High‐pressure polarized Raman spectra of Gd2(MoO4)3: phase transitions and amorphization

Polarized Raman spectra of a single crystal of gadolinium molybdate [Gd₂(MoO₄)₃] were obtained between 1 atm and 7 GPa. Using a mixture of alcohols as the pressure‐transmitting medium, YY, ZZ, XY components of scattering matrices were measured. The ZZ spectra were also obtained in argon. Five phase transitions and amorphization were identified. The first and second transitions are reversible, while amorphization is not. In alcohol, amorphization is observed above 6.5 GPa. With argon as the pressure‐transmitting medium, amorphization is progressive and begins above 3 GPa. The spectral changes with pressure affect the high wavenumber bands attributed to symmetric and antisymmetric MoO₄ stretching modes as well as the very low wavenumber modes such as librations of the tetrahedra. This means that both short‐range and long‐range organizations of the tetrahedra are involved in these phase transitions. The amorphization mechanism and its dependence on the pressure‐transmitting medium are discussed, and the steric hindrance between polyhedra is believed to be the most relevant mechanism. The TO and LO low wavenumber modes of A₁ symmetry, observed in the Y(ZZ)Y and Z(YY)Z geometries, respectively, below 50 cm⁻¹, soften continuously through the first three phases when increasing pressure. The strong A₂ mode observed in the Z(XY)Z spectra exhibits the same anomalous behavior by decreasing from 53 to 46 cm⁻¹ at 2 GPa. The softening of these modes is related to the orientation change of tetrahedra observed by ab initio calculations when the volume of the cell is decreased. These orientation changes can explain the wavenumber decrease of the Mo O stretching modes above 2 GPa, which indicates an increase of Mo coordination. Copyright © 2010 John Wiley & Sons, Ltd.     

Understanding guest and pressure‐induced porosity through structural transition in flexible interpenetrated MOF by Raman spectroscopy

Interpenetrating metal organic frameworks are interesting functional materials exhibiting exceptional framework properties. Uptake or exclusion of guest molecules can induce sliding in the framework making it porous or non‐porous. To understand this dynamic nature and how framework interaction changes during sliding, metal organic framework (MOF) 508 {Zn(BDC)( 4,4′‐Bipy)_0.5 · DMF(H₂O)_0.5} was selected for study. We have investigated structural transformation in MOF‐508 under variable conditions of temperature, pressure and gas loading using Raman spectroscopy and substantiated it with IR studies and density functional theory (DFT) calculations. Conformational changes in the organic linkers leading to the sliding of the framework result in changes in Raman spectra. These changes in the organic linkers are measured as a function of high pressure and low temperature, suggesting that the dynamism in MOF‐508 framework is driven by ligand conformation change and inter‐linker interactions. The presence of Raman signatures of adsorbed CO₂ and its librational mode at 149 cm⁻¹ suggests cooperative adsorption of CO₂ in the MOF‐508 framework, which is also confirmed from DFT calculations that give a binding energy of 34 kJ/mol. Copyright © 2015 John Wiley & Sons, Ltd.     

Pressure induced phase transition in mercurous chloride

Pressure induced phase transition in mercurous chloride has been studied by high pressure x-ray diamond anvil cell. The change in diffraction pattern started and ended at a pressure of about 5 kbar and 20 kbar respectively. The patterns recorded at 20 kbar could be indexed basing on an orthorhombic lattice, with lattice parametersa=4.23 Å,b=4.54 Å andc=10.44 Å.     

High‐pressure Raman and photoluminescence of highly anisotropic CdS nanowires

Raman and photoluminescence of CdS nanowires of diameter 80 nm and lengths up to several tens of micrometers were studied at pressure up to 60 kbar using a Jobin‐Yvon T64000 micro‐Raman system in conjunction with the diamond‐anvil cell technique. The phase transition pressure of wurtzite to rock salt was observed at 38 kbar, which is higher than that of bulk CdS. In contrast with the transition pressure of different‐sized CdS nanocrystal, this elevated phase transition pressure cannot be explained well by the size effect. Thus the contribution of particle morphology of such a system, which represents the low‐energy surface structure, should be considered. The pressure dependence of photoluminescence is also discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Resonance Raman spectroscopic study of shape‐induced phase transition in CdSe nanoclusters

We report an observation of shape‐induced phase transition from wurtzite to zinc blende phase of encapsulated CdSe nanoclusters in mesoporous silica. Presence of both the phases is also observed in the as‐grown sample before encapsulation. Role of interfacial energy in the energetic mesopores, as the possible origin of phase transition, is thus ruled out, as the samples are encapsulated subsequent to their synthesis in the soft chemistry route. Electron–phonon coupling in the resonant Raman spectroscopic studies, using different energies for clusters of different phase and shape, thereby confirms the presence of both the wurtzite and the zinc blende phases. Transmission electron microscopic studies are used for the direct evidence of the shape‐induced solid–solid phase transition between two crystalline phases, for the first time. Small fluctuation of energies, in the form of shape, during its growth may be the driving force for the observed phenomenon, as the surface energy of both the phases stabilizes to the same value. Thus, finally, specific shapes can be used as one of the ways to differentiate the resulting phases. Copyright © 2014 John Wiley & Sons, Ltd.     

Pressure‐induced structural transformations in In2‐xYx(MoO4)3 systems

In this work, we report results of high‐pressure Raman experiments (P < 8 GPa) on In_2‐xY_xMo₃O₁₂ for x = 0.0 and 0.5. A crystalline to crystalline structural phase transition and pressure‐induced amorphization (PIA) have been identified. The structural phase transition takes place at 1.5 and 1.0 GPa for In₂(MoO₄)₃ and In_1.5Y_0.5(MoO₄)₃, respectively, resulting in the change of structure from monoclinic P2₁/a to a more denser structure. The PIA started at 5 and 3.4 GPa for In₂Mo₃O₁₂ and In_1.5Y_0.5Mo₃O₁₂, respectively. The amorphization process takes place in two stages in the case of In_1.5Y_0.5Mo₃O₁₂ phase, while for In₂Mo₃O₁₂, it is not complete until the pressure is as high as 7 GPa. Our results also suggest that with increase of ionic size of the A³⁺ ions, the octahedral distortion increases and consequently larger local structural disorder is introduced in the A₂(MoO₄)₃ system, where A is a trivalent ion (In, Y³⁺, Sc³⁺, Fe³⁺, etc.). Copyright © 2015 John Wiley & Sons, Ltd.     

Pressure‐induced phase transitions in palmitic acid: C form

In this article, we report a high‐pressure Raman spectroscopy study of palmitic acid (PA, C form) from ambient pressure up to 21 GPa. The effects of hydrostatic pressure on the vibrational spectrum of PA are reported, and the data show that PA experiences a rich sequence of phase transformations. These changes in the crystal structure occur gradually as the pressure increases and they are related to the highly flexible crystalline structure. Copyright © 2011 John Wiley & Sons, Ltd.     

高温高压下方解石相转变的拉曼光谱原位实验研究

用热液金刚石压腔装置结合拉曼光谱技术研究了高温高压下方解石的相变过程及拉曼光谱特征。结果表明:常温条件下,体系压力增至1 666和2 127 MPa时,方解石的拉曼特征峰155cm-1消失,1 087cm-1峰分裂为1 083和1 090cm-1两个谱峰、282cm-1峰突然降至231cm-1,证明其转变为方解石-Ⅱ和方解石-Ⅲ。在起始压力为2 761MPa和低于171℃的升温过程中,方解石-Ⅲ的拉曼散射的各个特征振动峰没有变化。当温度达到171℃,方解石晶体完全变成不透明状,其对称伸缩振动峰1 087cm-1、面内弯曲振动峰713cm-1和晶格振动峰155和282cm-1均发生突变,说明方解石-Ⅲ相变生成一种碳酸钙新相。体系降至常温,该新相一直保持稳定不变,表明高温高压下方解石向碳酸钙新相的转变过程是不可逆的。方解石-Ⅲ与碳酸钙新相之间的相变线方程为P(MPa)=9.09.T(℃)+1 880。碳酸钙新相的对称伸缩振动峰(ν1 087)随压力、温度的变化率分别为dν/dP=5.1(cm-1.GPa-1),dν/dT=-0.055 3(cm-1.℃-1)。     

Hydrostatic-pressure induced phase transition of phonons in single-walled nanotubes

We study the effect of the hydrostatic pressure on the phonons in single-walled carbon nanotubes (SWNTs) in a magnetic field. We calculate the magnetic moments of the phonons using a functional integral technique, and find that the phonons in SWNTs undergo a pressure-induced phase transition from the paramagnetic phase to the diamagnetic phase under hydrostatic pressure 2 GPa. We explain the mechanism of generating this phase transition.     

X ray absorption spectroscopy investigation of phase transition in Ge,GaAs and GaP

Abstract

The phase transitions of GaAs, GaP and Ge under pressure have been investigated by x-ray absorption spectroscopy (XAS). At the onset of the transition the Debye-Waller factor increases and the x-ray absorption near edge structure (XANES) is progressively modified. A mixing of the low and high pressure phase can be determined by XAS as well as amorphization of the sample on pressure release.     

Peierls相变与磁场中碳纳米管的场发射

应用紧束缚模型和WKB方法研究碳纳米管的out-of-plane型Peierls相变,及其对碳纳米管的场发射的影响.结果发现Peierls相变会在室温出现,并使碳纳米管费米面附近出现能隙,导致碳纳米管发生金属—半导体转变,从而抑制碳纳米管的场发射.磁场也会抑制Peierls形变,Peierls相变和磁场相互竞争影响碳纳米管的能带结构,从而影响碳纳米管的场发射. 关键词： 场发射 碳纳米管 Peierls相变     

Preferred orientations of encapsulated C60 molecules inside single wall carbon nanotubes

A systematical study of the orientational behavior of C60 molecules in single wall carbon nanotubes (SWCNTs) with different chirality and diameter has been performed by using a model of an infinite long nanotube filled with two C60 (denoted as C60-1 and C60-2) molecules. We studied the preferred orientation of the C60-1 molecule when the neighboring C60-2 molecule was fixed at the pentagon, double-bond, and hexagon orientations respectively. Our results showed that the C60-1 molecule prefers the pentagon (hexagon) orientation when the tube diameter is smaller (larger) than 1.31nm (1.36nm). For the tube diameter in between, the preferred molecular orientation of C60-1 changes from pentagon to hexagon with the increasing tube diameter when the neighboring C60-2 molecule is fixed at the pentagon or double-bond orientation. A novel vertex orientation for the C60-1 molecule has been found when the C60-2 molecule is fixed at the hexagon orientation.     

Raman analysis of insulin denaturation induced by high‐pressure and thermal treatments

Raman spectroscopy has been used to investigate different conformational states of bovine pancreatic insulin: the native form and several structurally modified states with different extent of denaturation induced by thermo‐chemical treatment and by applying very high pressure (up to 8 GPa) using a diamond anvil cell. High‐pressure results confirm the peculiar strength to volume compression of insulin and largely extend the pressure range of its structural stability (0–4.2 GPa). Above 4.2 GPa, insulin undergoes an irreversible structural transition that, once pressure is released, leaves the sample in a new conformational state. The protein secondary structure after the pressure treatment results in a structure that is somewhat intermediate between that of the native and the thermo‐chemical fibrillar samples. The analysis of the pressure dependence of the Raman spectrum and of several specific spectroscopic markers allows us to follow the path from the native to new pressure‐denatured protein conformation. Copyright © 2011 John Wiley & Sons, Ltd.     

C_(60)高压相变的拉曼光谱研究

本文对非静水压淬火的 C_(60)样品进行了拉曼光谱研究。在14.8GPa 以上,C_(60)的1468cm~(-1)特征峰的频率和线宽保持不变,这是 fullerite 相己转变成“透明相”的拉曼光谱证据。在32GPa 以上,原 C_(60)的散射峰全部消失,只观察到两个新的弥散的弱散射峰。表明 C_(60)原子团簇已倒塌或严重畸变。     

Hexagonal germanium formed via a pressure‐induced phase transformation of amorphous germanium under controlled nanoindentation

We have studied the stable end phase formed in amorphous germanium (a‐Ge) films that have been subjected to a pressure‐induced phase transformation under indentation loading using a large (20 µm) spherical indenter. After indentation the samples have been annealed at room temperature to remove any residual unstable R8 and BC8 phases. Raman spectroscopy indicates a single broad peak centred around 292 cm^–1 and we have used first principles density functional perturbation theory calculations and simulated Raman spectra for nano‐crystalline diamond cubic germanium (DC‐Ge) to help identification of the final phase as hexagonal diamond germanium (HEX‐Ge). Transmission electron microscopy and selected area diffraction analysis confirmed the presence of a dominant HEX‐Ge end phase. These results help explain significant inconsistencies in the literature relating to indentation‐induced phase transitions in DC‐ and a‐Ge. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High‐pressure Raman spectra of deuterated L‐alanine crystal

Raman spectra of deuterated L ‐alanine have been obtained at high‐pressure conditions. A phase transition at ∼1.5 GPa associated with the splitting of some internal modes and increase of the wavenumber of the external modes was observed. Similarly to the hydrogenated L ‐alanine crystal, this first transition was related to a symmetry change. Moreover, further modifications of the Raman spectra were observed at 4.4 GPa, which may be associated to conformational changes of the molecule. To give further support to such a hypothesis, neutron powder diffraction measurements were performed. Information about the cell parameter at atmospheric pressure gave valuable information about the N D distances, shedding light on the behavior of the torsional vibration of ND₃⁺. Copyright © 2009 John Wiley & Sons, Ltd.     

单壁碳纳米管的高压拉曼研究

采用金刚石对顶砧装置对直径分布在1.3 nm左右的单壁碳纳米管进行了高压拉曼光谱研究.实验结果表明随压力的增加碳管的截面形状发生了由圆到椭圆再到扁平的变化,这和我们之前的研究结果一致.从31 GPa卸压至常压后碳管的结构得到了较好的保持,这个压力值明显高于传统的Sp2键结构的碳材料所能稳定存在的压力范围(20 GPa以...     

MgO高温高压特性及相变的第一性原理研究

应用第一性原理密度泛函理论计算了MgO在零温(0K)下和0~200GPa静水压范围内的晶体结构和弹性模量,以及B1、B4和B8相结构的MgO的声速随压力的变化。利用准简谐近似下的Debye模型,通过拟合三阶Birch-Murnaghan物态方程模拟了高温效应并对三个相在高温高压下的相稳定性做了研究。本工作的计算结果与前人的理论和实验结果符合较好,说明第一性原理结合准简谐Debye模型能够比较准确的模拟矿物如MgO在高温高压下的热力学性质。