Recent advances in the structures and properties of materials under high-pressure
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压力是基本的物理学参量之一, 可有效地使物质的原子间距缩短、相邻电子轨道重叠增加, 进而改变物质的晶体结构、电子结构和原子(分子)间的相互作用, 形成常压下难以存在的新物质状态. 这些物质多具有异于常压物质的结构、新颖的物理和化学性质. 在100万大气压力下每种物质平均可出现五个以上相变, 即高压可提供超出现有材料数倍的新物质, 为寻找特殊用途的新材料提供了丰富的来源. 在过去的十几年里, 高压研究已经推广应用到更为宽广的领域, 如能源科学、地质科学、材料工程、资源环境以及生物科学等, 取得了巨大的进展, 正在改变人们对周围世界的认识.
“近年来, 国内高压科学得到了迅猛的发展, 已经处于国际领先水平, 取得了突破性进展和成果. 本刊特组织“高压下物质的新结构与新性质研究进展”专题, 从高压下对物质的结构和性质研究两方面, 汇集了富氢材料、超导材料、超硬材料、有机分子材料等物质在高压作用下的研究论文和综述, 以帮助读者了解这个领域的最新进展, 推动对高压下物质的结构和性质的进一步深入研究.
2017, 66 (3): 033301.
doi: 10.7498/aps.66.033301
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In condensed phase, the dissociation mechanism of molecule is different from that of isolated molecule due to the effect of interaction between molecules. How to effectively trace the reaction process and products in condensed phase is a technical problem which needs to be solved urgently. In this paper, femtosecond transient grating spectroscopy is used to investigate dissociation dynamics in condensed phase. Transient grating spectroscopy, as a coherent spectral technique, has some advantages such as high signal-noise ratio and free background, thus it can identify trace numbers of reaction products in dissociation. The investigation about model molecules such as iodomethane and nitromethane demonstrates that the transient grating technique can observe relaxation in electronic excited state and also has ability to track reactants, products, and vibration of molecule or perssad. The dissociation dynamics in condensed phase material is significant for understanding the reaction mechanism in the fields of biochemistry and detonation. Thus the femtosecond transient grating has a wide application prospect in these fields. In addition, the transient grating technique, as a non-contact diagnostic approach, can be easily adapted to high temperature and high pressure conditions, etc. Thus, the transient grating technique also has a potential value in the fields of phase transform dynamics and high pressure synthesis, etc.
2017, 66 (3): 036102.
doi: 10.7498/aps.66.036102
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Metallic hydrogen can be realized theoretically at high pressure, which suggests that it will be a room-temperature superconductor due to the high vibrational frequencies of hydrogen atoms. However, the metallic state of hydrogen is not observed in experiment at up to 388 GPa. Scientists have been exploring various new ways to achieve hydrogen metallization. Hydrogen-rich compounds can be metallized at much lower pressures because of chemical pre-compression. Moreover, because such materials are dominated by hydrogen atoms, some novel properties can be found after metallization, such as high Tc superconductivity. Therefore, hydrogen-rich compounds are potential high-temperature superconductors, and this method is also believed to be an effective way to metalize hydrogen, which has aroused significant interest in lots of fields, such as physics, material science, etc. In a word, hydrogen-rich compounds are expected to become a new member of superconductor family:hydrogen-based superconductor. Very recently, the theoretical prediction and the successful experimental discovery of high-temperature superconductivity at 200 K in a sulfur hydride compound at high pressure have set a record, which inspired further efforts to study the superconductivity of hydrogen-rich compounds. The present review focuses on crystal structures, stabilities, interaction between atoms, metallization, and superconductivity of several typical hydrogen-rich compounds at high pressures. Furthermore, higher Tc superconductors can be expected to be found in hydrogen-rich compounds in the future.
2017, 66 (3): 036103.
doi: 10.7498/aps.66.036103
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Transition metal borides (TMBs) are hard or potential superhard materials due to abrasion resistant, corrosion preventive, oxidation resistance and high hardness. However, few TMBs are superhard materials, so, discussing the strength of TMBs to understand hardness mechanism is necessary. Moreover, there are superconductors, magnetic materials, and catalysts in TMBs. But uncovering more functions in TMBs is important for finding a new kind of functional hard or superhard material. While, high energy is necessary to synthesize TMBs due to strong BB covalent bonds and high melting of transition metal. Thus high temperature or extreme condition is necessary for synthesizing single crystal or bulk sample with high density, which is important for testing physical properties. Various ways of hybridizing boron atoms and high content of valence electron of transition metal are used to induce a large number of structures and potential new properties in TMBs. Boron atoms can form different substructures with different content of boron in TMBs, such as one-dimensional, two-dimensional and three-dimensional (3D) structures. These different boron atom substructures can affect the stability of structure and physical properties, especially hardness, because of the strong covalent bonds between boron atoms. Thus the structure and hardness of TMBs have always received much attention. The multiple electron transfer between transition metal and boron induces diverse chemical bonds in TMBs. All of covalent bonds, ionic bonds, and metal bonds in TMBs determine the mechanic performances, electricitic and magnetic properties, and chemical activity of TMBs. In this work, synthesis method, stability of structure, hardness, and functional properties of TMBs are discussed. The using of high pressure and high temperature is an effective method to prepare TMBs, because under high pressure and high temperature the electrons can transfer between transition-metal atoms and boron atoms in TMBs. There are not only stable TMBs which are even under very high pressure, but also many metastable structures in TMBs. Hardness values of TMBs are discussed by different content of boron, the high boron content or even 3D boron structure is not superhard material. Because insufficient electron transfer can form the distorted BB covalent bond which is weaker than directional covalent bonds like CC in diamond. Thus electron transfer is significant in TMBs for designing hard or even superhard materials. Besides high hardness, there are superconductor, magnetic material, and catalyzers in TMBs, but there are many potential properties of TMBs which are unknown. Further study to uncover the new properties of TMBs is significant for finding a new kind of functional hard material.
2017, 66 (3): 036202.
doi: 10.7498/aps.66.036202
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Stimulated by the extensive application and research value, the study of anhydrous magnesium carbonate (MgCO3) has been a subject of great concern recently, so that a basic problem in designing a method of effectively synthesizing MgCO3 is very worth considering. In previous studies, different methods were reported to synthesize MgCO3 successfully but they still have some obvious deficiencies. The micro-particle sizes are too small to satisfy the basic requirements of micro-analysis. Thus, it is needed to explore the new methods of artificially synthesizing MgCO3 with the simple process and the high efficiency.
By using magnesium oxalate dihydrate (MgC2O42H2O) as starting material, MgCO3 sample is successfully synthesized by a solid reaction under high temperature and high pressure for the first time in this work. The properties of as-synthesized sample are investigated by X-ray powder diffraction and Raman spectroscopy:neither of them shows any impurities existing in the sample. Significantly, the crystallinity quality is greatly improved in the terms of the maximum grain sizes up to 200 micrometers, which could provide a base for MgCO3 single crystal growth in the future. Moreover, compared with the results of previous studies, the reaction time of high pressure synthesis is controlled within 1 h so that the efficiency of the synthesis is greatly improved.
Based on thermogravimetric analyses and the results of high pressure experiment under the various pressures and temperatures, the P-T phase diagrams of MgC2O42H2O-MgCO3-MgO at high pressures of 0.5, 1.0 and 1.5 GPa are obtained, and in this case, it is reasonable to explain the principle of MgCO3 synthesis under high pressure strictly. From the P-T diagram, high pressure can greatly improve the thermal stability of material, and the decomposition temperature of MgCO3 obviously increases with pressure increasing. However, due to decomposition temperature of MgCO3 increasing more quickly than that of MgCO42H2O, the stable phase regions of MgC2O42H2O and MgCO3 are separated from each other, and hence, the corresponding temperature and pressure can be controlled to decompose the phase of MgC2O42H2O while stabilizing the phase of MgCO3 so as to obtain MgCO3 successfully. Besides, by using polarizing microscope, the morphology of MgCO3 sample as well as its crystal cleavage plane (1011) is observed clearly, and it is noted that as-synthesized MgCO3 has good optical properties and high-quality crystallinity. The electron probing analysis for MgCO3 thin section is performed to quantify the Mg content and the calculation indicates that the sample composition is Mg0.99CO3.
2017, 66 (3): 037401.
doi: 10.7498/aps.66.037401
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Magnetic quantum critical point (QCP) arises when a long-range magnetic order occurring at finite temperature can be suppressed to absolute zero temperature by using chemical substitutions or exerting high pressure. Exotic phenomena such as the non-Fermi-liquid behaviors or the unconventional superconductivity are frequently observed near the magnetic QCP. In comparison with chemical substitutions, the application of high pressure has some advantages in the sense that it introduces no chemical disorder and can approach the QCP in a very precise manner. In this article, our recent progress in exploring the unconventional superconductors in the vicinity of pressure-induced magnetic QCP is reviewed. By utilizing the piston-cylinder and cubic-anvil-cell apparatus that can maintain a relatively good hydrostatic pressure condition, we first investigated systematically the effect of pressure on the electrical transport properties of the helimagnetic CrAs and MnP. We discovered for the first time the emergence of superconductivity below Tc=2 K and 1 K near their pressure-induced magnetic QCPs at Pc0.8 GPa and 8 GPa for CrAs and MnP, respectively. They represent the first superconductor among the Cr- and Mn-based compounds, and thus open a new avenue to searching novel superconductors in the Cr- and Mn-based systems. Then, we constructed the most comprehensive temperature-pressure phase diagram of FeSe single crystal based on detailed measurements of high-pressure resistivity and alternating current magnetic susceptibility. We uncovered a dome-shaped magnetic phase superseding the nematic order, and observed the sudden enhancement of superconductivity with Tcmax=38.5 K accompanied with the suppression of magnetic order. Our results revealed explicitly the competing nature of nematic order, antiferromagnetic order, and superconductivity, and how the high-Tc superconductivity is achieved by suppressing the long-range antiferromagnetic order, suggesting the important role of antiferromagnetic spin fluctuations for the Cooper paring. These aforementioned results demonstrated that high pressure is an effective approach to exploring or investigating the anomalous phenomena of strongly correlated electronic systems by finely tuning the competing electronic orders.
2017, 66 (3): 038103.
doi: 10.7498/aps.66.038103
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Materials having Vickers hardness (HV) higher than 40 GPa are considered to be superhard. Superhard material is exclusively covalent and displays superior hardness, incompressibility, and wear resistance, which make this kind of material essential for a wide range of industrial applications, such as turning, cutting, boring, drilling, and grinding. Most of superhard materials are prepared under extreme pressure and temperature conditions, not only for scientific investigations, but also for practical applications. With the development of high pressure science and technology, the field of superhard composites is more active and more efficient, energy saving and environmental protection. Ultrahigh pressure and ultrahigh temperature method plays an important role in the scientific research and industrial production of superhard materials. It provides the driving forces for the light elements forming novel superhard phases and the way of sintering high-density nanosuperhard materials. In this paper, the recent achievements and progress in high-pressure synthesis and research of superhard materials are introduced mainly in the nanopolycrystalline diamond, nanopolycrystalline cubic boron nitride (cBN), ultrahard nanotwinned cubic boron nitride, submicron polycrystalline cubic boron nitride, cBN-Si composites material, cubic-Si3N4-diamond nanocomposites and diamond-cubic boron nitride superhard alloy (composite) material prepared under ultrahigh pressure and high temperature, by using multi-anvil apparatus based on the hinged-type cubic press. These superhard composite materials are successfully synthesized by high temperature and high pressure, and a variety of performance tests show that their hardness values and thermal stability properties exceed those of the traditional superhard materials. At the same time, some new ideas, approaches to the study of superhard composite materials in recent years have been introduced, such as nanostructuring approaches and special treatments of the starting material for high-performance superhard materials, using the formation of alloys or solid solution to fill the performance gap between different materials for enhancing comprehensive performance (i.e., hardness, fracture toughness, and thermal stability), or changing and optimizing the assembly method to improve the uniformity of performance. Finally, the prospect of superhard composite material is also discussed. In the research field of superhard materials, on the one hand, the relationship between macrohardness and microstructure of superhard materials is studied continuously to establish hardness models with atomic parameters, which can be used to guide the design or prediction of novel superhard crystals. On the other hand, highly comprehensive performance and larger size of super-hard composite materials are synthesized for practical application.
2017, 66 (3): 030701.
doi: 10.7498/aps.66.030701
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Recent advance in highly efficient solar cells based on organic-inorganic hybrid perovskites has triggered intense research efforts to ascertain the fundamental properties of these materials. In this work, we utilize diamond anvil cell to investigate the pressure-induced structural and optical transformations in methylammonium lead iodide (CH3NH3PbI3) at pressures ranging from atmospheric pressure to 7 GPa at room temperature. The synchrotron X-ray diffraction experiment shows that the sample transforms from tetragonal (space group I4cm) to orthorhombic (space group Imm2) phase at 0.3 GPa and amorphizes above 4 GPa. Pressure dependence of the unit cell volume of CH3NH3PbI3 shows that the unit cell volume undergoes a sudden reduction at 0.3 GPa, which can prove the observed phase transition. We provide the high-pressure optical micrographs obtained from a diamond anvil cell. Upon compression, we can visually observe that the opaque black sample gradually transforms into a transparent red one above 4 GPa. We analyze the pressure dependence of the band gap energy based on the optical absorption and photoluminescence (PL) results. As pressure increases up to 0.25 GPa, the absorption edge and PL peak move to the longer wavelength region of 9 nm. However, abrupt blueshifts of the absorption edge and PL peak occur at 0.3 GPa, followed by a gradual blueshift up to 1 GPa, these phenomena correspond to the previously observed phase transitions. Phase transition increases the band gap energy of CH3NH3PbI3 as a result of reductions in symmetry and tilting of the[PbI6]4- octahedral. Upon further compression, the sample exhibits pressure-induced amorphization at about 4 GPa, which significantly affects its optical properties. Further high pressure Raman and infrared spectroscopy experiments illustrate the high pressure behavior of organic CH3NH3+ cations. Owing to the presence of hydrogen bonding between organic cations and the inorganic framework, all of the bending and rocking modes of CH3 and NH3 groups are gradually red-shifted with increasing pressure. The transition of NH stretching mode from blueshift to redshift as a result of the attractive interactions between hydrogen atoms and iodine atoms is gradually strengthened. Moreover, all the observed changes are fully reversible when the pressure is completely released. In situ high pressure studies provide essential information about the intrinsic properties and stabilities of organic-inorganic hybrid perovskites, which significantly affect the performances of perovskite solar cells.
2017, 66 (3): 036101.
doi: 10.7498/aps.66.036101
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Energetic materials (EMs) including explosives, propellants and pyrotechnics have been widely used for the military and many other purposes. Solid nitrobenzene (an organic molecular crystal) could be considered as a prototype of energetic material. Up to now, numerous studies have been devoted to crystal structures, spectrum properties and decomposition mechanisms for solid nitrobenzene experimentally and theoretically. However there has been a lack of the comprehensive understanding of the anisotropic characteristics under different loading conditions. Thus we investigate the hydrostatic and uniaxial compressions along three different lattice directions to determine this anisotropic effect. In this work, the density functional theory calculations are performed based on Cambridge Sequential Total Energy Package (CASTEP) code using normconserving pseudo potentials and a kinetic energy cutoff of 700 eV. The generalized gradient approximation with the Perdew-Burke-Ernzerhof parameterization is used. Monkhorst-Pack k-point meshes with a density of 0.05 -1 are used for Brillouin-zone integration. The empirical dispersion correction by Grimme is taken to account for week intermolecular interactions. The hydrostatic compressions are applied from 0 GPa to 20 GPa. Cell volume, lattice shape and coordinates of the atoms could be fully relaxed. while uniaxial compression is applied up to 70% of the equilibrium cell volume in steps of 2% along their lattice directions respectively. At each compression step, only atomic coordinates are allowed to relax, with the lattice fixed. The equilibrium lattice structures under hydrostatic compressions are obtained by full relaxation at 0 K temperature. In ambient condition, the calculated volume and parameter of the unit cell are underestimated compared with the experimental data, and corresponding errors are -2.98%, 0.01%, -4.39%, 5.71% respectively. In contrast, the calculated lattice energy is overestimated compared with the range of experimental results with 5.71% of the error. In high pressure condition, the volume and cell parameter of the unit cell as a function of compression ratio are plotted and compared with the experimental data. The theoretical and experimental values are close with the increase of the pressure, for instant, the error decreases from -4.39% at 0 GPa to -1.93% at 4 GPa. On the other hand, the uniaxial compression is applied along the directions of three lattice vectors. The changes of stress tensor, band gap, energy per atom as a function of compression ratio are also plotted and discussed, which can characterize the anisotropic effect of solid nitrobenzene. The most noticeable effect of anisotropy in solid nitrobenzene is the metallization at V/V0=0.76 compressed along the X axis, while the solid nitrobenzene under hydrostatic pressure or other uniaxial compressions up to V/V0=0.76 remains semiconductor with band gap larger than 1.591 eV. By analyzing the local density of states and charge density distribution of nitrobenzene crystal, we confirm that the metallization is caused by the overlap of the electron from benzene ring. Through calculating different physical parameters, we find that X axis is the most sensitive direction of nitrobenzene crystal. The studies of anisotropic effects are expected to shed light on the physical and chemical properties of solid nitrobenzene on an atomistic scale and provide several insights for experiments.
2017, 66 (3): 036201.
doi: 10.7498/aps.66.036201
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In this review, we present our recent research progress in superhard materials, with specially focusing on two topics. One topic is to understand hardness microscopically and establish the quantitative relationship between hardness and atomic parameters of crystal, which can be used to guide the design of novel superhard crystals. The other topic is to identify the fundamental principle and technological method to enhance the comprehensive performances (i.e., hardness, fracture toughness, and thermal stability) of superhard materials, and to synthesize high-performance superhard materials. Starting from the chemical bonds associated with crystal hardness and electronic structure, we propose a microscopic understanding of the indentation hardness as the combined resistance of chemical bonds in a material to indentation. Under this assumption, we establish the microscopic hardness model of covalent single crystals and further generalize it to polycrystalline materials. According to the polycrystalline hardness model, we successfully synthesize nanotwinned cubic boron nitride and diamond bulks under high pressure and high temperature. These materials exhibit simultaneous improvements in hardness, fracture toughness, and thermal stability. We also clarify a long-standing controversy about the criterion for performing a reliable indentation hardness measurement. Our research points out a new direction for developing the high-performance superhard materials, and promises innovations in both machinery processing industry and high pressure science.
2017, 66 (3): 039101.
doi: 10.7498/aps.66.039101
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The ultimate goals of researches of one-dimensional (1D) nanomaterials, quasi-one-dimensional atomic/molecular chains are expected to exhibit their strong quantum effects and novel optical, electrical, magnetic properties due to their unique 1D structures. At present, synthesis and manipulation of 1D atomic/molecular chains on an atomic/molecular level in a controllable way have been the frontier subject of scientific research. The 1D atomic/molecular chains, which can be stable in ambient conditions, have been prepared successfully by using a confinement template, such as carbon nanotubes (CNTs), zeolite, etc.
High pressure can effectively tune the interatomic and intermolecular interactions over a broad range of conditions and thus to change the structures of materials. High pressure techniques have been recently adopted to investigate the 1D nanomaterials. In this paper, we briefly review some recent progress in the high pressure studies of 1D nanostructures, including iodine chains (I2)n confined in the 1D nanochannels of zeolite, multiwalled carbon nanotube (MWNT) arrays, and 1D carbon chains confined in CNTs. Particularly, polarized Raman spectroscopy combined with theoretical simulations has been used in the high pressure studies of 1D nanostructures. These studies reveal many interesting phenomena, including pressure-induced population increase and growth of 1D atomic/molecular chains. The underlying driven mechanisms have also been uncovered. Induced by pressure, the I2 molecules in zeolite 1D nanochannels rotates to the channel axial direction and the compression of the channel length in turn leads to a concomitant decrease of the intermolecular distance such that the iodine molecules come sufficiently close to the formation of longer (I2)n polymers. The novel polarized photoluminescence (PL) from the iodine chains and the pressure-induced PL enhancement due to the growth of 1D iodine chains under pressure. The depolarization effect vanishing in the polarized Raman spectra of compressed MWNT arrays. These are related to the pressure-induced enhancement of intertube interactions and inter/intratube sp3 bonding. The results obtained by polarized Raman spectroscopy overcome the difficulty:MWNTs have no obvious fingerprints for identifying the structural transformation under pressure.
Above all, the 1D nanostructures exhibit interesting and fantastic behaviors under pressure, which deserve further investigations in this research field. In addition, polarized Raman spectroscopy is an effective tool to study the structural transformations of 1D nanomaterials at high pressures, which can be extended to the studies of other analogous 1D nanostructures under pressure.
2017, 66 (3): 030201.
doi: 10.7498/aps.66.030201
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Strongly correlated electronic systems with ABO3 perovskite and/or perovskite-like structures have received much attention. High pressure is an effective method to prepare perovskites, in particular A-site and/or B-site ordered perovskites. In these ordered perovskites, both A and B sites can accommodate transition-metal ions, giving rising to multiple magnetic and electrical interactions between A-A, B-B, and A-B sites. The presence of these new interactions can induce a wide variety of interesting physical properties. In this review paper, we will introduce an A-site ordered perovskite with chemical formula AA3'B4O12 and two A- and B-site ordered perovskites with chemical formula AA3'B2B2'O12. All of these compounds can be synthesized only under high pressure. In the A-site ordered LaMn3Cr4O12 with cubic perovskite structure, magnetoelectric multiferroicity with new multiferroic mechanism is found to occur. This is the first observation of multiferroicity appearing in cubic perovskite, thereby opening the way to exploring new multiferroic materials and mechanisms. In the A- and B-site ordered perovskite CaCu3Fe2Os2O12, a high ferrimagnetic Curie temperature is observed to be around 580 K. Moreover, this compound exhibits semiconducting conductivity with an energy band gap of about 1 eV. The CaCu3Fe2Os2O12 thus provides a rare single-phase ferrimagnetic semiconductor with high spin ordering temperature well above room temperature as well as considerable energy band gap. Moreover, theoretical calculations point out that the introducing of A'-site Cu2+ magnetic ions can generate strong Cu-Fe and Cu-Os spin interactions. As a result, this A- and B-site ordered perovskite has a much higher Curie temperature than that of the B-site only ordered perovskite Ca2FeOsO6 (~320 K). In addition, we also for the first time prepare another A- and B-site ordered perovskite LaMn3Ni2Mn2O12. In the reported ordered perovskites with Mn3+ at the A' site, the A'-B intersite spin interaction is usually negligible. In our LaMn3Ni2Mn2O12, however, there exists the considerable A'-B interaction, which is responsible for the rare formation of B-site orthogonal spin structure with net ferromagnetic moment.
2017, 66 (3): 030505.
doi: 10.7498/aps.66.030505
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The equations of state (EOS) and the thermodynamics properties of plasma under high temperature are widely applied to the fields of astrophysics, controllable fusion, weapon design and damage. In this paper we mainly review the theoretical model and computing method of the EOS of hot plasma on different density scales and temperature scales. For an ideal plasma, the interaction between ions can be ignored, the EOS is simple and the theories turn matured. Under the condition of extremely high temperature, ions are ionized completely and the EOSs of ions and electrons can be approximated by the EOS of ideal gas. When the temperature is not very high and ions are just partly ionized, the EOS can be obtained by Saha model or its modified model. When atoms are strongly compressed, the EOS can be calculated by Thomas-Fermi model or its modified model. For the non-ideal plasma, there is a strong coupling between ions. No unified theoretical model can completely describe the interaction between ions at arbitrary density and arbitrary temperature. In principle, the quantum molecular dynamics (QMD) can accurately describe the EOS of plasma in large density range and large temperature range. However, due to the enormous computation and the difficulty in converging, it is difficult to apply QMD to the plasma under high temperature. With simple computing method and small computation, classical molecular dynamics using semi-empirical potential can calculate the EOS accurately at high temperature. However, it will produce great error at lower temperature. It is a simple and effective way to obtain a global EOS by using different theoretical models in different density range and different temperature range and by interpolating in the vacant density range and vacant temperature range.
2017, 66 (3): 036104.
doi: 10.7498/aps.66.036104
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Transition metals have special characteristics, such as a large number of valence electrons, multi valence states, high electron density, etc. Introducing a light element, such as boron, carbon, nitrogen, oxygen, etc. into a transition metal is an important means for searching the new multifunctional hard materials. With the development of ab intio calculation, advance in computer and the more in-depth understanding of the nature of hardness, it is possible to design new multifunctional ultra-hard transitional metal with using the advanced structure searching software, which could now serve as the experimental syntheses of these materials. In the present article, we introduce the design of ultra hard multi functional transition metal materials. We first introduce some basic ideas of hardness and material design, then conduct some studies, afterwards we discuss some difficulties in this kind of research. Hopefully these results in the present study could be helpful for designing and synthesizing the ultra-hard multifunctional materials.
2017, 66 (3): 036203.
doi: 10.7498/aps.66.036203
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A lot of great work has been done since the high pressure research carried out on synchrotron radiation facility almost 40 years ago. The history of high pressure single-crystal diffraction research on synchrotron radiation facility has also been more than 20 years. Recently, with the development of synchrotron X-ray optical techniques and high pressure technology, especially the invention and improvement of large opening diamond anvil cell (DAC), high pressure single-crystal X-ray diffraction (HPSXRD) method has become more and more popular in high pressure studies. The HPSXRD can be used to perform structure determination and refinement to obtain the information about lattice parameter, space group, atomic coordinate and site occupation. Compared with powder X-ray diffraction, the HPSXRD can not only obtain the three-dimensional diffraction information of samples, but also have much better signal-to-noise ratio. Furthermore, the HPSXRD data can be used to study the electron density distribution to obtain more information about chemical bonds and electron distribution. In this work, we introduce the HPSXRD method in synchrotron radiation facilities, including the knowledge of single-crystal X-ray diffraction experimental system, DAC for HPSXRD, sample loading, and HPSXRD data processing.
2017, 66 (3): 036401.
doi: 10.7498/aps.66.036401
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In this paper, we present in detail various theoretical models for studying the equation of state of warm dense matter, including the fluid variational theory, the chemical model, the ionization equilibrium model, the average atom model and INFERNO model. The method of calculating the equation of state of a mixture is also given. The results from the first principles molecular dynamics simulation and the quantum Monte Carlo simulation are also provided. Typical materials such as hydrogen, deuterium, helium, xenon, gold, tungsten, etc. are studied in warm dense region by using all the methods, showing the effects of dissociation and ionization in the equation of state.
2017, 66 (3): 037402.
doi: 10.7498/aps.66.037402
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As one of the independent control parameters, pressure plays an important role in finding new phenomena, testing related theories and guiding the explorations for new superconductors. In this review article, we will briefly review the progress achieved from high pressure studies on some main types of the iron pnictide superconductors, including 1111-type, 122-type, 111-type, 10-3-8 type and 112-type. A few typical results from high pressure studies are introduced in more detail, including the positive pressure effect on the superconducting transition temperature TC of 1111-type iron pnictide superconductors, which indicates a way to enhance the TC by using a smaller cation to replay La ion; the maximum TC of iron pnictide superconductors estimated by high-pressure studies on a series of 1111-type iron-based superconductors etc. More importantly, high pressure studies on the parent compounds of iron pnictide superconductors clearly demonstrate that pressure can suppress the transition temperatures of magnetic order and crystal structure, and then drive a superconducting transition. Furthermore, many examples are given in this review to reveal how the magnetic order competes with superconductivity under pressure, which provides new constrains for the establishment of the theory on superconductivity. These high pressure results are expected to be helpful for the studies of high-TC superconductors and for the exploring of new superconductors.
2017, 66 (3): 037403.
doi: 10.7498/aps.66.037403
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Temperature and pressure are the two most important thermodynamic elements, which determine the existent state of substance. Low temperature and high pressure are significant and key extreme conditions in the modern experimental science, providing new routes for many subjects such as physics, chemistry, materials and biology, and playing an important role in finding new phenomena. The magnetic research under extreme conditions is an important branch of the study of the extreme conditions, which not only presents the magnetic changes of the material under extreme conditions, but also is an important means to explore the high temperature superconductors. In this article, we elaborate the principle and method of measuring the magnetic susceptibility and superconducting transition temperature under high pressure. The in-situ magnetic measurement system under high pressure and low temperature is also briefly introduced, designed and installed by ourselves. Using the in-situ magnetic measurement system, the magnetic transition of iron and the superconducting transition temperature of the yttrium barium copper oxide sample under high pressure are measured.
