强范围内保持稳定。在整个加压过程中,反对称伸缩振动γ₃和面内弯曲振动γ₄处的拉曼峰出现强度减弱现象。同时也计算了各个峰频率对压力的依赖系数dγ/dP,结果显示[CO₃]2-基团内各个振动模式对压力的响应是不同的,这很可能与Ｃ-Ｏ键的键长有关。最后,对比发现,对称伸缩振动γ₁峰的强度比反对称伸缩振动γ₃和面内弯曲振动γ₄峰的强度大,并且[CO₃]2-基团对称伸缩振动γ₁受压力影响相对较小,可以用来区别不同种类的碳酸盐矿物。     

Xanes and raman spectroscopy under pressure of bromobenzene

Abstract

The behaviour of bromobenzene (BBe) compressed in a diamond anvill cell up to 30 GPa was studied by XANES and Raman spectroscopy. The liquid-solid transition and a solid-solid transition were observed at 0.9 GPa and 9 GPa respectively. Above 24 GPa, an irreversible transformation occurs to a solid orange-yellow compound which can be recovered at zero pressure. The polymerization mechanism, in connection with the occurence of Br-bonded Sp² and Sp³ carbons in the solid compound, is discussed.     

Sulfur-catalyzed phase transition in MoS2 under high pressure and temperature

We report phase transition and stability of MoS₂ with and without the presence of sulfur melt under high-pressure and high-temperature conditions. Rhombohedral (3R) phase is found to be a high-temperature phase of MoS₂ at high pressures. Excess sulfur melt catalyzes the hexagonal (2H) to rhombohedral (3R) phase transformation and lowers the conversion temperature by more than 280 K. Boundary between 2H and 3R phases has been delineated with a negative slope. Based on experimental observations, sulfur-catalyzed 2H→3R transformation mechanisms are proposed involving atomic exchange between MoS₂ and sulfur, which is different from the case of without excess sulfur that proceeds through rotation and translation of the S–Mo–S sandwich layers.     

Solvated fullerenes,a new class of carbon materials suitable for high-pressure studies: A review

As the list of known carbon compounds grows longer, solvated fullerenes have become a more important class of carbon materials. Their general properties and methods of synthesis have both attracted considerable attention. The study of the behavior of these compounds under high-pressure conditions has revealed several new phenomena that have never been observed in pure fullerene. This article is a review of all recent progress in this field.     

The pressure response of Raman active phonon modes of Tm3Al5O12

Abstract

We present the Raman spectrum of Tm₃Al₅O₁₂ single crystal and its pressure dependence for hydrostatic pressure up to 11GPa and room temperature. Tm₃Al₅O₁₂ belongs to the crystal family of rare earth garnets (Re₃A1₂(AlO₄)₃, Re: Gd, Tb, Dy, Er,…), which crystallize in the body-centered cubic lattice and contains eight molecular units in the conventional unit cell, Group theory predicts 25 Raman active modes for these compounds, while experimentally are observed 15 modes. As crystal volume decreases all Raman peaks exhibit pressure coefficients varying from 0.7 to 5.6cm^?1/ GPa. A large part of the vibrational spectra of these compounds could be explained taking into account the vibrational properties of molecular subunits, namely AlO₄.     

Raman spectroscopy study of Na2MoO4·2H2O and Na2MoO4 under hydrostatic pressure

Raman spectroscopy measurements of polycrystalline Na₂MoO₄·2H₂O (NMHO) and Na₂MoO₄ (NM) under hydrostatic pressure (from 0 to 10 GPa) were performed. This study allowed us to monitor the stretching and bending vibrations of MoO₄ ions as well as the translational modes as a function of pressure. The pressure dependence of the wavenumbers of the modes indicates that the Na₂MoO₄·2H₂O undergoes two phase transitions at about ∼3 and ∼4 GPa. When releasing pressure, we have observed that the original spectrum is recovered, thereby pointing to a reversible process. The Na₂MoO₄ (NM) starting phase was found to be stable up to 10 GPa. The pressure‐dependent Raman data for NM did not reveal any structural modification. The influence of the pressure‐transmitting medium on the phase transitions is also discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural, electronic and vibrational properties of InN under high pressure

The structural, electronic and vibrational properties of InN under pressures up to 20 GPa have been investigated using the pseudo-potential plane wave method (PP-PW). The generalized-gradient approximation (GGA) in the frame of density functional theory (DFT) approach has been adopted. It is found that the transition from wurtzite (B4) to rocksalt (B1) phase occurs at a pressure of approximately 12.7 GPa. In addition, a change from a direct to an indirect band gap is observed. The mechanism of these changes is discussed. The phonon frequencies and densities of states (DOS) are derived using the linear response approach and density functional perturbation theory (DFPT). The properties of phonons are described by the harmonic approximation method. Our results show that phonons play an important role in the mechanism of phase transition and in the instability of B4 (wurtzite) just before the pressure of transition. At zero pressure our data agree well with recently reported experimental results.     

常温及常压至1.2GPa条件下异辛烷的拉曼光谱研究

利用碳化硅压腔在室温(25℃)下,研究了异辛烷(2,2,4-三甲基戊烷)在常压至1.2GPa条件下的拉曼光谱特征。研究结果表明,异辛烷CH2和CH3的碳氢伸缩振动的拉曼位移随着压力的增大均呈线性向高频方向移动,其拉曼位移与压力的函数关系为:ν2 873=0.002 8P+2 873.3;ν2 905=0.004 8P+2 905.4;ν2 935=0.002 7P+2 935.0;ν2 960=0.012P+2 960.9。在1.0GPa附近,异辛烷的拉曼位移出现突变,与显微镜下观察发生的异辛烷液-固相变一致。结合异辛烷在常压下的熔点数据,获得了异辛烷的液-固两相相图,并根据克拉贝龙方程获得了液-固相转变过程中的摩尔体积变化量ΔVm=4.46×10-6 m3.mol-1和熵变ΔS=-30.32J.K-1.mol-1。     

Structural stability of tin clathrates under high pressure

High pressure Raman scattering experiments have been performed for Rb₈Sn₄₄□₂ in order to investigate the pressure induced phase transition. At pressures of 6.0 and 7.5 GPa, Raman spectrum was drastically changed, indicating the phase transitions. The irreversibility of the spectral change and the disappearance of Raman peak observed at 7.5 GPa strongly suggest the occurrence of irreversible amorphization.     

Diamond and cubic boron nitride under high pressure: Raman scattering,equation of state and high pressure scale

Abstract

The development of the diamond-anvil cell has stimulated Raman-scattering investigation of vibrational modes in covalent crystals. The linear pressure coefficient reported for diamond by Hanfland et at' (2.90 ± 0.05 cm-¹/GPa) agrees to within mutual experimental error with the result of Boppard et aL² (2.87±0.01 cm-¹/GPa). As to cubic boron nitride, the only work by Sanjurjo ef aL3 reports 3.45 ± 0.07 cm-¹/GPa for LO- and 3.39 ± 0.08 cm-¹/GPa for TO- modes. Since no compressibility data are availablel^1?3, the mode Griineisen parameter γ = ‐ δ In γ/δ is defined as y = K/Y·dv/aP and depends on the bulk modulus K and the calibration of the ruby scale. The above papers report y= 0.96 and y=0.95±O.O3fordiamondand γ_Lo=1.21,γTo=1.51 forBN.     

常温高压条件下硬石膏相变的原位拉曼光谱研究

硬石膏（CaSO₄）是地球上分布最广的硫酸盐矿物之一,为研究硬石膏向高压硬石膏转变的压力条件和相变机理、确定硬石膏拉曼光谱压标的适用范围,实验结合水热金刚石压腔和激光拉曼光谱实验技术,研究了常温高压条件下硬石膏的相变过程以及硬石膏和高压硬石膏的拉曼光谱特征。实验结果显示,常温条件下硬石膏向高压硬石膏发生相变的压力在2.3 GPa左右,但是该相变压力在增压和降压过程中存在较大差异,表明硬石膏与高压硬石膏的转变过程存在明显滞后性,证实了该相变过程属于重建型相变。由于重建型相变的控制因素除了温度和压力之外,还包括相变的速率以及矿物结构的亚稳定性等,从而很好地解释了不同实验者获得的硬石膏与高压硬石膏的相变压力之间存在的巨大差异。与硬石膏相比,高压硬石膏的拉曼光谱特征表现为SO₄对称伸缩振动（ν₁）从1 128.28 cm-1突然下降至1 024.39 cm-1,同时对称弯曲振动（ν₂）分裂为441,459和494 cm-1三个峰,反对称伸缩振动（ν₃）分裂为1 136,1 148,1 158和1 173 cm-1四个峰,反对称弯曲振动（ν₄）也分裂为598,616,646和671 cm-1四个峰,可以作为判定硬石膏进入高压相态的有效标志。与硬石膏相比,高压硬石膏SO₄振动产生的拉曼峰数量更多、强度更低,表明影响SO₄振动的原子更多、分布更加复杂,这与高压硬石膏晶体结构（独居石结构,单斜晶系）的对称性比硬石膏（斜方晶系）更低相吻合。在硬石膏结构稳定的压力范围内（常压至2.3 GPa）,硬石膏SO₄拉曼振动中除了ν_2,416的振动频率变化不显著以外,其余振动均随着压力的升高以稳定的速率向高波数方向移动,同时谱峰的强度、形态和半高宽没有明显改变,从而保证了不同压力下硬石膏的拉曼峰具有一致的拟合误差和压力标定精度。同时,还通过方解石ν_{1,1 085}拉曼峰随压力的变化速率、方解石向CaCO₃-Ⅱ以及CaCO₃-Ⅱ向CaCO₃-Ⅲ的相变压力对硬石膏压力标定结果进行检验,确定了硬石膏压标的可靠性。     

Behavior of decomposed ammonia borane at high pressure

High pressure behavior of ammonia borane after thermal decomposition was studied by Raman spectroscopy at high pressure up to 10 GPa using diamond anvil cell (DAC). The ammonia borane was decomposed at around 140 °C under the pressure at ∼0.7 GPa. Raman spectra show the hydrogen was desorbed within 1 h. The hydrogen was sealed in DAC well and cooled down to room temperature. Applying higher pressure up to ∼10 GPa indicates interactions between the products and loss of dihydrogen bonding. No rehydrogenation was detected in the pressure range investigated.     

HRTEM and Raman study of onion-like fullerenes encapsulated Fe

Onion-like fullerenes (OLFs) encapsulated Fe have been synthesized by chemical vapor deposition (CVD). The samples have been characterized by HRTEM and Raman methods. The diameters of OLFs encapsulated Fe are in the range from 15 to 40 nm with pure Fe particles. HRTEM and Raman spectra show that OLFs are highly graphite. XRD spectrum shows Fe nanoparticles are protected in OLFs and are not oxidized by air. It is the stain of graphene shells and the uneven size of OLFs encapsulated Fe that causes the Raman spectra to shift downward slightly from 1580 cm⁻¹.     

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.     

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。     

High-pressure Raman and X-ray studies of barite,BaSO4

High-pressure Raman spectroscopic and X-ray diffraction experiments of barite, BaSO₄, were carried out in a diamond anvil cell up to 25?GPa at room temperature. On the basis of the changes in the diffraction patterns and the variation of lattice parameters with pressure, it is inferred that barite undergoes a phase transformation at 10?GPa. The phase transition accompanies the change in the force constant of vibrational modes in barite. Further compression beyond the phase transition causes the distortion of SO₄ tetrahedron as indicated by the splitting in the SO₄ stretching modes. Both X-ray and Raman data support that the phase transition in BaSO₄ is reversible. The compression data yield a bulk modulus of 63?±?2?GPa for barite. Barite shows anisotropic compressibility along three crystallographic axes with c being the most compressible axis.