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.     

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.     

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.     

Structural stability of tin clathrates under high pressure

High pressure Raman scattering experiments have been performed for Rb₈Sn₄₄□₂ in order to investigate the pressure induced phase transition. At pressures of 6.0 and 7.5 GPa, Raman spectrum was drastically changed, indicating the phase transitions. The irreversibility of the spectral change and the disappearance of Raman peak observed at 7.5 GPa strongly suggest the occurrence of irreversible amorphization.     

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.      

Structural and elastic properties of TiN under high pressure

We have investigated the structural and elastic properties of TiN at high pressures by the first-principles plane wave pseudopotential density functional theory method at applied pressures up to 45.4 GPa. The obtained normalized volume dependence of the resulting pressure is in excellent agreement with the experimental data investigated using synchrotron radial x-ray diffraction (RXRD) under nonhydrostatic compression up to 45.4 GPa in a diamond-anvil cell. Three independent elastic constants at zero pressure and high pressure are calculated. From the obtained elastic constants, the bulk modulus, Young's modulus, shear modulus, acoustic velocity and Debye temperature as a function of the applied pressure are also successfully obtained.     

Structural stability and vibrational characteristics of CaB_6 under high pressure

In situ Raman spectroscopy and x-ray diffraction measurements are used to explore the structural stability of CaB_6 at high pressures and room temperature. The results show no evidence of structural phase transitions up to at least 40 GPa.The obtained equation of state with smooth pressure dependencies yields a zero-pressure isothermal bulk modulus B0=170(5) GPa, which agrees well with the previous measurements. The frequency shifts for A_(1g), Eg, and T_(2g) vibrational modes of polycrystalline CaB_6 are obtained with pressure uploading. As the pressure increases, all the vibration modes have smooth monotonic pressure dependence. The Gr¨uneisen parameter of Eg modes is the largest, indicating its largest dependence on the volume of a crystal lattice.     

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, electronic and vibrational properties of InN under high pressure

The structural, electronic and vibrational properties of InN under pressures up to 20 GPa have been investigated using the pseudo-potential plane wave method (PP-PW). The generalized-gradient approximation (GGA) in the frame of density functional theory (DFT) approach has been adopted. It is found that the transition from wurtzite (B4) to rocksalt (B1) phase occurs at a pressure of approximately 12.7 GPa. In addition, a change from a direct to an indirect band gap is observed. The mechanism of these changes is discussed. The phonon frequencies and densities of states (DOS) are derived using the linear response approach and density functional perturbation theory (DFPT). The properties of phonons are described by the harmonic approximation method. Our results show that phonons play an important role in the mechanism of phase transition and in the instability of B4 (wurtzite) just before the pressure of transition. At zero pressure our data agree well with recently reported experimental results.     

Structural stability of ultra-high temperature refractory material MoSi_2 and Mo_5Si_3 under high pressure

In-situ angle dispersive x-ray diffraction(ADXRD) with synchrotron radiation source is performed on an ultra-high temperature refractory of MoSi_2 and Mo_5Si_3 by using a diamond anvil cell(DAC) at room temperature. While the pressureinduced volume reduction is almost constant, the value of the bulk modulus increases with the decrease of molybdenum content in the system. According to the Brich–Murnaghan equation, the bulk modulus 222.1(2.1) GPa with its pressure derivative 4 of MoSi_2, and the bulk modulus 308.4(7.6) GPa with its pressure derivative 0.7(0.1) of Mo_5Si_3 are obtained.The experimental data show that MoSi_2 has distinct anisotropic behavior, Mo_5Si_3 is less anisotropic than MoSi_2. The result shows that MoSi_2 and Mo_5Si_3 have the structural stabilities under high pressure. When the pressure reaches up to 41.1 GPa, they can still maintain their body-cantered tetragonal structures.     

Structural, electronic, and magnetic properties of CrN under high pressure

The structural,electronic and magnetic properties of CrN under high pressure are investigated by first-principles calculations.The antiferromagnetic orthorhombic structure is identified to be the preferred ground state structure.It possesses a bulk modulus of 252.8 GPa and the nonzero magnetic moment of 2.33 μ B per Cr ion,which agree well with the experimental results.CrN undergoes structural and magnetic transitions from an antiferromagnetic rocksalt structure to a non-magnetic Pnma phase at 132 GPa.Under compression,the magnetic moment of the Cr ion reduces rapidly near the equilibrium and phase transition point,and the distribution of the density of states is broadened,but the form of overlap between the orbitals of Cr d and N p remains unchanged.The broadening of the band induces spin flipping,which consequently results in the smaller magnetic moment of the Cr ion.     

Structural stability and electronic properties of AgInS<Subscript>2</Subscript> under pressure

We employ state-of-the-art ab initio density functional theory techniques to investigatethe structural, dynamical, mechanical stability and electronic properties of the ternaryAgInS₂ compoundsunder pressure. Using cohesive energy and enthalpy, we found that from the six potentialphases explored, the chalcopyrite and the orthorhombic structures were very competitive aszero pressure phases. A pressure-induced phase transition occurs around 1.78 GPa from the low pressure chalcopyritephase to a rhombohedral RH-AgInS₂ phase. The pressure phase transition around 1.78 GPa isaccompanied by notable changes in the volume and bulk modulus. The calculations of thephonon dispersions and elastic constants at different pressures showed that thechalcopyrite and the orthorhombic structures remained stable at all the selected pressure(0, 1.78 and 2.5 GPa), where detailed calculations were performed, while the rhombohedralstructure is only stable from the transition pressure 1.78 GPa. Pressure effect on thebandgap is minimal due to the small range of pressure considered in this study. Themeta-GGA MBJ functional predicts bandgaps which are in good agreement with availableexperimental values.     

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.     

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.     

Structural transitions in NaNH_2 via recrystallization under high pressure

Multiple phase transitions are detected in sodium amide(NaNH_2), an important hydrogen storage material, upon compression in diamond anvil cells(DAC) by using Raman spectroscopy and x-ray diffraction(XRD) measurements.Additional Bragg reflections appear on lower and higher angle sides of the original ones at ～1.07 GPa and 1.84 GPa,accompanied by obvious changes in Raman spectroscopy, respectively.It reveals that NaNH_2 undergoes the high-pressure phase sequence(α–β–γ) up to 20 GPa at room temperature.Spectral analysis indicates an orthorhombic structure with PBAN space group for the γ phase.We also experimentally observe high pressure induced recrystallization in alkaline amide compounds for the first time.     

高温高压下钙钛矿晶体结构变化

最近高压单晶X射线衍射实验明确显示正交晶系（Pbnm或Pnma）（钙钛矿）ABO3的高压压缩行为完全由晶体框架结构（framework）中的BO6八面体和AO12多面体的相对压缩性所确定,即对一些钙钛矿晶体,BO6较AO12多面体更易压缩时,压力导致晶体结构向高对称性结构演变;对另外一些钙钛矿,BO6较AO12多面体更不易压缩时,压力导致晶体结构向低对称性结构演变.基于价键匹配关系假设,建立多面体压缩率之间关系：βB/βA=MA/MB,由价键参数和常压晶体结构数据计算参量Mi,可以估计BO6和AO12多面体的相对压缩率,从而正确地预言高压压缩行为.文章最后引入高温高压钙钛矿倾斜相变的一般性规律,即MA/MB＞1时,相变温度和高压关系为dTc/dP＜0,当MA/MB＜1时,dTc/dP＞0。     

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.