Phase stability and electronic properties of silver halides

In this work, we study the phase stability and electronic properties of silver halides ( AgBr, AgCl and AgI) using the full-potential linearized augmented plane wave method within the density functional theory. In this approach, the Wu–Cohen generalized gradient approximation was used for the exchange–correlation potential. Moreover, the modified Becke–Johnson approximation was also used for band-structure calculations. Various structural phase transitions were considered here in order to confirm the most stable structure and to predict the phase transition under hydrostatic pressure. In addition, we have studied the band structures of the stable phases of these compounds which reveal that the three compounds exhibit semiconducting behavior. The results obtained are compared with other calculations and experimental measurements.     

MgS和MgSe带隙的GW近似修正

应用基于密度泛函理论下的局域密度近似（DFT-LDA）方法研究了岩石矿结构的 MgS和MgSe的晶体结构和能带结构。本文得到稳定结构的晶格常数与已知实验数据相吻合,MgS和MgSe都是间接带隙半导体且带隙值分别为2.74 eV和1.70 eV。尽管本文利用LDA计算的带隙值与之前的理论值很接近,但是局域密度近似常常低估带隙值。因此应用准粒子GW（G是格林函数,W是库伦屏蔽相互作用）近似 对MgS和MgSe的带隙值进行了修正,其结果分别为4.15eV和2.74 eV。GW近似的结果应该是合适的值。     

MgS和MgSe带隙的GW近似修正

应用基于密度泛函理论下的局域密度近似（DFT-LDA）方法研究了岩石矿结构的 MgS和MgSe的晶体结构和能带结构。本文得到稳定结构的晶格常数与已知实验数据相吻合,MgS和MgSe都是间接带隙半导体且带隙值分别为2.74 eV和1.70 eV。尽管本文利用LDA计算的带隙值与之前的理论值很接近,但是局域密度近似常常低估带隙值。因此应用准粒子GW（G是格林函数,W是库伦屏蔽相互作用）近似 对MgS和MgSe的带隙值进行了修正,其结果分别为4.15eV和2.74 eV。GW近似的结果应该是合适的值。     

Phase transition,electronic and optical properties of III-Sb compounds under pressure

III-V semiconductors are the backbone of optoelectronic industry. Here, we have performed first principle calculations to investigate the structural, electronic and optical properties of III-Sb (III = B, Al, Ga, Sb) compounds under the effect of pressure. The structural phase transition from zincblende to rocksalt phases is determined by the common tangent of the two E–V curves. The obtained results are in good agreement with the available literature. Compounds make electronic transition from semiconductors to metals under pressure. The calculated band structure in zincblende structure was compared with experimental and theoretical findings. Optical properties including real and imaginary parts of the complex dielectric function, frequency-dependent reflectivity and optical conductivity are explained to characterize the optical nature of these compounds in both phases.     

First-principles investigation of the electronic, elastic and thermodynamic properties of VC under high pressure

An investigation of the electronic,elastic and thermodynamic properties of VC under high pressure has been conducted using first-principles calculations based on density functional theory (DFT) with the plane-wave basis set,as implemented in the CASTEP code. At elevated pressures,VC is predicted to undergo a structural transition from a relatively open NaCl-type structure to a more dense CsCl-type one. The predicted transition pressure is 520 GPa. The elastic constant,Debye temperature and heat capacity each as a function of pressure and/or temperature of VC are presented for the first time.     

纳米尺度下Sm-Co合金体系中相组成与相稳定性的研究

建立了纳米晶合金相的热力学模型,可定量描述纳米尺度下合金体系中化合物相的热力学性质,并预测合金相的稳定性及其转变规律.利用该模型全面计算了纳米晶Sm-Co合金体系中各化合物相在不同晶粒尺寸下的摩尔吉布斯自由能随温度的变化关系,预测了纳米尺度下Sm-Co合金体系中各物相的相对稳定性及转变规律.模型预测结果示出,在室温附近,随着纳米晶粒尺寸的减小,某些纳米晶合金相的摩尔吉布斯自由能将由负值变为正值,预示着将向其他更稳定的纳米晶合金相转变,这是与传统粗晶材料中合金相的稳定性仅依赖于温度条件而完全不同的纳米晶合金 关键词： 纳米晶材料热力学 Sm-Co合金 相稳定性 相变     

高压下MgS的弹性性质、电子结构和光学性质的第一性原理研究

采用基于密度泛函理论(density functional theory)基础上的第一性原理赝势平面波方法, 计算研究了MgS晶体B2构型在不同压强下的几何结构、弹性性质、电子结构和光学性质. 计算结果表明, 在高压作用下, 该结构的导带能级有向高能级移动的趋势, 而价带能级有向低能级移动的趋势. 同时, 对照态密度分布图及高压下能级的移动情况, 分析了MgS B2构型在高压作用下的光学性质, 发现高压作用下, 吸收光谱发生了明显的蓝移.     

Rb吸附石墨烯纳米带电子性质和光学性质的研究

本文基于密度泛函理论的第一性原理方法了计算了Rb、O和H吸附石墨烯纳米带的差分电荷密度、能带结构、分波态密度和介电函数,调制了石墨烯纳米带的电子性质和光学性质,给出了不同杂质影响材料光学特性的规律.结果表明本征石墨烯纳米带为n型直接带隙半导体且带隙值为0.639 eV;Rb原子吸附石墨烯纳米带之后变为n型简并直接带隙半导体,带隙值为0.494eV;Rb和O吸附石墨烯纳米带变为p型简并直接带隙半导体,带隙值增加为0.996eV;增加H吸附石墨烯纳米带后,半导体类型变为n型直接带隙半导体,且带隙变为0.299eV,带隙值相对减小,更有利于半导体发光器件制备.吸附Rb、O和H原子后,石墨烯纳米带中电荷密度发生转移,导致C、Rb、O和H之间成键作用显著.吸附Rb之后,在费米能级附近由C-2p、Rb-5s贡献;增加O原子吸附之后,O-2p在费米能级附近贡献非常活跃,杂化效应使费米能级分裂出一条能带;再增加H原子吸附之后,Rb-4p贡献发生蓝移,O-2p在费米能级附近贡献非常强,费米能级分裂出两条能带.Rb、O和H的吸附后,明显调制了石墨烯纳米带的光学性质.     

Phase transition and high pressure behavior of Zirconium and Niobium carbides

We have predicted the phase transition pressure (P _T )and high pressure behavior of Zirconium and Niobium carbide (ZrC, NbC). The high pressure structural phase transitions in ZrC and NbC has been studied by using a two body inter-ionic potential model, which includes the Coulomb screening effect, due to the semi-metallic nature of these compounds. These transition metal carbides have been found to undergo NaCl (B1) to CsCl (B2)-type structural phase transition, at high pressure like other binary systems. We predict such structural transformation in ZrC and NbC at a pressure of 98GPa and 85GPa respectively. We have also predicted second order elastic constant and bulk modulus. The present theoretical work has been compared with the corresponding experimental data and prediction of LAPW and GGA and LDA theories.      

Phase stability and temperature effect in ScX (X=S,Se and Te) compounds

Based on first-principles calculations, the structural stability and temperature effect in ScX (X=S, Se and Te) compounds are studied with three typical structures of B1 (NaCl-type), hcp (NiAs-type) and β-hcp (inverse Li₂O₂-type). Their dynamic stability has been verified using phonon mode analysis and molecular dynamics simulations. From the total energy calculations, we find that the most stable ground state structures are B1 for ScS, and hcp for ScSe and ScTe, respectively. Moreover, structural stabilities at finite temperature are studied with the combination of phonon dependent dynamics analysis and first-principles calculations, which reveals a phase transition from hcp to B1 in ScSe around 230 K and a phase transition from hcp to β-hcp in ScTe around 460 K, in accordance with experimental findings. The energy barrier and pathway along the phase transformation from hcp to β-hcp ScTe are also calculated and analyzed by the solid-state nudged elastic band method.     

钍基碳化物ThC的高压结构稳定性研究

钍基碳化物ThC是重要的核材料,基于第一性原理计算,我们研究了ThC的结构稳定性及相变.声子色散曲线表明基态结构的晶格能够稳定到42 GPa附近,当压力继续升高时虚频的出现表明了动力学的不稳定.经比较基态Fm-3m及高压相Pnma结构的自由能,发现零温下,相变压力为22.4 GPa,符合声子色散曲线的结果,同时预测了两种结构的相变边界.     

NMR parameters in column 13 metal fluoride compounds (AlF3, GaF3, InF3 and TlF) from first principle calculations

The relationship between the experimental ¹⁹F isotropic chemical shift and the ¹⁹F isotropic shielding calculated using the gauge including projector augmented-wave (GIPAW) method with PBE functional is investigated in the case of GaF₃, InF₃, TlF and several AlF₃ polymorphs. It is shown that the linear correlation between experimental and DFT-PBE calculated values previously established on alkali, alkaline earth and rare earth of column 3 basic fluorides (Sadoc et al., Phys. Chem. Chem. Phys. 13 (2011) 18539–18550) remains valid in the case of column 13 metal fluorides, indicating that it allows predicting ¹⁹F solid state NMR spectra of a broad range of crystalline fluorides with a relatively good accuracy. For the isostructural α-AlF₃, GaF₃ and InF₃ phases, PBE-DFT geometry optimization leads to noticeably overbended M–F–M bond angles and underestimated ²⁷Al, ⁷¹Ga and ¹¹⁵In calculated quadrupolar coupling constants. For the studied compounds, whose structures are built of corner shared MF₆ octahedra, it is shown that the electric field gradient (EFG) tensor at the cationic sites is not related to distortions of the octahedral units, in contrast to what previously observed for isolated AlF₆ octahedra in fluoroaluminates.     

Al-Ni-RE非晶合金的晶化行为和热稳定性

本文制备了一系列Al-Ni-RE (RE=La, Ce, Y) 非晶合金薄带, 利用差示量热扫描仪和X射线衍射仪考察了非晶合金的晶化行为和初生相, 并分析了其与合金成分和原子特性间的关系. 结果表明: 在拓扑不稳定参数λ以有效原子半径修正为λ'后, 每一Al-Ni-RE非晶合金体系可由其两个临界值划分为纳米晶、纳米玻璃和玻璃三类; Al-Ni-RE非晶合金的晶化开始温度和混合焓与λ'成良好的线性关系, 即λ'能很好的表征Al基非晶合金的热稳定性. 关键词： Al基非晶 玻璃转变 初生相 热稳定性     

Electronic properties and topological phases of ThXY(X= Pb,Au, Pt and Y= Sb,Bi, Sn) compounds

The electronic properties and topological phases of Th XY(X = Pb, Au, Pt, Pd and Y = Sb, Bi, Sn) compounds in the presence of spin–orbit coupling, using density functional theory are investigated. The Th Pt Sn compound is stable in the ferromagnetic phase and the other Th XY compounds are stable in nonmagnetic phases. Band structures of these compounds in topological phases(insulator or metal) and normal phases within generalized gradient approximation(GGA) and Engel–Vosko generalized gradient approximation(GGA EV) are compared. The Th Pt Sn, Th Pt Bi, Th Pt Sb, Th Pd Bi, and Th Au Bi compounds have topological phases and the other Th XY compounds have normal phases. Band inversion strengths and topological phases of these compounds at different pressure are studied. It is seen that the band inversion strengths of these compounds are sensitive to pressure and for each compound a second-order polynomial fitted on the band inversion strengths–pressure curves.     

Quantum Phase Transitions,Entanglement Spectrum,and Schmidt Gap in Bond-Alternative S=1 Antiferromagnetic Chain

Bipartite entanglement, entanglement spectrum, and Schmidt gap in S=1 bond-alternative antiferromagnetic Heisenberg chain are investigated by the infinite time-evolving block decimation （iTEBD） method. The quantum phase transition （QPT） from the singlet-dimer phase to the Haldane phase can be detected by the singular behavior of bipartite entanglement, the sudden change of the entanglement spectrum, and the completely vanishing of the Schmidt gap. The critical point is determined to be around rc ~- 0.587, and the second-order character of the QPT is verified. Doubly degenerate entanglement spectra of both even and odd bonds are observed in the Haldane phase, by which one can distinguish the Haldane phase from the singlet-dimer phase easily. Nearest-neighbor antiferromagnetic correlations and next-nearest-neighbor ferromagnetic correlations are found in the whole parameter region. At the critical massless point, although exponentially decaying antiferromagnetie correlation is observed, it approaches to a constant value finally. Therefore, long-range correlations exist and the correlation length becomes divergent at the critical point.     

Phase stability,mechanical and thermodynamic properties of orthorhombic and trigonal MgSiN2: an ab initio study

Structural stability and mechanical and thermodynamic properties of the orthorhombic and trigonal MgSiN₂ polymorphs (or-MgSiN₂ and tr-MgSiN₂) were investigated through density functional theory and quasi-harmonic Debye model (QHDM). Our calculations show that or-MgSiN₂ is energetically the stable polymorph at low pressure, in agreement with previous experimental and theoretical study. Under pressure, a crystallographic transition from the orthorhombic structure to the trigonal one occurs around 25, 17.45 and 19.05 GPa as obtained from the generalized gradient approximation of Perdew-Wang (GGA-PW91), the generalized gradient approximation parameterized recently by Perdew et al (GGA-PBEsol) and the local density approximation developed by Ceperley and Alder and parameterized by Perdew and Zunger (LDA-CAPZ), respectively. Single-crystalline and polycrystalline elastic constants and related properties, namely Vickers hardness, acoustic Grüneisen parameter, minimum thermal conductivity, isotropic sound velocities and Debye temperature, were numerically estimated for both or-MgSiN₂ and tr-MgSiN₂. We have showed that the hardness of tr-MgSiN₂ is comparable to that of the harder materials like c-BN and B₆O. Temperature and pressure dependencies of volume, bulk modulus, thermal expansion, Grüneisen parameter, heat capacities and Debye temperature were investigated using QHDM.     

Phase transition, structural and thermodynamic properties of Mg2Si polymorphs

The plane-wave pseudo-potential method within the framework of first principles is used to investigate the structural and elastic properties of Mg 2 Si in its intermediate pressure(Pnma) and high pressure phases(P 6 3 /mmc).The lattice constants,the band structures.The bulk moduli of the Mg 2 Si polymorphs are presented and discussed.The phase transition from anti-cotunnite to Ni 2 In-type Mg 2 Si is successfully reproduced using a vibrational Debye-like model.The phase boundary can be described as P = 24.02994 + 3.93 × 10 3 T 4.66816 × 10 5 T 2 2.2501 × 10 9 T 3 + 2.33786 × 10 11 T 4.To complete the fundamental characteristics of these polymorphs we have analysed thermodynamic properties,such as thermal expansion and heat capacity,in a pressure range of 0-40 GPa and a temperature range of 0-1300 K.The obtained results tend to support the available experimental data and other theoretical results.Therefore,the present results indicate that the combination of first principles and a vibrational Debye-like model is an efficient scheme to simulate the high temperature behaviours of Mg 2 Si.     

Prediction of scandium tetraboride from first-principles calculations:Crystal structures,phase stability,mechanical properties,and hardness

Using the evolutionary methodology for crystal structure prediction, we have predicted the orthorhombic Cmcm and Pnma phases for ScB_4. The earlier proposed Cr B_4~-, Fe B_4~-, Mn B_4~-, and Re P_4~- type structures for ScB_4 are excluded.It is first discovered that the Cmcm phase transforms to the Pnma phase at about 18 GPa. Moreover, both phases are dynamically and mechanically stable. The large bulk modulus, shear modulus, and Young's modulus of the two phases make it an optimistic low compressible material. Moreover, the strong covalent bonding nature of ScB_4 is confirmed by the ELF analysis. The strong covalent bonding contributes greatly to its stability.     

High pressure structural,electronic and vibrational properties of InN and InP

A first-principles plane wave self-consistent method with the Ultrasoftpseudopotential scheme in the framework of density functional theory is performed to study the high pressure structural, electronic and vibrational properties of InX (X = N, P) for the zinc-blende (ZnS/B3), rock-salt (NaCl/B1) and cesium-chloride (CsCl/B2) phases. We also calculate the phase transition pressures among these different phases. Conclusions based on electronic energy band structure, phonon dispersion and phonon density of states at high pressure phases along phase transition regions are outlined.     

第一性原理研究硫化镉高压相变及其电子结构与弹性性质

采用第一性原理研究了CdS的六方纤锌矿(WZ), 立方闪锌矿(ZB) 和岩盐矿(RS)相在高压条件下的相稳定性、 相变点、电子结构以及弹性性能.WZ相与RS 相可以在相应的压强范围内稳定存在, 而ZB相不能稳定存在.压强大于2.18 GPa时, WZ相向RS相发生金属化相变.WZ相中S原子电负性大于Cd, 且电负性差值小于1.7, CdS的WZ相为共价晶体.高压作用下, S原子半径被强烈压缩, 有效核电荷增加, 对层外电子吸引能力提高, 电负性急剧增大, 导致S与Cd的电负性差值大于1.7, CdS的RS相以离子晶体存在. WZ相的C₄₄随压强增加呈下降趋势, 导致WZ相力学不稳定, 并向RS相转变.当压强大于2.18 GPa时, RS相C₁₁, C₁₂随压强增加而增大, 并且C₄₄保持稳定, 说明RS相具有良好的高压稳定性与力学性能. 关键词： 第一性原理 相变 电子结构 弹性性质