Theoretical study of novel B–C–O compounds with non-diamond isoelectronic

Two novel non-isoelectronic with diamond(non-IED)B–C–O phases(tI16-B₈C₆O₂and mP16-B₈C₅O₃)have been unmasked.The research of the phonon scattering spectra and the independent elastic constants under ambient pressure(AP)and high pressure(HP)proves the stability of these non-IED B–C–O phases.Respective to the common compounds,the research of the formation enthalpies and the relationship with pressure of all non-IED B–C–O phases suggests that HP technology performed in the diamond anvil cell(DAC)or large volume press(LVP)is an important technology for synthesis.Both tI16-B₈C₆O₂and tI12-B₆C₄O₂possess electrical conductivity.mP16-B₈C₅O₃is a small bandgap semiconductor with a 0.530 eV gap.For aP13-B₆C₂O₅,mC20-B₂CO₂and tI18-B₄CO₄are all large gap semiconductors with gaps of 5.643 eV,6.113 eV,and 7.105 eV,respectively.The study on the relationship between band gap values and pressure of these six non-IED B–C–O phases states that tI16-B₈C₆O₂and tI12-B₆C₄O₂maintain electrical conductivity,mC20-B₂CO₂and tI18-B₄CO₄have good bandgap stability and are less affected by pressure.The stress-strain simulation reveals that the max strain and stress of 0.4 GPa and 141.9 GPa respectively,can be sustained by tI16-B₈C₆O₂.Studies on their mechanical properties shows that they all possess elasticity moduli and hard character.And pressure has an obvious effect on their mechanical properties,therein toughness of tI12-B₆C₄O₂,aP13-B₆C₂O₅,mC20-B₂CO₂and tI18-B4CO4 all increases,and hardness of mP16-B₈C₅O₃continue to strengthen during the compression.With abundant hardness characteristics and tunable band gaps,extensive attention will be focused on the scientific research of non-IED B–C–O compounds.     

Structural, Electronic Properties and Chemical Bonding of Borate Li4CaB2O6 under High Pressure： an Ab Initio Investigation

We calculate structural, electronic properties and chemical bonding of borate Li4CaB2O6 under high pressure by means of the local density-functional pseudopotential approach. The equilibrium lattice constants, density of states, Mulliken population, bond lengths, bond angles as well as the pressure dependence of the band gap are presented. Analysis of the simulated high pressure band structure suggests that borate Li4CaB2O6 can be used as the semi-conductor optical material. Based on the Mulliken population analysis, it is found that the electron transfer of the Li atom is very different from that of other atoms in the studied range of high pressures. The charge populations of the Li atom decrease with the pressure up to 60 GPa, then increase with the pressure.     

High-pressure polymorphs of LiPN2: A first-principles study

In this work, high-pressure phase behavior of LiPN₂ within 0–300 GPa was studied by using an unbiased structure searching method in combination with first-principles calculations. Three pressureinduced phase transitions were predicted, as tI16→hR4→cF64→oP8 at 44, 136, and 259 GPa, respectively. The six-fold coordination environments were found for all high-pressure polymorphs, which are substantially different from the four-fold coordination environments observed in the tI16 structure. The hR4 and cF64 structures consist of close-packed PN₆ and LiN₆ octahedra connected by edge-sharing, whereas the oP8 structure is built up from edge- and face-sharing PN₆ and LiN₆ octahedra with N lying in the center of the trigonal prisms. The electronic structure analysis reveals that LiPN₂ is a semiconductor within the pressure range studied and P-N and Li-N bonds are covalent and ionic, respectively. The results obtained are expected to provide insight and guidance for future experiments on LiPN₂ and other alkali metal nitridophosphates.     

高压下钨弹性和热力学性质的第一性原理研究

本文利用密度泛函理论研究了高压下bcc结构钨的弹性和热力学性质,计算得到钨的晶格常数、体弹模量以及其对压强的一阶偏导与实验值符合较好;在常压下弹性常数计算值与实验值符合较好的基础上,预测了其高压数据.针对钨的固相结构稳定性问题,根据力学稳定判断标准得到0~600 GPa范围内bcc结构是力学稳定的.此外,通过体模量和剪切模量的计算得到bcc结构钨在压力低于600 GPa时的力学性能表现为韧性.最后,基于准简谐德拜模型,成功预测了钨的热膨胀系数、等压热容、等容热容和熵随着压强和温度的变化关系,为钨及其合金的进一步设计及应用提供参考.     

高压下钨弹性和热力学性质的第一性原理研究

本文利用密度泛函理论研究了高压下bcc结构钨的弹性和热力学性质,计算得到钨的晶格常数、体弹模量以及其对压强的一阶偏导与实验值符合较好;在常压下弹性常数计算值与实验值符合较好的基础上,预测了其高压数据.针对钨的固相结构稳定性问题,根据力学稳定判断标准得到0~600 GPa范围内bcc结构是力学稳定的.此外,通过体模量和剪切模量的计算得到bcc结构钨在压力低于600 GPa时的力学性能表现为韧性.最后,基于准简谐德拜模型,成功预测了钨的热膨胀系数、等压热容、等容热容和熵随着压强和温度的变化关系,为钨及其合金的进一步设计及应用提供参考.     

Revelation of the role of impurities and conduction electron density in the high resolution photoemission study of ferromagnetic hexaborides

We investigate the temperature evolution of the electronic structure of ferromagnetic CaB6 using ultrahigh resolution photoemission spectroscopy; the electronic structure of paramagnetic LaB6 is used as a reference. High resolution spectra of CaB6 reveal a finite density of states at the Fermi level E(F) at all the temperatures and evidence of impurity induced localized features in the vicinity of E(F), which are absent in the spectra of LaB6. Analysis of the high resolution spectra suggests that disorder in the B sublattice inducing partial localization in the mobile electrons and low electron density at E(F) is important to achieve ferromagnetism in these systems.     

高压下RhB的相变、弹性性质、电子结构及硬度的第一性原理计算

采用密度泛函理论中的赝势平面波方法系统地研究了高压下RhB的结构相变、弹性性质、电子结构和硬度.分析表明,RhB在25.3 GPa时从anti-NiAs结构相变到FeB结构,这两种结构的弹性常数、体弹模量、剪切模量、杨氏模量和弹性各向异性因子的外压力效应明显.电子态密度的计算结果显示,这两种结构是金属性的,且费米能级附近的峰随着压强的增大向两侧移动,赝能隙变宽,轨道杂化增强,共价性增强,非局域化更加明显.此外,硬度计算结果显示,anti-NiAs-RhB的金属性比较弱,有着较高的硬度,属于硬质材料.     

高温高压下碳化钨晶体的结构、力学、电子、光学以及热力学性能的第一性原理计算

本文利用密度泛函理论中的广义梯度近似对碳化钨晶体的三种结构(碳化钨相、闪锌矿相以及纤锌矿相)进行了优化,得到能量最低的稳定构型,并在此基础上计算了它的力学、电子、光学和高温高压下的热力学性质.研究表明:在0～300 GPa压力范围内,碳化钨相具有最高的稳定性.同时,高压下碳化钨相的弹性常数满足Born-Huang准则,且0 GPa和300 GPa下的声子色散没有虚频,证明了高压下碳化钨相的静力学稳定性和动力学稳定性.电子性质表明了碳化钨的金属性.光学性质表明碳化钨在高能区很难吸收光.热力学性质的研究表明:体积比V/V_0对压强的变化更敏感;高温时C_V曲线近似一条直线;给定压强下热膨胀系数α在600 K温度以上增长非常缓慢;压强对德拜温度Θ_D的影响较大;在低压下格林艾森系数γ的变化较大.     

ZnS结构相变、电子结构和光学性质的研究

运用第一性原理平面波赝势和广义梯度近似方法, 对闪锌矿结构(ZB)和氯化钠结构(RS) ZnS的状态方程及其在高压下的相变进行计算研究, 分析相变点附近的电子态密度、能带结构和光学性质的变化机理. 结果表明: 通过状态方程得到ZB相到RS相的相变压强值为18.1 GPa, 而利用焓相等原理得到的相变压强值为18.0 GPa; 在结构相变过程中, sp³轨道杂化现象并未消失, RS相ZnS的金属性明显增强; 与ZB相ZnS相比, RS相ZnS的介电常数主峰明显增强, 并向低能方向出现了明显偏移, 使得介电峰向低能方向拓展, 在低能区电子跃迁大大增强. 关键词： 硫化锌 相变 电子结构 光学性质     

A study of the optical properties of titanium oxide films prepared by dc reactive magnetron sputtering

TiO₂ thin films were deposited on the glass substrates by dc reactive magnetron sputtering technique at different sputtering pressures (2 × 10⁻³ to 2 × 10⁻² mbar). The films prepared at low pressures have an anatase phase, and the films prepared at high pressures have an amorphous phase. The optical properties were studied by measuring the transmittance and the ellipsometric spectra. The optical constants of the films in the visible range were obtained by fitting the transmittance combined with the ellipsometry measurements using the classical model with one oscillator. The refractive index of the films decreases from 2.5 until 2.1 as the sputtering pressure increases from 2 × 10⁻³ to 2 × 10⁻² mbar. The films prepared at the pressure higher than 6 × 10⁻³ mbar show a volume inhomogeneity. This volume inhomogeneity has been calculated by fitting the transmittance and the ellipsometric spectra. The volume inhomogeneity of the film prepared at the highest sputtering pressure is about 10%. Although the films prepared at high pressures show a large volume inhomogeneity, they have low extinction coefficients. It is suggested that the anatase phase results in more light scattering than amorphous phase does, and then a high extinction coefficient.     

Structural, electronic and optical properties of high pressure stable phases of ZnTe

We have performed full potential linear augmented plane wave calculations to investigate the pressure induced phase transition in ZnTe. Total energies of three phases (zinc-blende, cinnabar and Cmcm) are calculated using density functional theory formalism under generalized gradient approximation and Engel-Vosko generalized gradient approximation for the exchange correlation potential approximation. The pressure stability corresponding to zinc-blende, cinnabar and Cmcm phases of ZnTe are computed. We find that cinnabar phase could be formed as a metastable phase by releasing pressure from the high pressure Cmcm phase. The obtained structural, electronic and optical results are compared with previous calculations and available experimental data. Overall good agreement is found.     

Electronic,elastic and dynamical properties of MgSe under pressure: rocksalt and iron silicide phase

The electronic, elastic and dynamical properties of MgSe in the rocksalt (B1) and iron silicide (B28) phase and the effects of pressure on these properties are investigated using first-principles method. The calculated electronic band structure indicates that the B1 phase of MgSe presents an indirect band-gap feature and the band gaps initially increase with pressure and subsequently decrease upon compression. Remarkably, an indirect-to-direct band-gap transition has been observed at the phase transition pressure. The elastic constants, bulk modulus, shear modulus, Young’s modulus, elastic anisotropy and B/G ratio of MgSe in the B1 and B28 phase at high pressure have also been investigated. The bulk modulus, shear modulus and Young’s modulus all increase monotonously with the increasing of pressure for the B1 and B28 phase of MgSe. The calculated phonon frequencies of the B1 phase at zero pressure agree well with available theoretical results. And the transverse acoustic phonon TA(X) mode of this phase completely softening to zero at 82 GPa. The phonon curves of the B28 phase under pressure have also been successfully investigated.     

Phase Transition and Physical Properties of InS

Using the crystal structure prediction method based on particle swarm optimization algorithm, three phases(P nnm, C2/m and Pm-3 m) for InS are predicted. The new phase Pm-3m of InS under high pressure is firstly reported in the work. The structural features and electronic structure under high pressure of InS are fully investigated. We predicted the stable ground-state structure of InS was the P nnm phase and phase transformation of InS from P nnm phase to P m-3 m phase is firstly found at the pressure of about 29.5 GPa. According to the calculated enthalpies of InS with four structures in the pressure range from 20 GPa to 45 GPa, we find the C2/m phase is a metastable phase. The calculated band gap value of about 2.08 eV for InS with P nnm structure at 0 GPa agrees well with the experimental value. Moreover, the electronic structure suggests that the C2/m and P m-3m phase are metallic phases.     

Elastic,electronic and thermal properties of topological insulator SmB6: first-principles calculations

Abstract

The pressure dependence of the structural, elastic, electronic and thermal properties of Kondo insulator SmB₆ have been systematically studied by density functional theory combined with the quasi-harmonic Debye model. The calculated structure at zero pressure is in good agreement with the available experimental results at low temperature. The obtained elastic constants, bulk modulus and shear modulus indicate that SmB₆ is mechanically stable and behaves in a brittle manner under the applied pressure 0–20 GPa, consistent with available experimental data. In addition, the elastic-relevant properties, Young’s modulus and the Poisson ratio manifest that increasing pressure results in an enhancement in the stiffness of the compound. It is found that unlike temperature, pressure has little effect on the heat capacity of SmB_6. What more important is that we observed an insulator to metal phase transition at about 5.5 GPa through the disappearance of the band gap, well consistent with the experimental data. This transition has little effect on the physical properties of SmB₆.     

First-principles study of the elastic and thermodynamic properties of thorium hydrides at high pressure

The high pressure behaviors of Th_4H_(15) and ThH_2 are investigated by using the first-principles calculations based on the density functional theory(DFT). From the energy–volume relations, the bct phase of ThH_2 is more stable than the fcc phase at ambient conditions. At high pressure, the bct ThH_2 and bcc Th_4H(15) phases are more brittle than they are at ambient pressure from the calculated elastic constants and the Poisson ratio. The thermodynamic stability of the bct phase ThH_2 is determined from the calculated phonon dispersion. In the pressure domain of interest, the phonon dispersions of bcc Th_4H(15) and bct ThH_2 are positive, indicating the dynamical stability of these two phases, while the fcc ThH_2 is unstable. The thermodynamic properties including the lattice vibration energy, entropy, and specific heat are predicted for these stable phases. The vibrational free energy decreases with the increase of the temperature, and the entropy and the heat capacity are proportional to the temperature and inversely proportional to the pressure. As the pressure increases, the resistance to the external pressure is strengthened for Th_4H_(15) and ThH_2.     

GaAs相变和热力学性质的密度泛函理论研究

利用平面波赝势密度泛函方法,对GaAs从闪锌矿结构到CsCl 结构的相变进行了理论研究.通过Birch-Murnaghan状态方程拟合闪锌矿结构GaAs的能量和体积,得到了GaAs的热力学性质.我们发现对 GaAs来说,闪锌矿结构通常比CsCl结构稳定,由闪锌矿结构到CsCl结构的相变压力在37.019 GPa左右.计算所得到的晶格常数、体弹模量及体弹模量对压强的一阶导数与实验值以及其他作者的计算值相符合.     

GaN结构相变、电子结构和光学性质

运用第一性原理平面波赝势和广义梯度近似方法,对纤锌矿结构和氯化钠结构GaN的状态方程及其在高压下的相变进行计算研究,分析相变点附近的电子态密度、能带结构和光学性质的变化机制.通过状态方程和焓相等原理得到GaN从纤锌矿到氯化钠结构的相变压强分别为43.9 Gpa和46.0 Gpa;在相变的过程中,GaN由典型的直接带隙半导体转变为间接带隙半导体材料;氯化钠结构GaN相比于纤锌矿结构,介电函数主峰值增强,本征吸收边明显往高能方向移动,氯化钠结构GaN在低能区域的光学性质差于纤锌矿结构.     

GaP相变及热力学性质的理论研究

利用基于密度泛函理论的第一性原理方法研究了闪锌矿和氯化钠结构的GaP的相变及热力学性质.对两种结构的能量体积曲线做公切线,得到了从闪锌矿到氯化钠结构的相变压力约为26.2GPa,与实验结果一致.通过准谐德拜模型得到了不同温度下体积和热膨胀系数与压强的关系,以及不同压强下热容与温度的关系.     

Equations of state and the parameters of interatomic interaction potentials for six metals which experience phase transformations in stress waves

For six metals which experience phase transformations in stress waves, Fe, Cr, Hf, Pb, Ti, and Zr, equations of state have been obtained with the use of the Lennard-Jones interatomic interaction potential for low pressure and high pressure phases. For the low pressure phases, the parameters of the potential were calculated with the use of a complex of physical characteristics which are determined for the unstrained sample under normal conditions. The parameters of therelations D(u) and P(x) for the high pressure phases were determined from the data of a shock-wave experiment with observance of requirements for their consistency. The approximations obtained for the high pressure phases enable one to calculate the compressibility with the density increment measured with an error no more than 6% and the velocity of propagation of the shock wave with an error of 4% up to about twofold compression. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 57–60, October, 2006.     

钝感高能炸药三氨基三硝基苯高温高压下热力学性质的分子动力学模拟研究

热力学性质是钝感高能炸药1, 3, 5-三氨基-2, 4, 6-三硝基苯(TATB)爆轰性质和安全性评估分析的重要参数. 由于结构的复杂性, TATB炸药尚缺乏系统的实验和理论计算结果. 结合全原子力场和分子动力学的方法, 本文系统研究了不同温度和压力条件下TATB的力学性质和热力学参数, 得到了弹性模量、德拜温度等随温度、压力的变化情况, 并与实验进行了对比分析. 结果表明: 在 0-50 GPa外部压力下, TATB晶体保持力学稳定, 弹性常数和弹性模量随压力升高而增大, 各向异性程度随压力升高而减小, 泊松比和延展性则受压力的影响较小; 随温度的升高, TATB的力学稳定性逐渐下降, 有发生力学失稳的可能, 各弹性常数随温度升高而逐渐减小, 各向异性程度也随之减小; TATB 的声速和德拜温度同样随着压力升高而增大, 平均声速从0 GPa下的1833 m/s, 增加到10 GPa 下的3143 m/s, 德拜温度由0 GPa下的254 K增加到10 GPa的587 K. TATB 热膨胀系数的计算表明, 在200-500 K 温度常压情况下, 其体热膨胀系数为35.9×10^-5 K^-1, 与实验数据符合较好.