BeO高压相变的从头算分子动力学模拟

摘要利用从头算分子动力学方法研究了BeO的高压诱导相变，发现，在360GPa压强下纤锌矿（wz）结构的BeO平稳地相变为岩盐（Rs）结构，且没有经过闪锌矿（zB）结构。相变机制包含模拟胞的单斜变形、沿[2110]方向的原子膨胀以及沿[0110]和[0001]方向的原子收缩。比较研究了闪锌矿（zB）结构BeO的高压诱导相...     

高压强下GaN的结构相变及电子结构的第一性原理研究

基于密度泛函理论的平面波赝势方法,选择广义梯度近似(GGA)下的PBE算法-关联泛函对GaN晶体结构、能带结构以及电子态密度随压强的变化进行了研究,并计算出GaN材料的相变点压强值。研究结果表明:随着压强增加,常见的纤锌矿与闪锌矿GaN会发生结构相变成岩盐矿结构,并且其能带结构均由直接带隙转变成间接带隙。其中,通过焓相等原理得到纤锌矿到岩盐矿结构的相变压强为44.4GPa,而闪锌矿到岩盐矿结构的相变压强为43.6GPa。此外,随着压强增大,GaN纤锌矿、闪锌矿和岩盐矿的价带态密度均向低能方向偏移,而导带态密度向高能方向偏移,从而导致GaN共价性增强及带隙随压强增大而展宽。     

碳酸钡在高温及高压下的相变行为

为了研究BaCO_3在高温及高压下的相变行为,利用差示扫描量热法观察了BaCO_3在常压下从室温到1 200℃的相变过程,之后结合拉曼光谱技术研究了BaCO_3从常温常压到15. 2 GPa压强的相变过程。由差热分析可知,BaCO_3在808℃和955℃处存在两个吸热峰,分别对应斜方晶相到三方晶相和三方晶相到立方晶相的转变。当压强从常压升高到10. 6 GPa时,BaCO_3的各个模向高波数段移动,且未发生相变;其斜方晶系结构(Pmcn)到三方晶系结构(P31c)的相变发生在10. 6 GPa附近;而高于10. 6 GPa时也未观察到相变。从释放压强之后的BaCO_3的拉曼光谱图上发现,[CO_3]基团对称伸缩振动模v1与其初始相(斜方晶系结构)不对应,而对应其高压相(三方晶系结构)。     

分子动力学方法模拟压强对Bi4Ti3O12铁电相变行为的影响

在壳模型的基础上, 通过分子动力学方法模拟了压强对Bi₄Ti₃O₁₂(BIT)铁电相变行为的影响. 为了提高模拟的准确性, 在原有势参数的基础上增加了Ti-Ti短程相互作用势. 计算得出了温度为300K时BIT单晶的铁电正交B2cb相在x方向和z方向的自发极化强度分别为39.4μC/cm²和0, 与实验结果较好的吻合. 然后模拟了压强对BIT相变行为的影响. 模拟结果表明: BIT单晶在压强从-2 GPa到24 GPa范围内, 经历了两次结构相变, 分别发生在 6 GPa和20 GPa处. 这种对称性的改变类似于在环境压力条件下温度导致BIT单晶对称性的改变. 因而模拟结果为研究压强引起BIT的相变行为提供了理论依据.     

TiN振动和热力学性质的第一性原理研究

基于平面波赝势的第一性原理方法,对TiN的B1(NaCl)和B2(CsCl)以及B3(闪锌矿)复合结构的力学性能以及焓与压力之间的关系进行了研究,并讨论了它们的相对稳定性.通过研究分析可知,TiN在347 GPa左右时会发生从B1到B2的结构相变,在-17.5 GPa左右时会发生从B1到B3的结构相变.由声子频率对压强的依赖关系可知频率和带隙随着压力的增加而增加.另外高温区TiN的热膨胀系数受温度的影响较小.     

碳纳米管中的准高压效应及其应用

碳纳米管是具有中空管结构的新型纳米碳素材料,其直径一般在1nm至数10nm之间。详细论证了碳纳米管中存在的准高压效应,研究表明该压强超过了1.9GPa,而在电子辐射的作用下更可能高达40GPa,可以起到诱导填充物相变与化学反应以及调控自身和填充物电子状态的作用。同时,还介绍了该准高压效应在储气等方面的应用,并指出目前存在的问题以及展望了未来的发展。     

Ce3Th合金高压相变的第一性原理研究

为探究Ce_3Th合金在高压下的相变规律及物理性质,本文利用基于密度泛函理论的虚晶近似(VCA)对Ce_3Th进行了系统的理论研究。通过对计算结果的分析,发现了Ce_3Th合金在压力下的相变规律。当压强增加到25 GPa附近,轴向比c/a突然从■跃迁到1.68左右,fcc结构开始向bct结构转变。同时,通过计算弹性性质进一步说明了Ce_3Th的相变,发现得到的弹性常数、弹性模量、弹性德拜温度和声速的线性关系在相变压力附近有"软化"的趋势,这也从另一角度反映了结构相变的信息。最后,根据准谐德拜模型,预测了在0～90 GPa的压力范围和0～1 200 K的温度范围内的热容(C_V)和热膨胀系数α等热力学性质,Ce_3Th在0 GPa, 300 K时的热膨胀系数α约为4.27×10~(-5) K~(-1)。     

金属钛拉伸的分子动力学研究

本文利用分子动力学的研究方法,采用了钛的嵌入式原子势,建立了沿[0001]晶向和[0110]方向拉伸模型,结果表明:两种方向的拉伸均包含弹性变形阶段、屈服阶段、颈缩阶段、断裂阶段.沿[0001]方向拉伸时,滑移系少,取向偏离软取向.变形时屈服强度为3.55GPa,屈服应变为0.063,断裂时的应变达到0.55.沿[01...     

Al纳米晶在Fe表面熔化行为的分子动力学模拟

本文应用分子动力学(MD)方法,采用嵌入势模型,在Al纳米晶熔点以上、Fe熔点以下的温度范围内,对Al原子在Fe(001)、(110)和(111)面上的扩散现象进行了系统研究。结果发现,在模拟时间内Al原子的扩散主要发生在Al和Fe直接接触的第一原子层上,且大部分Al原子沿着Fe表面即x-y平面扩散,仅有极少数Al原子沿着z方向向下扩散。Al原子在不同Fe表面上的主要扩散通道并不相同:在Fe(001)面上Al原子沿[110]和[1-10]的扩散几率大致相同;在Fe(110)面上Al原子主要在Fe表面沿[001]方向扩散;在Fe(111)面上Al原子沿[1-10]、[1-01]和[01-1]的扩散几率大致相同。     

外磁场对Ni-Mn-Ga磁性形状记忆合金相变应变及显微组织的影响

研究了外磁场对Ni52Mn24.6Ga23.4(%,原子分数)单晶马氏体相变及其相变应变的影响,并对磁场增强相变应变的微观机制进行了探讨。研究结果表明无外加磁场时,NiMnGa合金发生马氏体相变时可产生约0.3%的收缩形变,沿单晶[100]方向施加外磁场,其相变应变随磁场的增加而呈近线性增加。当外磁场强度为6.37×105A/m时,应变量达到最大值(3.5%)。磁场作用下冷却形成的马氏体虽然孪晶亚结构不变,但自协作组态消失,并伴随有孪晶板条的增厚。磁场对马氏体相变应变的增强效应来自于磁场作用下的马氏体变体的择优取向。     

Ab initio molecular dynamics simulation of pressure-induced phase transformation in BeO

Ab initio molecular dynamics (MD) method has been used to study high pressure-induced phase transformation in BeO based on the local density approximation (LDA) and the generalized gradient approximation (GGA). Both methods show that the wurtzite (WZ) and zinc blende (ZB) BeO transforms to the rocksalt (RS) structure smoothly at high pressure. The transition pressures obtained from the LDA method are about 40 GPa larger than the GGA result for both WZ → RS and ZB → RS phase transformations, and the phase transformation mechanisms revealed by the LDA and GGA methods are different. For WZ → RS phase transformations both mechanisms obtained from the LDA and GGA methods are not comparable to the previous ab initio MD simulations of WZ BeO at 700 GPa based on the GGA method. It is suggested that the phase transformation mechanisms of BeO revealed by the ab initio MD simulations are affected remarkably by the exchange–correlation functional employed and the way of applying pressure.     

Electronic structure and properties of beryllium oxide

This review focuses on computer simulation studies of the nature of chemical bonding in BeO and its electronic structure and physicochemical properties. The capabilities of modern quantum-chemical methods are analyzed with application to various structural defects in BeO, phase equilibria in the Be-O system, pressure-induced polymorphic transformations of BeO, and its mechanical, thermal, and spectroscopic properties.     

High pressure phase transitions for CdSe

The structure and pressure-induced phase transitions for CdSe are investigated using first-principles calculations. The pressure-induced phase transition sequence WZ/ZB → Rs → Cmcm → CsCl for CdSe is drawn reasonably for the fist time, the corresponding transition pressures are 3.8, 29 and 107 GPa, respectively and the intermediate states between the Cmcm structure and the CsCl structure should exist.     

First-Principles Studies on Magnetic Stability of SrC and BaC in Rocksalt, Zincblende, and Nickel Arsenide Phases Under Pressure

Using the first-principles method based on the density functional theory, we investigated the magnetic stability of sp half-metal ferromagnets SrC and BaC in rocksalt (RS), zincblende (ZB), and nickel arsenide (NA) structures under external pressure. The magnetic moments, total energy of magnetic and nonmagnetic phases, and lattice constants are calculated as a function of the applied pressure. The calculations show the occurrence of pressure-induced magnetic phase transitions which are mainly resulted from the band widening of anion p states. It is also confirmed that for both SrC and BaC, the rocksalt structure is the most stable phase among the three phases.     

The effect of the phase transition on the elasticity of cubic platinum carbide

A study of the high-pressure elastic properties of ideal stoichiometric platinum carbide (PtC) in the rock-salt (RS) and zinc-blende (ZB) structures was conducted using first-principles calculations based on density functional theory, in which we employ the generalized gradient approximation of the Perdew–Burke–Eruzerhof form together with plane-wave basis sets for expanding the crystal orbitals and periodic electron density. Our calculation shows that the recently synthesized compound PtC possess a high-bulk modulus value in the RS phase and the ZB phase is more stable. The investigation of the elastic stability under pressure indicated that the transition pressure from ZB to RS structure of PtC is about 30 GPa and the high-pressure RS phase is stable up to 100 GPa. Our conclusions are consistent with the other theoretical predictions but are reversed with the diamond anvil cell experimental results. Therefore, the experimental observation of the RS structure in PtC remains a puzzle and our study indicates that more experimental and theoretical works need to be performed to ascertain the true nature of the newly discovered PtC material. In addition, the pressure dependence of the bulk modulus K, the shear modulus G, the Young’s modulus E, the Poisson’s ratio υ, the Debye temperature Θ _D, the compressional wave velocity V _p, the shear wave velocity V _s, and the elastic anisotropy factor A for the ZB and RS structures of PtC are all successfully obtained. Moreover, the pressure dependence of the longitudinal and the shear wave velocities in three directions [100], [110], and [111] for cubic PtC are also predicted for the first time.     

A new type face-centered cubic zirconium phase in pure zirconium

So far,only two orientation relationships (OR) between hexagonal close-packed (HCP) (α phase) and face-centered cubic (FCC) structures in zirconium and titanium alloys have been reported.Here a new type FCC phase (named γphase) with OR:< 11(2)0 >αll< 100 >γ and {0001}αll{002}γ was observed for the first time in annealed pure zirconium by means of transmission electron microscopy (TEM) technique.The α→γphase transformation can be accomplished via expansion along[1(1)00]direction and slip of Shockley partial dislocation with 1/3[1(1)00]on (0001) basal planes.     

Temperature stability of magnetic field-induced strain and field-controlled shape memory effect on Ni/sub 52/Mn/sub 16.4/Fe/sub 8/Ga/sub 23.6/ single crystals

The temperature dependence of the magnetic field-induced strain (MFIS) and the field-controlled shape memory effect in Ni/sub 52/Mn/sub 16.4/Fe/sub 8/Ga/sub 23.6/ single crystals were investigated by measuring the MFIS and measuring the magnetic field-enhanced transformation strain with a field bias applied in the [001] and [010] directions of the parent phase, respectively. The results show that such material combined with the martensitic transformation can product large field-enhanced transformation strain and large MFIS. The strain accompanying the martensitic transformation is -1.61% in zero field and can be enhanced to -3.30% by a field of 960 kA/m. A MFIS of 1.04% has been induced along [001] in unstressed crystals with saturated magnetic field of 600 kA/m applied along the same direction at near martensitic transformation temperature. It was found that the MFIS is almost temperature independent; the maximum decrease of the saturated MFIS is less than 10%, from 265 K to 100 K. This well-behaved temperature response makes this alloy particularly valuable for industrial and military smart actuators and transducers. Furthermore, it was found that the direction in which the MFIS has the largest value is always the [001], namely, the growth direction of the crystals.     

Zinc blende and wurtzite crystal phase mixing and transition in indium phosphide nanowires

Indium phosphide (InP) nanowires, which have crystal phase mixing and transition from zinc blende (ZB) to wurtzite (WZ), are grown in intermediate growth conditions between ZB and WZ by using selective-area metalorganic vapor phase epitaxy (SA-MOVPE). The shape of InP nanowires is tapered unlike ZB or WZ nanowires. A growth model has been developed for the tapered nanowires, which is simply described as the relationship between tapered angle and the ratio of ZB and WZ segments. In addition, the peak energy shift in photoluminescence measurement was attributed to the quantum confinement effect of the quantum well of the ZB region located in the polytypic structure of ZB and WZ in nanowires.     

Zinc blende GaAsSb nanowires grown by molecular beam epitaxy

We report the growth of GaAsSb nanowires (NWs) on GaAs(111)B substrates by Au-assisted molecular beam epitaxy. The structural characteristics of the GaAsSb NWs have been investigated in detail. Their Sb mole fraction was found to be about?25%. Their crystal structure was found to be pure zinc blende (ZB), in contrast to the wurtzite structure observed in GaAs NWs grown under similar conditions. The ZB GaAsSb NWs exhibit rotational twins around their [111]B growth axis, with twin-free segments as long as 500?nm. The total volumes of GaAsSb segments with twinned and un-twinned orientations, respectively, were found to be equal by x-ray diffraction analysis of NW ensembles.     

Stress-induced wurtzite to hexagonal phase transformation in zinc oxide nanowires

The stress-induced wurtzite to hexagonal phase transformation in [0110] oriented zinc oxide nanowires were investigated using a molecular dynamics simulation and reactive force field potentials. The yield strength of the 2.13 x 1.93 nm wurtzite nanowires is 12 GPa at 50 K. The wurtzite to hexagonal phase transformation was successfully observed at stress plateaus (5-5.5 GPa at 50 K) located after the yield point of the wurtzite phase. The wurtzite to hexagonal phase transformation was a result of the propagation of {0111} twinning boundaries. During the phase transformation, the wurtzite and hexagonal phases were clearly separated by the {0111} twinning boundaries. To analyze the difference between ceramic and metallic systems, all the calculation data of wurtzite to hexagonal transformation were compared with stress-induced phase transformation in metallic nanowires such as CuZr and NiA1. As the result of the [0110] tensile loading of the ZnO nanowires, the hexagonal phase was obtained.