第一原理对铝的静态结构和相变的计算

 在密度泛函理论（DFT）和广义梯度近似（GGA）下,用缀加平面波加局域轨道（APW+lo）方法对铝的晶格常数、体弹模量以及在静态高压下的固态相变进行了计算。计算得出面心立方晶格结构（fcc）向六角密堆积结构（hcp）和体心立方结构（bcc）的相变分别发生在220 GPa和330 GPa,hcp向体心结构bcc的相变发生在380 GPa。计算结果和实验数据以及其它理论计算符合较好。     

Evidence of polymorphic transformations of Sn under high pressure

The high-pressure polymorphs and structural transformation of Sn were experimentally investigated using angledispersive synchrotron x-ray diffraction up to 108.9 GPa. The results show that at least at 12.8 GPa β-Sn→bct structure transformation was completed and no two-phase coexistence was found. By using a long-wavelength x-ray, we resolved the diffraction peaks splitting and discovered the formation of a new distorted orthorhombic structure bco from the bct structure at 31.8 GPa. The variation of the lattice parameters and their ratios with pressure further validate the observation of the bco polymorph. The bcc structure appears at 40.9 GPa and coexists with the bco phase throughout a wide pressure range of40.9 GPa–73.1 GPa. Above 73.1 GPa, only the bcc polymorph is observed. The systematically experimental investigation confirms the phase transition sequence of Sn as β-Sn→bct→bco→ bco + bcc→bcc upon compression to 108.9 GPa at room temperature.     

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     

Phase Transition of Graphitic-C3N4 under High Pressure by In Situ Resistance Measurement in a Diamond Anvil Cell

In situ resistance measurement of Graphitic-C3N4 has been performed under high pressure in a diamond anvil cell. The result reveals that there are changes of electron transport behaviour. As the pressure increases from ambient to 30 GPa, three abnormal resistance changes can be found at room temperature and two are found at 77K. The abnormal resistance dropped at 5 GPa is close to the phase transition pressure from the P6m2 structure to the p structure predicted by Lowther et al. [Phys. Reg. B 59 (1999) 11683] Another abnormal change of resistance at 12 GPa is related to the phase transition from g-C3N4 to cubic-C3N4 [Teter and Hemley, Science 271 (1990) 53].     

外压下ZrN结构相变的第一性原理研究

利用基于密度泛函理论的赝势平面波方法,研究了面心立方(fcc)和体心立方(bcc)结构ZrN的平衡态性质以及不同压力下的弹性性质,计算了fcc和bcc结构ZrN的焓-压关系,讨论了其相对稳定性。通过对总能、焓-压关系、弹性性质以及声子色散关系的分析,推测fcc结构到bcc结构的相变发生在205~235GPa之间。     

过渡金属钼高压物性的第一原理研究北大核心CSCD

在平面波赝势密度泛函理论的框架下,利用广义梯度近似(GGA)计算了体心立方(bcc)和双层密排六方(dhcp)结构的Mo在不同体积下的总能和焓值,算得的等温压缩线与前人的计算结果符合较好。对焓值作差,预测了620GPa压强附近bcc→dhcp的结构相变。根据声子谱的计算结果可知,在高压下,bcc结构可能会向dhcp或9R结构转变。力学稳定性的计算结果进一步显示,在620GPa以上,dhcp-Mo是能够稳定存在的相。结合准谐德拜模型研究了Mo在高压下的热力学性质,计算结果表明,在620GPa附近,bcc和dhcp结构Mo的热力学性质并无显著差异。     

First-Principles Investigations of the Phase Transition and Optical Properties of Solid Oxygen

Using density-functional-theory calculations, a monoclinic metallic post-ζ phase （space group C2/c） is predicted at 215 GPa. The calculated phonon dispersion curves suggest that this structure is stable at least up to 310 GPa. Oxygen remains a molecular crystal and there is no dissociation in the related pressure range. Moreover, it is found that the phase transition from （ to post-ζ phase is attributed to phonon softening, The significant change in the optical properties can be used to identify the phase transition.     

过渡金属钼高压物性的第一原理研究

在平面波赝势密度泛函理论的框架下,利用广义梯度近似(GGA)计算了体心立方(bcc)和双层密排六方(dhcp)结构的Mo在不同体积下的总能和焓值,算得的等温压缩线与前人的计算结果符合较好。对焓值作差,预测了620GPa压强附近bcc→dhcp的结构相变。根据声子谱的计算结果可知,在高压下,bcc结构可能会向dhcp或9R结构转变。力学稳定性的计算结果进一步显示,在620GPa以上,dhcp-Mo是能够稳定存在的相。结合准谐德拜模型研究了Mo在高压下的热力学性质,计算结果表明,在620GPa附近,bcc和dhcp结构Mo的热力学性质并无显著差异。     

Structural phase transition,strength,and texture in vanadium at high pressure under nonhydrostatic compression

The structural phase transition, strength, and texture of vanadium have been studied under nonhydrostatic compression up to 70 GPa using an angle-dispersive radial x-ray diffraction technique in a 2-fold paranomic diamond anvil cell and up to 38 GPa using an angle-dispersive x-ray diffraction technique in a modified Mao–Bell diamond anvil cell at room temperature. We have confirmed a phase transition from body-centered cubic structure to rhombohedral structure at 27–32 GPa under nonhydrostatic compression. The radial x-ray diffraction data yields a bulk modulus K_0= 141(5) GPa and its pressure derivative K_0′= 5.4(7) for the bcc phase and K_0= 154(13) GPa with K_0′= 3.8(3) for the rhombohedral phase at ψ = 54.7°. The nonhydrostatic x-ray diffraction data of both bcc and rhombohedral phases yields a bulk modulus K_0= 188(5) GPa with K_0′= 2.1(3). Combined with the independent constraints on the high-pressure shear modulus, it is found that the vanadium sample can support a differential stress of ～1.6 GPa when it starts to yield with plastic deformation at ～36 GPa. A maximum differential stress as high as ～ 1.7 GPa can be supported by vanadium at the pressure of ～ 47 GPa.In addition, we have investigated the texture up to 70 GPa using the software package MAUD. It is convinced that the bodycentered cubic to rhombohedral phase transition and plastic deformation due to stress under high pressures are responsible for the development of texture.     

Detailed structural,mechanical,and electronic study of five structures for CaF2 under high pressure

Detailed density functional theory(DFT)calculations of the structural,mechanical,thermodynamic,and electronicproperties of crystalline CaF2 with five different structures in the pressure range of 0 GPa–150 GPa are performed byboth GGA(generalized gradient approximation)-PBE(Perdew–Burke–Ernzerhof)and LDA(local density approximation)-CAPZ(Cambridge Serial Total Energy Package).It is found that the enthalpy differences imply that the fluorite phase→PbCl2-type phase→Ni2In-type phase transition in CaF2 occurs at PGGA1=8.0 GPa,PGGA2=111.4 GPa by usingthe XC of GGA,and PLDA1=4.5 GPa,PLDA2=101.7 GPa by LDA,respectively,which is consistent with previousexperiments and theoretical conclusions.Moreover,the enthalpy differences between PbCl2-type and Ni2In-type phases inone molecular formula become very small at the pressure of about 100 GPa,indicating the possibility of coexistence of twophase at high pressures.This may be the reason why the transition pressure of the second phase transition in other reportsis so huge(68 GPa–278 GPa).The volume changed in the second phase transition are also consistent with the enthalpydifference result.Besides,the pressure dependence of mechanical and thermodynamic properties of CaF2 is studied.Itis found that the high-pressure phase of Ni2In-type structure has better stiffness in CaF2 crystal,and the hardness of thematerial has hardly changed in the second phase transition.Finally,the electronic structure of CaF2 is also analyzed withthe change of pressure.By analyzing the band gap and density of states,the large band gap indicates the CaF2 crystal isalways an insulator at 0 GPa–150 GPa.     

微区Raman光谱在TiO_2高压结构相变研究中的应用

本文以金红石单晶TiO2和锐钛矿多晶TiO2为研究对象,应用金刚石小压机和原位拉曼光谱测量技术,系统研究了室温高压下TiO2的结构相变。原位拉曼测量表明,金红石单晶TiO2在压力达到12.91GPa时开始发生由金红石结构向斜锆石结构(MI)的相变,当压力达到14.16 GPa时,相变完成;继续加压到21.65 GPa,没有发现进一步的相变;卸压时由斜锆石结构转变为PbO2结构,相变发生在大约7.11 GPa处。锐钛矿多晶TiO2在压力达到4.26 GPa时开始向PbO2结构转变,当压力达到8.34 GPa时相变完成;继续加压到12.94 GPa,样品开始发生由PbO2结构向斜锆石结构的相变,当压力达到18.74 GPa时相变完成;继续加压到21.39 GPa,没有发现进一步的相变;卸压时也由斜锆石结构转变为PbO2结构,起始相变压力点应高于8 GPa。     

First-principles study of structural stability, electronic and elastic properties of ZrC compounds

We theoretically study the possible pressure-induced structural phase transition, electronic and elastic properties of ZrC by using first-principles calculations based on density functional theory (DFT), in the presence and absence of spin-orbit coupling (SOC). The calculations indicate that there exists a phase transition from the NaCl-type (B1) structure to CsCl-type (B2) structure at the transition pressure of 313.2 GPa (without SOC) and 303.5 GPa (with SOC). The detailed structural changes during the phase transition were analyzed. The band structure shows that B1-ZrC is metallic. A pseudogap appears around the Fermi level of the total density of states (DOS) of the B1 phase of ZrC, which may contribute to its structural stability.     

First-principles study of high-pressure phase transformations in LaBi

An investigation into the structural stability and the electronic properties of LaBi under high pressure was conducted using first-principles calculations based on density functional theory (DFT), in the presence and absence of spin–orbit coupling (SOC). Our results demonstrate that there exists a structural phase transition from the NaCl-type (B1) structure to a primitive tetragonal (PT) structure at the transition pressure of 11.2 GPa (without SOC) and 12.9 GPa (with SOC). The chemical bond between La and Bi is mainly ionic. The band structure shows that B1-LaBi is metallic. A pseudogap appears around the Fermi level of the total density of states (DOS) of the B1 phase of LaBi, which may contribute to its stability.     

First principles study of structural phase transition of YSb and ScSb compounds

High pressure induced phase transition of YSb and ScSb compounds have been studied using Density Functional Theory method within Generalized Gradient Approximation. It was found that the phase transition from the NaCl-type (B1) to a CsCl-type structure (B2) began to occur at around 29 GPa for YSb compound, agreeing well with available experiments and theoretical calculations. For ScSb compound it was suggested that structural phase transition from B1 to B2 will occur at about 40 GPa, differing greatly with experimental and theoretical results. The finding that the transition pressures increase with decreasing lattice constant in the NaCl-type structure for YSb and ScSb compounds was found to be similar to the phenomena observed for LnSb (Ln: lanthanide) compounds. Mulliken charge and overlap population analysis revealed that YSb and ScSb compounds in B1 structure show similar interaction between anion and cation, while in B2 structure a higher degree of covalency was found for ScSb than that in YSb. Also, DOS and band structure of these two compounds in B1 and B2 structures were presented and analyzed.     

十种稀土金属电阻的压力效应

 使用Bundy和Dunn发展起来的带有烧结金刚石砧的Drickamer型高压装置，用固定点测压法标定实验压力，在室温及0~43 GPa的压力范围内测量了稀土金属中Pr、Nd、Sm、Gd、Tb、Dy、Ho、Tm、Lu和Yb的电阻随压力的变化。在各稀土元素的电阻随压力变化的曲线上，观测到了若干“凸起”和斜率突变点，根据Jayaraman提出的三价稀土在压力作用下的相变顺序，得到了这些突（凸）变点分别对应着hcp→Sm-type→dhcp→fcc相变顺序中的某一类型的相变压力。此外还观测到了Pr、Gd、Tb的fcc相随着压力再增高而发生的相变，根据已报导的关于Pr的工作，推测Gd和Tb的这一相变应为fcc→dfcc相变，它们分别发生在22.0和24.5 GPa。在本工作所得结果基础上对Johansson的三价稀土总相图进行了修正。     

New high-pressure polymorphs in sodium halides

Static compression experiments to 50 GPa, employing X-ray diffraction through a diamond cell, were made on NaF, NaBr and NaI. NaF was found to transform from its initial B1 (NaCl-type) to the B2 (CsCl-type) structure at 27 ± 1GPa on the ruby fluorescence scale with a volume change at the transition of ?8.9%. New high-pressure polymorphs showing birefringence under microscope were found both for NaBr and NaI at 29 ± 1 GPa and 26 ± 1 GPa, respectively. X-ray diffraction patterns of these high-pressure polymorphs could not be indexed as the B2 structure. The GeS-type structure (the distorted NaCl structure) was tentatively assigned to the high-pressure polymorph of NaI.     

Equal compressibility structural phase transition of molybdenum at high pressure

We have studied the high-pressure compression behavior of molybdenum up to 60 GPa by synchrotron radial x-ray diffraction (RXRD) in a diamond anvil cell (DAC). It is found that all diffraction peaks of molybdenum undergo a split at around 27 GPa, and we believe that a phase transition from a body-centered cubic structure to a rhombohedral structure at room pressure has occurred. The slope of pressure-volume curve shows continuity before and after this phase transition, when fitting the pressure-volume curves of the body-centered cubic structure at low pressure and the rhombohedral structure at high pressure. A bulk modulus of 261.3 (2.7) GPa and a first-order derivative of the bulk modulus of 4.15 (0.14) are obtained by using the nonhydrostatic compression data at the angle ψ = 54.7° between the diffracting plane normal and stress axis.     

Structural stability and phase transitions in K8Si46 clathrate under high pressure

The structural stability of type-I K8Si46 clathrate has been investigated at high pressure by synchrotron x-ray diffraction. In contrast to that observed in the Na-doped structure-II analogue [A. San-Miguel, Phys. Rev. Lett. 83, 5290 (1999)]], no phase separation into the beta-Sn Si structure was identified at 11 GPa. Instead, K8Si46 is found to undergo a transition to an isostructural positional disordered phase at around 15 GPa. Ab initio phonon band structure calculations reveal a novel phenomenon of phonon instabilities of K atoms in the large cavities is responsible for this transition. Above 32 GPa, the new structure transforms into an amorphous phase.     

SmN晶体的电子结构和高压相变的第一性原理

利用基于密度泛函理论的第一性原理,研究了SmN晶体的电子结构和高压相变. SmN晶体的电子结构具有半金属特征,多数自旋电子显示金属导电性,少数自旋电子显示半导体导电性. 高压相变的结果显示,SmN晶体经历从NaCl型(B1)到CsCl型(B2)结构转变的压致结构相变,相变压力117 GPa. 弹性系数的结果显示,在环境压力下SmN晶体的弹性系数满足玻恩稳定条件,标志着B1相是力学稳定结构. 声子谱结果显示,在环境压力下B1相是热力学稳定结构,与弹性系数的计算结果一致.     

Phase transitions of YbX (X = P,As and Sb) with a NaCl-type structure at high pressures

The powder X-ray diffraction of YbX (X?=?P, As and Sb) with a NaCl-type structure has been studied with synchrotron radiation up to 63?GPa at room temperature. YbSb undergoes the first-order structural phase transition from the NaCl-type (B1) to the CsCl-type (B2) structure at around 13?GPa. The structural change to the B2 structure occurs with the volume collapse of about 1% at 13?GPa. The transition pressure of YbSb is surprisingly lower than that of any other heavier LnSb (Ln?=?Dy, Ho, Er, Tm and Lu). The pressure-induced phase transitions in YbP and YbAs are observed at around 51?GPa and 52?GPa respectively. The transition pressure of both compounds is much higher than that of YbSb. The high-pressure structural behaviour of YbX (X?=?P, As and Sb) is discussed. The volume versus pressure curve for YbX with the NaCl-type structure is fitted by a Birch equation of state. The bulk moduli of these compounds with the NaCl-type structure are 104?GPa for YbP, 85?GPa for YbAs and 52?GPa for YbSb.