Phase diagram of silicon from atomistic simulations

In this Letter we present a calculation of the temperature-pressure phase diagram of Si in a range of pressures covering from -5 to 20 GPa and temperatures up to the melting point. The phase boundaries and triple points between the diamond, liquid, beta-Sn, and Si34 clathrate phases are reported. We have employed efficient simulation techniques to calculate free energies and to numerically integrate the Clausius-Clapeyron equation, combined with a tight-binding model capable of an accuracy comparable to that of first-principles methods. The resulting phase diagram agrees well with the available experimental data.     

Melting curve of silicon to 15 GPa determined by two-dimensional angle-dispersive diffraction using a Kawai-type apparatus with X-ray transparent sintered diamond anvils

The melting curve of silicon has been determined up to 15 GPa using a miniaturized Kawai-type apparatus with second-stage cubic anvils made of X-ray transparent sintered diamond. Our results are in good agreement with the melting curve determined by electrical resistivity measurements [V.V. Brazhkin, A.G. Lyapin, S.V. Popova, R.N. Voloshin, Nonequilibrium phase transitions and amorphization in Si, Si/GaAs, Ge, and Ge/GaSb at the decompression of high-pressure phases, Phys. Rev. B 51 (1995) 7549] up to the phase I (diamond structure)—phase II (β-tin structure)—liquid triple point. The triple point of phase XI (orthorhombic, Imma)—phase V (simple hexagonal)—liquid has been constrained to be at 14.4(4) GPa and 1010(5) K. These results demonstrate that the combination of X-ray transparent anvils and monochromatic diffraction with area detectors offers a reliable technique to detect melting at high pressures in the multianvil press.     

高温高压下碳的状态方程及相变理论研究

 研究了高温高压下三相碳（石墨、金刚石、液相碳）的状态方程，包括高压下石墨到金刚石的固-固相变以及高温下石墨和金刚石的熔解曲线。计算所得到的金刚石熔解曲线具有正的斜率，石墨-金刚石-液相三相点为4 400 K，14 GPa左右。     

Structure of liquid iron at pressures up to 58 GPa

We report structural data on liquid iron at pressures up to 58 GPa measured by x-ray scattering in a laser heated diamond anvil cell. The determined structure factor preserves essentially the same shape along the melting curve. Our data demonstrate that liquid iron at high pressures is a close-packed hard-sphere liquid. The results place important constraints on the thermodynamic and transport properties of liquid iron and the melting curve of iron.     

Gold under high pressure

Abstract

First principle predictions for the equation of state of gold using solid and liquid state theories are compared up to combined pressures and temperatures of 600 GPa and 17 000 K with static diamond anvil cell compression, ultrasonic measurements and shock Hugoniot data which include a recent laser driven shock Hugoniot points at 600 GPa. Excellent agreement between theoretical and experimental data is observed. The theoretically estimated 300 K isotherm agrees to within 2 GPa with the isotherm that has been measured to 70 GPa using the diamond anvil cell. The structural energy estimates show that the normal f.c.c. phase remains stable under pressure. The estimate of the shock Hugoniot temperature of gold at 600 GPa based on a liquid state model is consistent with the measurements of laser induced shock luminescence, which in fact provides an experimental determination of the temperature of gold above its Hugoniot melting point. The powerful means provided by theory in the prediction of material properties of gold at ultra high pressures and temperatures is significant because gold is an efficient converter of laser energy into soft X-rays and is a potential candidate as a standard for high pressure, high temperature work.     

Melting behaviors of carbon underhigh pressures

Abstract

The melting behaviors of graphite and diamond were investigated at pressures up to 25 GPa using flash-heating method. By rapid heating, the metastable graphite was melted in the diamond stable P-T field, competing with its conversion to diamond in the rate of reaction. For the diamond the pressure dependence of inserted energy required to reach the molten state suggested that the melting temperature of diamond increases with pressure.     

High-pressure melting curve of nitrogen and the liquid-liquid phase transition

The melting curve of nitrogen was measured up to 71 GPa, a fourfold increase in pressure over previous measurements. The measurements were made using the laser-heated diamond anvil cell and melting was detected in situ by the laser speckle method. The melting temperature rises linearly up to a maximum at 50 GPa and 1920 K, and with increasing pressure suddenly decreases linearly to 1400 K at 71 GPa. This sharp drop in the melting slope (dT/dP) above 50 GPa indicates the appearance of a liquid denser than the solid and of a liquid-liquid phase transition. The sharpness of the changes suggests that the transition is first order and is a liquid-liquid polymer transition. This conclusion is consistent with earlier theoretical studies and experimental evidence that pressure transforms molecular nitrogen into a chainlike polymeric form.     

A molecular dynamics study of boron and nitrogen in diamond

Based upon molecular dynamics simulation via the Tersoff many-body potential, we proposed the co-doping method for fabricating n-type diamond. We calculated the optimal co-doping configurations of n-type (nitrogen) and p-type (boron) dopants, the stable structure of a boron atom in diamond is associated with four nitrogen atoms placed at the nearest neighbour positions, the total energy of the system with the stable structure is 136 MeV lower than that of the system with the nitrogen atoms placed in others positions. The results indicated that the co-dopants of nitrogen and boron were the perfect candidates to make n-type diamond, and additional boron would increase the solubility limit of nitrogen in diamond, reduce the lattice-relaxation energy of crystal and improve its doping efficiency in diamond.     

Metastable extensions of phase equilibrium lines and singular points of simple substance

The thermodynamic properties of crystal, liquid, and gas in the stable and metastable states have been determined by molecular dynamics simulation of a system of 2048 Lennard-Jones particles. The spinodals of a superheated crystal, a superheated liquid, and a supersaturated vapor have been approximated; the spinodal for a supercooled liquid turns out to be nonexistent. The liquid-vapor, liquid-crystal, and crystal-vapor equilibrium curves and their extensions beyond the triple point have been calculated. It has been shown that, as distinct from the metastable extension of the saturation curve, which terminates at the zero isotherm, the metastable melting and sublimation curves terminate at, respectively, the stretched liquid and superheated crystal spinodals. The properties of the critical end points of metastable equilibrium of extended phases are considered.     

Ib型金刚石大单晶的限形生长

金刚石的限形生长有利于其后续加工.对于磨料级金刚石限形生长的研究已经比较透彻,但金刚石大单晶的限形生长尚缺乏全面系统的研究.本文以Fe Ni(64wt%:36wt%)合金为触媒,利用高温高压下的温度梯度法在5.6 GPa时对不同温度下分别沿(100)面和(111)面生长的Ib型金刚石大单晶的晶形进行了研究.研究表明:随着温度的升高,沿(100)晶面生长的金刚石大单晶的晶形分别为板状、塔状直至尖塔状,而沿(111)面生长的金刚石大单晶的晶形则分别为塔状和板状;分析了不同温度下分别沿(100)面和(111)面生长金刚石大单晶不同晶形高径比的变化情况.利用不同压力和温度下的金刚石大单晶合成实验绘制了沿(100)和(111)面生长金刚石大单晶的晶形在V形生长区域内的分布示意图,表明沿(111)面生长的金刚石大单晶V形区温度下限明显比以(100)面生长的高,而沿这两面生长金刚石大单晶的V形区温度上限差别并不明显.对不同生长面V形区温度上下限的差别进行了解释,据此实现了Ib型金刚石大单晶的限形生长.     

Singular point of a system of Lennard-Jones particles at negative pressures

The method of molecular dynamics is used in a system of 2048 Lennard-Jones particles to determine the spinodal of a stretched liquid and crystal and the lines of their phase equilibrium at negative pressures. It is shown that a metastable extension of the melting line does not reach the zero isotherm, and ends on the spinodal of a stretched liquid. The point of termination of metastable liquid-crystal phase equilibrium is the singular point at a thermodynamic surface of states.     

Study of partial melting at high-pressure using in situ X-ray diffraction

The high-pressure melting behavior of different iron alloys was investigated using the classical synchrotron-based in situ X-ray diffraction techniques. As they offer specific advantages and disadvantages, both energy-dispersive (EDX) and angle-dispersive (ADX) X-ray diffraction methods were performed at the BL04B1 beamline of SPring8 (Japan) and at the ID27-30 beamline of the ESRF (France), respectively. High-pressure vessels and pressure ranges investigated include the Paris–Edinburgh press from 2 to 17 GPa, the SPEED-1500 multi-anvil press from 10 to 27 GPa, and the laser-heated diamond anvil cell from 15 to 60 GPa. The onset of melting (at the solidus or eutectic temperature) can be easily detected using EDX because the grains start to rotate relative to the X-ray beam, which provokes rapid and drastic changes with time of the peak growth rate. Then, the degree of melting can be determined, using both EDX and ADX, from the intensity of diffuse X-ray scattering characteristic of the liquid phase. This diffuse contribution can be easily differentiated from the Compton diffusion of the pressure medium because they have different shapes in the diffraction patterns. Information about the composition and/or about the structure of the liquid phase can then be extracted from the shape of the diffuse X-ray scattering.     

B2O3添加宝石级金刚石单晶的生长特性

利用温度梯度法, 在5.3-5.7 GPa压力、1200-1600 ℃的温度条件下, 将B₂O₃粉添加到FeNiMnCo+C合成体系内, 进行B₂O₃添加宝石级金刚石单晶的合成. 研究得到了FeNiMnCo触媒生长B₂O₃添加宝石级金刚石单晶的相图分布规律. 结果表明B₂O₃添加会使晶体生长的“V”形区上移和低温六面体单晶生长区间变宽. 通过晶体生长实验, 研究合成了不同形貌的B₂O₃添加宝石级金刚石单晶. 研究同时证实, B₂O₃的过量添加会对宝石级金刚石单晶生长带来不利影响. 当B₂O₃的添加量高于约3 wt‰、生长时间超过20 h时, 很难实现优质B₂O₃添加宝石级金刚石单晶的生长. 但B₂O₃的适量添加(不超过1 wt‰), 有助于提高低温板状六面体宝石级金刚石单晶的成品率. 通过对晶体生长速度的研究发现, B₂O₃的添加使得优质晶体的生长速度明显降低, 随着晶体生长时间的延长, B₂O₃添加剂对晶体生长的抑制作用会越发明显. 扫描电镜测试结果表明, 合成体系内B₂O₃添加剂的引入, 导致晶体表面的平整度明显下降.     

Shock compressing diamond to a conducting fluid

Laser generated shock reflectance data show that diamond undergoes a continuous transition from optically absorbing to reflecting between Hugoniot pressures 600相似文献   

First-principles study on the mechanics,optical, and phonon properties of carbon chains

Besides graphite, diamond, graphene, carbon nanotubes, and fullerenes, there is another allotrope of carbon, carbyne,existing in the form of a one-dimensional chain of carbon atoms. It has been theoretically predicted that carbyne would be stronger, stiffer, and more exotic than other materials that have been synthesized before. In this article, two kinds of carbyne, i.e., cumulene and polyyne are investigated by the first principles, where the mechanical properties, electronic structure, optical and phonon properties of the carbynes are calculated. The results on the crystal binding energy and the formation energy show that though both are difficult to be synthesized from diamond or graphite, polyyne is more stable and harder than cummulene. The tensile stiffness, bond stiffness, and Young's modulus of cumulene are 94.669 eV/?A,90.334 GPa, and 60.62 GPa, respectively, while the corresponding values of polyyne are 94.939 eV/?A, 101.42 GPa, and60.06 GPa. The supercell calculation shows that carbyne is most stable at N = 5, where N is the supercell number, which indicates that the carbon chain with 10 atoms is most stable. The calculation on the electronic band structure shows that cumulene is a conductor and polyyne is a semiconductor with a band gap of 0.37 eV. The dielectric function of carbynes varies along different directions, consistent with the one-dimensional nature of the carbon chains. In the phonon dispersion of cumulene, there are imaginary frequencies with the lowest value down to-3.817 THz, which indicates that cumulene could be unstable at room temperature and normal pressure.     

Mo高压熔化的分子动力学模拟

 采用第一原理方法计算了钼在零温下的结构,表明钼在500 GPa以下一直保持bcc结构（常温）,与实验一致。在零压附近计算了E-V关系,利用Murnaghan物态方程拟合得到了零压体积及其模量,与实验结果符合得很好。采用第一原理分子动力学模拟了钼的高压熔化性质。采用NVT系综计算了128个原子的系统,初始构形为bcc结构,体积分别为0.015 48、0.012 19、0.010 98、0.009 84、0.009 10 nm³/atom,计算了几个温度点,拟合得到了熔化曲线,熔化温度明显高于金刚石压砧（DAC）实验结果;将初始构形改变为fcc结构,模拟其熔化特性,得到的熔化温度明显下降,与激光加载DAC实验结果一致,认为可能的原因是钼熔化后形成的液体结构类似于fcc结构,而不是常态时的bcc结构。     

Formation of diamond powders from melamine under high pressure and high temperature

High pressure pyrolysis of melamine has been attracting great interest recently, due to it being considered as a suitable precursor to realize the g-C₃N₄ and even superhard C₃N₄. In this work, we studied the detailed pyrolysis behavior of melamine at 22 GPa. Melamine was stable at 800 ℃, and decomposed to diamond in the form of powder at 1500-2000 ℃ under this pressure condition. At 2000 ℃, the pure cubic diamond powders with 0.1-0.5 μm grain size were obtained. The diamond particles exhibited euhedral forms and dispersed to each other, we proposed that these novel features were caused by the presence of liquid N₂ and NH₃ during diamond formation. The high pressure pyrolysis of melamine may provide a new means of producing micrometer-sized diamond powders.     

Modeling the phase diagram of carbon

We determined the phase diagram involving diamond, graphite, and liquid carbon using a recently developed semiempirical potential. Using accurate free-energy calculations, we computed the solid-solid and solid-liquid phase boundaries for pressures and temperatures up to 400 GPa and 12 000 K, respectively. The graphite-diamond transition line that we computed is in good agreement with experimental data, confirming the accuracy of the employed empirical potential. On the basis of the computed slope of the graphite melting line, we rule out the hotly debated liquid-liquid phase transition of carbon. Our simulations allow us to give accurate estimates of the location of the diamond melting curve and of the graphite-diamond-liquid triple point.     

纳米晶Si在高压下的电学性质与金属化相变

 在金刚石压砧装置上,采用电阻和电容测量方法,研究了粒径为15~18 nm和80 nm的纳米晶Si在室温下、24 GPa内的电阻、电容与压力的关系。实验结果表明,它们分别在19~17 GPa和14 GPa左右发生了金属化相变。     

Dynamic ionization of water under extreme conditions

Raman spectroscopy in a laser heated diamond anvil cell and first principles molecular dynamics simulations have been used to study water in the temperature range 300 to 1500 K and at pressures to 56 GPa. We find a substantial decrease in the intensity of the O-H stretch mode in the liquid phase with pressure, and a change in slope of the melting line at 47 GPa and 1000 K. Consistent with these observations, theoretical calculations show that water beyond 50 GPa is "dynamically ionized" in that it consists of very short-lived (<10 fs) H2O, H3O+, and OH- species, and also that the mobility of the oxygen ions decreases abruptly with pressure, while hydrogen ions remain very mobile. We suggest that this regime corresponds to a superionic state.