High pressure phase transitions and depressurization amorphization of Bi2Fe4O9

High pressure structural behavior of Bi₂Fe₄O₉ has been studied by in situ angular-dispersive X-ray diffraction (ADXD) measurements up to 51.3 GPa. Two phase transitions have been observed at 7.6 and 22.6 GPa, respectively. A second high pressure structure (HP2) involving the tripling of lattice parameter c has been identified. An unusual amorphization occurs after releasing pressure. The high pressure phase transitions can be understood in terms of the increase in the coordination number of Fe³⁺ ion. The depressurization amorphization results from the appearance of the metastable HP2 and its collapse after releasing pressure. The results extend our understanding of pressure-induced amorphization.     

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.     

PbTiO3在压力下的异常相变和极强压电效应

运用第一性原理计算预言了在一纯化合物中可由压力诱导出顺序为四方晶－单斜体－菱形体－立方体的新的相变,而且存在有变形相界面.在相变区有可与在复杂的单晶固溶体压电材料,如人们期待在机电应用方面引起革命性变化的Pb(Mg1/3Nb2/3)O3-PbTiO3可比的,极大的介电和压电耦合常数.我们的结果表明变形相界面和巨压电效应并不需要内禀的无序,并打开了在简单系统中研究这一效应的可能性.     

PbTiO3在压力下的异常相变和极强压电效应

运用第一性原理计算预言了在一纯化合物中可由压力诱导出顺序为叫方晶-单斜体-菱形体-立方体的新的相变，而且存在有变形相界面。在相变区有可与在复杂的单晶固溶体压电材料，如人们期待在机电应用方面引起革命性变化的Pb（Mg1/3Nb2/3）O3-PbTiO3可比的，极大的介电和压电耦合常数。我们的结果表明变形相界面和巨压电效应并不需要内禀的无序，并打开了在简单系统中研究这一效应的可能性，     

Pressure-induced phase transitions for goethite investigated by Raman spectroscopy and electrical conductivity

We reported two pressure-induced phase transitions of goethite up to ～35?GPa using a diamond anvil cell in conjunction with ac impendence spectroscopy, Raman spectra at room temperature. The first pressure-induced phase transition at ～7.0?GPa is manifested in noticeable changes in six Raman-active modes, two obvious splitting phenomena for the modes and the variations in the slope of conductivity. The second phase transition at ～20?GPa was characterized by an obviously drop in electrical conductivity and the noticeable changes in the Raman-active modes. The variations in activation energy with increasing pressure were also discussed to reveal the electrical properties of goethite at high pressure.     

Pressure-induced phase transition of crystalline and amorphous silicon and germanium at low temperatures

Abstract

X-ray diffraction has been measured for crystalline silicon, crystalline germanium, amorphous silicon and amorphous germanium at temperatures down to 100 K and pressures up to 20 GPa using a diamond anvil cell and synchrotron radiation. The structural phase transitions, including amorphization, take place in the pressure-temperature range. It has been found that the structures after the phase transitions strongly depend on the path in the pressure-temperature diagram through which the system undergoes the phase transitions. For any of the aforementioned four materials, the high-pressure phase with the p-Sn structure is quenched during a release of pressure at 100 K, and transforms into an amorphous state when heated up to around 2 GPa. The path dependence of the states is discussed in relation to the pressure dependence of the heights of the energy barriers which have to be overcome when phase transitions occur. The effect of a structural disorder on the phase transition is also discussed by comparing the experimental results for the crystalline and amorphous materials.     

Pressure-induced phase transition and structural properties of CrO2

The structural properties and pressure-induced phase transitions of CrO₂ have been investigated using the pseudopotential plane-wave method based on the density functional theory (DFT). The rutile-type (P4₂/mnm), CaCl₂-type (Pnnm), pyrite-type (Pā3), and CaF₂-type (Fm-3m) phases of CrO₂ have been considered. The structural properties such as lattice parameters, bulk moduli and its pressure derivative are consistent with the available experimental data. The second-order phase-transition pressure of CrO₂ from the rutile phase to CaCl₂ phase is 10.9?GPa, which is in good agreement with the experimental result. The sequence of these phases is rutile-type?→?CaCl₂-type?→?pyrite-type?→?CaF₂-type with the phase-transition pressures 10.9, 23.9, and 144.5?GPa, respectively. The equation of state of different phases has also been presented. It is more difficult to compress with the increase of pressure for different phases of CrO₂.     

纳米晶粒在高压下的相变

文章报道了纳米晶粒的晶粒尺寸对压力诱导相变的影响的最新研究进展．采用热力学理论揭示了纳米晶体材料的相变压力与同种大块材料不同的主要因素是体积变化率、表面能差和内能差．通过估算这3个因素的具体大小，可解释文献中报道的实验结果，并且可以确定同种大块材料和纳米晶体材料之间的相变压力发生差异的控制因素．在纳米晶体材料中，晶粒尺寸对结构稳定性和相变压力的影响与体系本身有关．     

Pressure-induced phase transitions in the ZrXY (X= Si,Ge, Sn;Y= S,Se, Te) family compounds

Pressure is an effective and clean way to modify the electronic structures of materials, cause structural phase transitions and even induce the emergence of superconductivity. Here, we predicted several new phases of the ZrXY family at high pressures using the crystal structures search method together with first-principle calculations. In particular, the ZrGeS compound undergoes an isosymmetric phase transition from P4/nmm-I to P4/nmm-II at approximately 82 GPa. Electronic band structures show that all the high-pressure phases are metallic. Among these new structures, P4/nmm-II ZrGeS and P4/mmm ZrGeSe can be quenched to ambient pressure with superconducting critical temperatures of approximately 8.1 K and 8.0 K, respectively. Our study provides a way to tune the structure, electronic properties, and superconducting behavior of topological materials through pressure.     

Ba3Bi3Ti4NbO18的压致结构相变

 利用Mao-Bell型金刚石对顶砧装置（DAC）,使用4∶1的甲醇-乙醇混合液作传压介质,研究了层状铁电固溶体Ba₃Bi₃Ti₄NbO₁₈的在位高压拉曼光谱和压致结构相变（0~8.87 GPa）。观测到了Ba₃Bi₃Ti₄NbO₁₈的一个典型的压致结构相变。发现了赝钙钛矿结构A₄B₅O₁₆的A位和B位联合置换对Ba₃Bi₃Ti₄NbO₁₈的拉曼振动模式及压致相变点的调制作用。并使用内模方法对Ba₃Bi₃Ti₄NbO₁₈的内模进行了指认。通过对加压下的拉曼光谱的分析,得到了压力作用下样品中BO₆八面体的结构畸变的演化方向。     

A complete Raman mapping of phase transitions in Si under indentation

Crystalline Si substrates are studied for pressure‐induced phase transformation under indentation at room temperature (RT) using a Berkovich tip. Raman spectroscopy, as a nondestructive tool, is used for the identification of the transformed phases. Raman lines as well as area mapping are used for locating the phases in the indented region. Calculation of pressure contours in the indented region is used for understanding the phase distribution. We report here a comprehensive study of all the phases of Si, reported so far, leading to possible understanding of material properties useful for possible electromechanical applications. As a major finding, distribution of the amorphous phase in the indented region deviates from the conventional wisdom of being in the central region alone. We present phase mapping results for both Si(100) and Si(111) substrates. Copyright © 2009 John Wiley & Sons, Ltd.     

静电纺丝法制备Bi2Fe4O9及其磁学性能的研究

采用静电纺丝法合成了纤维状的Bi₂Fe₄O₉前驱体,再对前驱体进行热处理得到了棒状的Bi₂Fe₄O₉.通过X射线衍射、扫描电子显微镜及透射电子显微镜表征了合成样品的物相及形貌特征.结果表明合成的样品为Bi₂Fe₄O₉单相,属于正交晶系;退火处理导致纤维状的前驱体转变为棒状的Bi₂Fe₄O₉.紫外-可见吸收光谱表明制备的Bi₂Fe₄O₉对光的吸收范围广,不仅对紫外光具有较强吸收,而且对可见光也有一定的吸收.通过振动样品磁强计测定Bi₂Fe₄O₉磁滞回线研究其磁学特性,相应的矫顽力H_C≈82 Oe（1 Oe=79.5775 A/m）,剥离顺磁信号后的剩磁M_r≈0.25 emu/g,研究发现Bi₂Fe₄O₉样品具有弱铁磁性,并且软磁性能有所提高.     

In situ Raman and photoluminescence study on pressure‐induced phase transition in C60 nanotubes

Single crystalline C₆₀ nanotubes having face‐centered‐cubic structure with diameters in the nanometer range were synthesized by a solution method. In situ Raman and photoluminescence spectroscopy under high pressure were employed to study the structural stabilities and transitions of the pristine C₆₀ nanotubes. A phase transition, probably because of the orientational ordering of C₆₀ molecules, from face‐centered‐cubic structure to simple cubic structure occurred at the pressure between 1.46 and 2.26 GPa. At above 20.41 GPa, the Raman spectrum became very diffuse and lost its fine structure in all wavenumber regions, and only two broad and asymmetry peaks initially centered at 1469 and 1570 cm^–1 were observed, indicating an occurrence of amorphization. This amorphous phase remained to be reversible until 31.1 GPa, and it became irreversible to the ambient pressure after the pressure cycle of 34.3 GPa was applied. Copyright © 2012 John Wiley & Sons, Ltd.     

High pressure structural phase transitions of TiO_2 nanomaterials

Recently, the high pressure study on the TiO_2 nanomaterials has attracted considerable attention due to the typical crystal structure and the fascinating properties of TiO_2 with nanoscale sizes. In this paper, we briefly review the recent progress in the high pressure phase transitions of TiO_2 nanomaterials. We discuss the size effects and morphology effects on the high pressure phase transitions of TiO_2 nanomaterials with different particle sizes, morphologies, and microstructures. Several typical pressure-induced structural phase transitions in TiO_2 nanomaterials are presented, including size-dependent phase transition selectivity in nanoparticles, morphology-tuned phase transition in nanowires, nanosheets,and nanoporous materials, and pressure-induced amorphization(PIA) and polyamorphism in ultrafine nanoparticles and TiO_2-B nanoribbons. Various TiO_2 nanostructural materials with high pressure structures are prepared successfully by high pressure treatment of the corresponding crystal nanomaterials, such as amorphous TiO_2 nanoribbons, α-PbO_2-type TiO_2 nanowires, nanosheets, and nanoporous materials. These studies suggest that the high pressure phase transitions of TiO_2 nanomaterials depend on the nanosize, morphology, interface energy, and microstructure. The diversity of high pressure behaviors of TiO_2 nanomaterials provides a new insight into the properties of nanomaterials, and paves a way for preparing new nanomaterials with novel high pressure structures and properties for various applications.     

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.     

Behaviors of Zn_2GeO_4 under high pressure and high temperature

The structural stability of Zn_2GeO_4 was investigated by in-situ synchrotron radiation angle dispersive x-ray diffraction. The pressure-induced amorphization is observed up to 10 GPa at room temperature. The high-pressure and hightemperature sintering experiments and the Raman spectrum measurement firstly were performed to suggest that the amorphization is caused by insufficient thermal energy and tilting Zn–O–Ge and Ge–O–Ge bond angles with increasing pressure,respectively. The calculated bulk modulus of Zn_2GeO_4 is 117.8 GPa from the pressure-volume data. In general, insights into the mechanical behavior and structure evolution of Zn_2GeO_4 will shed light on the micro-mechanism of the materials variation under high pressure and high temperature.     

Pressure-induced phase transition and stability of EuO and EuS with NaCl structure

We have predicted the phase transition pressures and corresponding relative volume changes of EuO and EuS having NaCl-type structure under high pressure using three-body interaction potential (TBIP) approach. In addition, the conditions for relative stability in terms of modified Born criterion has been checked. Our calculated results of phase transitions, volume collapses and elastic behaviour of these compounds are found to be close to the experimental results. This shows that the inclusion of three-body interaction effects makes the present model suitable for high pressure studies.      

高压下的铁基超导体:现象与物理

始于2008年的铁基超导体研究续写了高温超导发展史的新篇章.回顾过去十年对铁基超导体的研究,在理论、实验及应用方面都取得了辉煌的成绩,丰富了人们对高温超导电性的认识,为突破高温超导机理研究、最终实现超导材料的人工设计与更广泛的应用奠定了坚实的基础.本文主要介绍了通过高压实验研究手段在铁基超导体的研究中取得的一些重要进展及呈现出的新现象和新物理,例如压致超导现象、压力导致的超导再进入现象、压力对超导转变温度的提升效应、压力研究对铁基超导体超导转变温度的预测、相分离结构对超导电性的影响及反铁磁-超导双临界点的发现等.希望这些高压研究结果与本文报道的其他各类实验与理论研究成果一起,为全面、深入地理解铁基超导体勾画出一幅较为完整的物理图像.