Structural stability and electrical properties of AlB2-type MnB2 under high pressure

The structural stability and electrical properties of AlB2-type MnB2 were studied based on high pressure angledispersive x-ray diffraction, in situ electrical resistivity measured in a diamond anvil cell(DAC) and first-principles calculations under high pressure. The x-ray diffraction results show that the structure of AlB2-type MnB2 remains stable up to 42.6 GPa. From the equation of state of MnB2, we obtained a bulk modulus value of 169.9±3.7 GPa with a fixed pressure derivative of 4, which indicates that AlB2-type MnB2 is a hard and incompressible material. The electrical resistance undergoes a transition at about 19.3 GPa, which can be explained by a transition of manganese 3d electrons from localization to delocalization under high pressure.     

Structural stability and mechanical properties of Be3N2 under high pressure

Abstract

A theoretical investigations on the structural stability and mechanical properties of Be₃N₂ crystallising in α and β phases was performed using first-principles calculations based on density functional theory. The obtained ground state structure and mechanical properties are in excellent agreement with the available experimental and theoretical data. A full elastic tensor and crystal anisotropy of Be₃N₂ in two phases are determined in the wide pressure range. Results indicated that the two phases of Be₃N₂ are mechanically stable and strongly pressure dependent in the range of pressure from 0 to 80 GPa. The superior mechanical properties show that the two phases of Be₃N₂ are potential candidate structures to be the hard material. And the α-Be₃N₂ has better mechanical properties than β-Be₃N₂. By the calculated B/G ratio, it is predicted that both phases are intrinsically brittleness and strongly prone to ductility when the pressure is above 65.6 and 68.5 GPa, respectively. Additionally, the pressure-induced elastic anisotropy analysis indicates that the elastically anisotropic of Be₃N₂ in both phases is strengthening with increasing pressure, and strongly dependent on the propagation direction.     

Structural and electrical transport properties of charge density wave material LaAgSb_2 under high pressure

Layered lanthanum silver antimonide LaAgSb_2 exhibits both charge density wave(CDW) order and Dirac-cone-like band structure at ambient pressure.Here,we systematically investigate the pressure evolution of structural and electronic properties of LaAgSb_2 single crystal.We show that the CDW order is destabilized under compression,as evidenced by the gradual suppression of magnetoresistance.At P_C～22 GPa,synchrotron x-ray diffraction and Raman scattering measurements reveal a structural modification at room-temperature.Meanwhile,the sign change of the Hall coefficient is observed at 5 K.Our results demonstrate the tunability of CDW order in the pressurized LaAgSb_2 single crystal,which can be helpful for its potential applications in the next-generation devices.     

The electrical properties of HgTe and HgSe under very high pressure

The electrical properties of HgTe and HgSe have been investigated at pressures up to 200 kbar in an octahedral apparatus. Measurements of the electrical resistivity at room temperature showed that, beyond the well-known transition from the semimetallic to semiconductive state, both become metallic, at 84 kbar and 155 kbar, respectively. The energy gap at various fixed pressures was obtained from the resistance-temperature relationships. The energy gap of semiconducting HgTe decreases monotonically with pressure, the coefficient being ?l.53 × 10^?5$\text{eV}\text{bar}$. The energy gap of HgSe is rather insensitive to pressures up to 75 kbar, above which it decreases continuously ($\text{dE}\text{dP}$ = ?1.59 × 10^?5$\text{eV}\text{bar}$) before vanishing around 150 kbar. At high pressures the temperature coefficient of the resistance in the metallic state is 3.25 ~ 4.70 × $\text{10}{}^{\mathrm{?3}}\text{deg}$ for HgTe, and 5.7 ~ 5.9 × $\text{10}{}^{\mathrm{?3}}\text{deg}$ for HgSe.     

Structural and electrical transport properties of FeH x under high pressures and low temperatures

We present the first successful in situ simultaneous measurement of the electrical resistance and X-ray diffraction of FeH _x (x～ 1) under high-pressure H₂ up to 25.5 GPa and low temperatures down to 9 K. The electrical resistivity ρ showed a sharp increase with the formation of iron-hydride FeH _x (x～ 1) at 3.5 GPa. The ?′-phase of FeH _x was found to be metallic up to 25.5 GPa. The ρ vs. T curves up to 16.5 GPa approximately follow Fermi-liquid law below 25 K. However, T ⁵ was found to be better fitting at 25.5 GPa. This change can be considered to be related to the previously reported ferromagnetism collapse at corresponding pressure.     

First-principles calculations of structural stability and mechanical properties of tungsten carbide under high pressure

The structural stability and mechanical properties of WC in WC-, MoC- and NaCl-type structures under high pressure are investigated systematically by first-principles calculations. The calculated equilibrium lattice constants at zero pressure agree well with available experimental and theoretical results. The formation enthalpy indicates that the most stable WC is in WC-type, then MoC-type finally NaCl-type. By the elastic stability criteria, it is predicted that the three structures are all mechanically stable. The elastic constants C_ij, bulk modulus B, shear modulus G, Young?s modulus E and Poisson?s ratio ν of the three structures are studied in the pressure range from 0 to 100 GPa. Furthermore, by analyzing the B/G ratio, the brittle/ductile behavior under high pressure is assessed. Moreover, the elastic anisotropy of the three structures up to 100 GPa is also discussed in detail.     

"Devil's staircase"-type phase transition in NaV2O5 under high pressure

The "devil's staircase"-type phase transition in the quarter-filled spin-ladder compound NaV2O5 has been discovered at low temperature and high pressure by synchrotron radiation x-ray diffraction. A large number of transitions are found to successively take place among higher-order commensurate phases with 2a x 2b x zc type superstructures. The observed temperature and pressure dependence of modulation wave number q(c), defined by 1/z, is well reproduced by the axial next nearest neighbor Ising model. The q(c) is suggested to reflect atomic displacements presumably coupled with charge ordering in this system.     

极端条件下锆的动力学稳定性研究

过渡金属Zr具有优良的物理、化学及力学性能, 具有广泛的应用价值. 主要通过新近发展的自洽晶格动力学方法, 充分考虑声子间的相互作用, 成功获得了β-Zr的高温高压声子色散曲线, 预测了β-Zr在相图中能够稳定存在的区域, 进一步比较α-Zr, ω-Zr和β-Zr的自由能, 获得了α-β 及ω-β 相变的相边界, 构建了Zr的参考相图. 同时, 也获得了β-Zr的高温状态方程及热膨胀系数, 能够为构建Zr的全区物态方程提供有益的参考.     

高温高压下钼的结构稳定性研究

以钼为代表的一系列过渡金属,在高温高压的相变及结构稳定性研究是实验和理论研究的热点.钼在常温常压下是bcc结构,但是在高温高压下可能的相结构一直未能确定.本文首先预测了几种高压下的结构,并计算了其自由能及力学性质.针对可能的hcp结构,我们通过新近发展的自洽晶格动力学方法,充分考虑声子间相互作用,成功获得了hcp结构高温高压声子色散曲线,结果表明hcp相在热力学及动力学上都是能够稳定存在的结构,是一种可能的高压相.     

硼硫协同掺杂金刚石的高压合成与电学性能研究

FeNiMnCo-C体系中,在压力6.5 GPa、温度1280—1300℃的极端物理条件下,采用温度梯度法成功合成了硼(B)、硫(S)协同掺杂金刚石大单晶.通过傅里叶红外光谱测试对高温高压所制备金刚石中的杂质进行了表征.借助霍尔效应对典型金刚石样品的电输运性能进行了测试,测试结果表明:硼硫协同掺杂有利于提高p型金刚石的电导率,而且硼硫在合成体系中的添加比例可以决定金刚石的p, n特性.此外,第一性原理计算结果表明,合成体系中不同比例的硼硫协同掺杂对金刚石的p, n特性以及电导率有着直接的影响,计算结果与实验测试结果相吻合.     

Investigation of the electrical properties and stability of HfInZnO thin-film transistors

In this study, amorphous HfInZnO (a-HIZO) thin films and related thin-film transistors (TFTs) were fabricated using the RF-sputtering method. The effects of the sputtering power (50–200 W) on the structural, surface, electrical, and optical properties of the a-HIZO films and the performance and NBIS stability of the a-HIZO TFTs were investigated. The films’ N_e increased and resistivity decreased as the sputtering power increased. The 100 W deposited a-HIZO film exhibited good optical and electrical properties compared with other sputtering powers. Optimization of the 100 W deposited a-HIZO TFT demonstrated good device performance, including a desirable μ_FE of 19.5 cm²/Vs, low SS of 0.32 V/decade, low V_th of 0.8 V, and high I_on/I_off of 10⁷, respectively. The 100 W deposited a-HIZO TFT with Al₂O₃ PVL also exhibited the best stability, with small V_th shifts of -2.2 V during NBIS testing. These high-performance a-HIZO thin films and TFTs with Al₂O₃ PVL have practical applications in thin-film electronics.     

The superconducting and the electrical properties of single crystals Bi2Sr2CuCa2oXat high pressure

Abstract

We have investigated the effect of pressure on lattice parameters and Raman-active phonons of the molecular crystal xenon difluride by powder x-ray diffraction and first-order Raman scattering, respectively. The ambient pressure tetragonal crystal structure remains stable up to the maximum pressure of 50 GPa. The compressibility is found to be strongly anisotropic. The frequencies of the intramolecular A1_g stretching mode and the E_g libration increase sublinearly under pressure. ding the a-and c-axis compression data, we have determined the dependence of phonon frequencies on lattice parameters. The high-frequency A1_g mode shows a pronounced nonlinear scaling with c-axis lattice parameter, with particularly small changes during initial compression. This effect is attributed to the interplay between intra-and intermolecular interactions during compression     

Electronic and thermodynamic properties of ReB2 under high pressure and temperature

First-principles calculations of the crystal structures of rhenium diboride (ReB₂) have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The quasi-harmonic Debye model, using a set of total energy versus molar volume obtained with the first-principles calculations, is applied to the study of the thermal and vibrational effects. The structural parameters, thermal expansions, heat capacities, Grüneisen parameters and Debye temperatures dependence on the temperature and pressure are obtained in the whole pressure range from 0 to 70 GPa and temperature range from 0 to 2000 K as well as compared with available data.     

Zr4Al2高压下热动力学性质的第一性原理研究

本文主要通过平面波赝势密度泛函理论的第一性原理计算方法,采用广义梯度近似（GGA）交换相关函数,研究了六角结构的Zr₄Al₂的EOS态方程和热动力学性质.通过采用了声子效应的准谐近似德拜模型,我们成功得到了晶胞体积V随温度以及V/V₀、德拜温度（?）、热熔C_V随压强P的变化关系.另外,我们还研究了热膨胀系数α及格林参数γ随温度和压强的变化关系,计算结果表明格林参数和热膨胀系数随温度的升高而增加,但是在较高的温度下,热膨胀系数几乎不受温度的影响.     

Zr4Al2高压下热动力学性质的第一性原理研究

本文主要通过平面波赝势密度泛函理论的第一性原理计算方法,采用了广义梯度近似（GGA）交换相关函数,研究了六角结构的Zr4Al2的EOS态方程和热动力学性质。通过采用了声子效应的准谐近似德拜模型,我们成功得到了晶胞体积V随温度以及V/V0、德拜温度Θ、热熔CV 随压强P的变化关系。另外,我们还研究了热膨胀系数α及格林参数γ随温度和压强的变化关系,计算结果表明格林参数和热膨胀系数随温度的升高而增加,但是在较高的温度下,热膨胀系数几乎不受温度的影响。     

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.     

Structural stability of TiO and TiN under high pressure

The high pressure phase transition and elastic behavior of Transition Metal Compounds (TiO and TiN) which crystallize in NaCl-structure have been investigated using the three body potential model (TBPM) approach. These interactions arise due to the electron-shell deformation of the overlapping ions in crystals. The TBP model consists of a long range Coulomb, three body interactions, and the short-range overlap repulsive forces operative up to the second neighboring ions. The authors of this paper estimated the values of the phase transition pressures, associated volume collapses, and elastic constants, all of which were found to be closer to available experimental data than other calculations. Thus, the TBPM approach promises to predict the phase transition pressure and pressure variations of elastic constants of Transition Metal compounds.      

高压下LiF和NaF的结构稳定性及其电子和光学性质的第一性原理研究

基于密度泛函理论，采用全势线性缀加平面波加局域轨道方法，计算模拟了LiF高压下的相变行为，预测其在450GPa附近发生由NaCl结构(B1)到CsCl结构(B2)的结构相变.同时还计算了高压下LiF不同相的电学特性，LiF的复介电函数以及介电常数随压强变化关系.通过比较能带结构的变化行为，得出LiF在53GPa附近还存在等结构相变，即由直接带隙结构变为间接带隙结构.将LiF的计算结果与另外一个同构化合物NaF进行了比较讨论. 关键词： LiF 压致相变 从头计算     

First-principle study of phase stability,electronic structure and thermodynamic properties of cadmium sulfide under high pressure

By employing first principle and a quasi-harmonic Debye model, we study the phase stability, phase transition, electronic structure and thermodynamic properties of cadmium sulfide (CdS). The results indicate that CdS is a typical ionic crystal and that the zinc-blende phase in CdS is thermodynamically unstable. Moreover, the heat capacity of the wurtzite and rocksalt phases of CdS decreases with pressure and increases with temperature, obeying the rule of the Debye T³ law at low temperature and the Dulong–Petit limit at high temperature.     

高压下金属Ba的结构稳定性以及热动力学的第一原理研究

采用第一原理方法计算了高压下金属Ba的三个高压相 Ba-I, Ba-Ⅱ和Ba-V的稳定性及热动力学性质.结果表明, Ba的三个高压相在0 K时在其压力范围内都是动力学和力学稳定的;但随压力增加, Ba-I 和Ba-Ⅱ 的声子谱频率出现异常"软化",而Ba-V则出现"硬化".虽然 Ba-Ⅱ 和 Ba-V 同为六方密堆(hcp)结构,计算表明它们在高压下表现出了不同的弹性各向异性.计算同时发现 Ba-Ⅱ 在更高的压力下仍满足力学稳定条件,但声子谱有虚频存在, 表明动力学失稳是Ba-Ⅱ在压力下向Ba-I!V相转变的原因. 计算和比较了同为六方密堆(hcp)结构的Ba-Ⅱ和Ba-V在高压下的声速、 德拜温度、体模量、剪切模量等力学和热学性质, 展现了金属Ba在压力下的稳定机制和热动力学性质.