第一性原理计算Fe从bcc到hcp结构的相变路径及其磁性边界

本文采用基于密度泛函理论的第一性原理方法,计算了压力作用下Fe从bcc到hcp结构相变的势能面、相变路径以及相变过程中的磁性相边界.结果表明:与Burgers路径不同,相变过程中bcc结构(110)bcc面的剪切和相对滑移相互耦合,并伴随有(110)bcc面间距的减小;这一相变机制可以解释Fe的静高压实验中在相变初期观察到的hcp结构异常.因此,并不需要像Wang和Ingalls提出的那样,在相变过程中引入一个亚稳定的fcc相来解释这些实验结果.对相变势能面的计算表明剪切对相变的发生有激活作用.此外,分析表明相变过程中涉及复杂的磁性转变,相变过渡态位置正好位于磁性相边界上,并对原子磁性对结构转变影响的物理机制进行了讨论.     

单轴应变条件下Fe从α到ε结构相变机制的第一性原理计算

采用基于密度泛函理论的平面波赝势方法,研究了沿［001］方向单轴应变条件下Fe从体心立方结构(bcc,α相)到六角密排结构(hcp,ε相)相变的临界压力、相变路径、相变势垒以及相变过程中原子磁性的变化.结果发现:单轴应变条件下Fe从α到ε结构的相变路径与以前理论计算模拟给出的静水压力条件下的相变路径明显不同;原子磁矩沿着相变路径突然降低,同时伴随着能量和体积的突然变化,是典型的一阶磁性相转变,表明原子磁性的丧失导致了bcc结构不稳定而向hcp结构转变.对单轴应变下吉布斯自由能的计算表明,相变势垒随着单轴应 关键词： 相变 单轴应变 第一性原理 铁     

非静水压条件下铁从α到ε结构相变的第一性原理计算

采用基于密度泛函理论的平面波赝势方法,研究了三轴加载的非静水压力和静水压力对铁从体心立方结构(bcc,α相)到六角密排结构(hcp,ε相)相变压力和磁性的影响,结果发现:在0-18 GPa压力范围内,相对静水压力条件,随着压力的升高,bcc结构的原子磁矩在非静水压力下降低得更快;在非静水压力下,相变更容易发生,相变压力随着非静水压力程度的增加而降低;并且对非静水压力对相变压力影响的物理机理进行了讨论.     

非静水压条件下铁从α到ε结构相变的第一性原理计算

采用基于密度泛函理论的平面波赝势方法,研究了三轴加载的非静水压力和静水压力对铁从体心立方结构（bcc,α相）到六角密排结构（hcp,ε相）相变压力和磁性的影响,结果发现：在0—18 GPa压力范围内,相对静水压力条件,随着压力的升高,bcc结构的原子磁矩在非静水压力下降低得更快;在非静水压力下,相变更容易发生,相变压力随着非静水压力程度的增加而降低;并且对非静水压力对相变压力影响的物理机理进行了讨论. 关键词： 相变 非静水压力 第一性原理 铁     

高温高压下过渡金属Ru的结构相变

采用基于密度泛函理论的第一性原理和准简谐晶格动力学方法对Ru的六角密排 (hcp)、面心立方 (fcc)、体心四方 (bct) 和体心立方 (bcc) 结构的磁性、晶格结构稳定性和高温高压下的相变进行了系统的研究. 计算获得了各相结构的磁性基态及其稳定性范围, 结果表明: 零温下在计算的压力范围内, NM-hcp 结构是Ru最稳定的结构, 压力的单独作用下并没有相变的发生; NM-fcc结构是Ru的亚稳定结构, 而NM-bcc和FM-bct结构在动力学上并不稳定. 高温高压下Ru将发生从NM-hcp到NM-fcc结构的相变, 并给出了Ru的温度压力相图. 关键词： 相变 晶格稳定性 磁性 第一性原理     

金属Li相稳定性的第一性原理研究

Li在常温常压下为体心立方结构(bcc),随着压力和温度的变化会发生结构转变.本文应用第一性原理方法研究了Li的9R,fcc,hcp和bcc四种不同结构相在基态和有限温度下的相对稳定性.计算表明Li在低温时的最稳定相为六角堆垛的9R相,而且随着温度的变化会发生结构相变,最终在高温时(370K)转变为bcc相.     

单轴应变驱动铁bcc—hcp相转变的微观模拟

用分子动力学方法模拟了沿〈001〉晶向应变加载和卸载情况下单晶铁中体心立方(bcc)与六方密排(hcp)结构的相互转变,分析了相变的可逆性和微结构演化特征.微观应力的变化显示样品具有超弹性性质,而温度变化表明在相变和逆相变过程中均出现放热现象.相变起始于爆发式均匀形核,晶核由块状颗粒迅速生长为沿{011}晶面的片状分层结构; 而卸载逆相变则从形核开始就呈现片状形态,且相界面晶面指数与加载相变完全一致,表现出形态记忆效应.在两hcp晶核生长的交界面易形成面心立方(fcc)堆垛层错. fcc通过在hcp晶粒内     

纳米多晶铁的冲击相变研究

利用分子动力学方法研究了不同晶粒度的纳米多晶铁在冲击压缩下的结构相变过程,模拟结果表明:纳米多晶铁的冲击结构相变(由体心立方(bcc)结构 α 相到六角密排(hcp)结构 ε 相)发生的临界冲击应力在15 GPa左右.纳米多晶铁在经过弹性压缩变形后,晶界导致的塑性变形开始发生,然后大多数相变从晶界成核并最终发展为大规模相变.不同变形过程在应力和粒子速度剖面上能得到清晰的体现,并通过微观原子结构分析分辨.冲击压缩后的微观结构以晶界原子和以fcc结构原子充当孪晶界的hcp原子为主.晶粒度明显影响晶界变形及相变 关键词： 冲击相变 纳米多晶铁 冲击波 分子动力学     

Fe1-xPdx合金电子结构和磁性的理论研究

Fe1-xPdx合金的磁性强烈地依赖于其结构以及Pd的相对含量.从第一性原理出发，用线性缀加平面波(LAPW)方法，分别计算了x=000，025，050，075，100的情况下，面心立方(fcc)和体心立方(bcc)结构的Fe1-xPdx合金的电子结构和基态磁性.随x的增大，fcc结构的Fe1-xPdx合金的磁性从铁磁性或者反铁磁性变为亚铁磁性，再从亚铁磁性变为铁磁性和顺磁性；bcc结构的Fe1-xPdx合金从铁磁性减弱到顺磁性，预言了fcc结构的Fe1-xPdx合金可能存在亚铁磁相.并较好地解 关键词： 合金 电子结构 磁性     

第一性原理对金的高压相变和零温物态方程的计算

在密度泛函理论下，用缀加平面波加局域轨道方法，分别采用广义梯度近似(GGA)和局域密度近似(LDA)对金的面心立方晶格结构(fcc)、体心立方晶格结构(bcc)和六角密堆积结构(hcp)的结构能量进行了计算.在GGA下，计算得出fcc向hcp和hcp向bcc的相变分别发生在380GPa 和1250GPa；而LDA下相变分别发生在490GPa和790GPa.当计算压强达到2TPa时，bcc在这两种近似下仍然保持稳定的结构.根据不同体积下不同结构的电子态密度的特征，对发生相变的物理原因进行了定性的分析，在此基础上得到了金的零温状态方程. 关键词： 缀加平面波方法 固态相变 电子态密度 物态方程     

Study of theoretical tensile strength of Fe by afirst-principles computational tensile test

This paper employs a first-principles total-energy method to investigate the theoretical tensile strengths of bcc and fcc Fe systemically. It indicates that the theoretical tensile strengths are shown to be 12.4, 32.7, 27.5~GPa for bcc Fe, and 48.1, 34.6, 51.2~GPa for fcc Fe in the [001], [110] and [111] directions, respectively. For bcc Fe, the [001] direction is shown to be the weakest direction due to the occurrence of a phase transition from ferromagnetic bcc Fe to high spin ferromagnetic fcc Fe. For fcc Fe, the [110] direction is the weakest direction due to the formation of an instable saddle-point `bct structure' in the tensile process. Furthermore, it demonstrates that a magnetic instability will occur under a tensile strain of 14%, characterized by the transition of ferromagnetic bcc Fe to paramagnetic fcc Fe. The results provide a good reference to understand the intrinsic mechanical properties of Fe as a potential structural material in the nuclear fusion Tokamak.     

Effect of pressure on itinerant magnetism and spin disorder in cubic FeGe

The results of ab initio calculations of the pressure dependence of Fe magnetism in cubic FeGe are presented. We find that when the pressure-volume scale is set by means of generalized gradient approximation total energies and magnetism is described by means of the local density approximation, the critical pressure at which the magnetic phase transition occurs is estimated at ≈18 GPa, which is in good agreement with experiments. Using the disordered local moment method we find a localized to itinerant model cross-over of Fe magnetism in cubic FeGe, as a function of volume. Moreover, our calculations also suggest subtle signatures of longitudinal spin fluctuations in cubic FeGe, and that the stiffness parameter softens with increasing pressure. We associate the retention of metallicity in FeGe under pressure with the spin-disorder scattering. The effect of spin-orbit coupling on the electronic structure is also discussed.     

Fe-Al系统磁性及其压力效应

采用紧束缚线性muffin-tin轨道方法,研究了Fe-Al系统的磁性与Al组分的关系及其压力效应. bcc Fe-Al合金的铁磁磁矩随Al组分增大而单调减小,且会出现Al组分引起的铁磁—顺磁相变. bcc Fe-Al合金的铁磁磁矩随体积压缩单调减小,但与γ-Fe及某些fcc铁基合金不同的是bcc Fe-Al合金不会出现压力诱导的铁磁—顺磁相变.计算结果与实验符合. 关键词：     

Electronic structure, magnetism and superconductivity of layered iron compounds

The layered iron superconductors are discussed using electronic structure calculations. The four families of compounds discovered so far, including Fe (Se, Te) have closely related electronic structures. The Fermi surface consists of disconnected hole and electron cylinders and additional hole sections that depend on the specific material. This places the materials in proximity to itinerant magnetism, both due to the high density of states and due to nesting. Comparison of density functional results and experiment provides strong evidence for itinerant spin fluctuations, which are discussed in relation to superconductivity. It is proposed that the intermediate phase between the structural transition and the SDW transition in the oxy-pnictides is a nematic phase.     

STUDY OF IRON OVERLAYER ON SULPHUR PASSIVATED GaAs(100) BY SYNCHROTRON RADIATION PHOTOEMISSION

We have studied the interface electronic structures and the chemical reaction of the Fe overlayer deposited on S-passivated GaAs(100). The chemical bond and electronic structure are different from Fe/GaAs, and the reaction between As and Fe is weakened by S atoms. This is beneficial to the magnetism in the interface. In the first stage of deposition, Fe clusters is form near S atoms due to the large electronegativity of S. The S atoms remain at the interface with Fe coverage. Magnetic ordering feature is found at a coverage higher than 0.6 nm. According to the large exchange splitting in valence band spectra, we suggest that Fe phase transition from bcc to fcc occurs with increasing coverage.     

Phase transitions in cerium at high pressures (up to 15 GPa) and high temperatures

Phase transitions in cerium have been studied by the electrical resistance method in the 15-GPa pressure range at high temperatures. At pressures above 10 GPa, cerium represents a mixture of stable and metastable phases, the composition of this mixture being dependent on the trajectory in the P-T plane that leads to a given point. Transformations in both stable and metastable components of the mixture proceeding rather independently display a complicated picture of phase transitions. It was assumed that only the α (fcc) and α′ (α-U) phases are stable at pressures above the well-known γ-α transition, the other phases being metastable. The proposed bow-shaped equilibrium phase diagram includes an extremely wide hysteresis region, where stable and metastable phases can coexist. The fcc α phase alone survives upon heating above 50°C at 15 GPa.     

Martensitic fcc-to-hcp transformation observed in xenon at high pressure

Angle-resolved x-ray diffraction patterns of Xe to 127 GPa indicate that the fcc-to-hcp transition occurs martensitically between 3 and 70 GPa in diamond-anvil cells without an intermediate phase. These data also reveal that the transition occurs by the introduction of stacking disorder in the fcc lattice at low pressure, which grows into hcp domains with increasing pressure. The small energy difference between the hcp and the fcc structures may allow the two phases to coexist over a wide pressure range. Evidence of similar stacking disorder and incipient growth of an hcp phase are also observed in solid Kr.