Structural stability,electronic structure and mechanical properties of 4d transition metal nitrides TMN (TM=Ru,Rh, Pd)

Ab initio calculations are performed to investigate the structural stability, electronic, structural and mechanical properties of 4d transition metal nitrides TMN (TM=Ru, Rh, Pd) for five different crystal structures, namely NaCl, CsCl, zinc blende, NiAs and wurtzite. Among the considered structures, zinc blende structure is found to be the most stable one among all three nitrides at normal pressure. A structural phase transition from ZB to NiAs phase is predicted at a pressure of 104 GPa, 50.5 GPa and 56 GPa for RuN, RhN and PdN respectively. The electronic structure reveals that these nitrides are metallic. The calculated elastic constants indicate that these nitrides are mechanically stable at ambient condition.     

First principles calculations of mechanical properties of cubic 5d transition metal monocarbides

The electronic and elastic properties of cubic 5d transition metal monocarbides in rocksalt, cesium chloride, and zinc blende structures have been studied by first principles calculations. The calculations show that the incompressibility for ReC in cesium chloride structure is even higher than that of diamond under pressure (above 89 GPa). The transformation pressure from zinc blende structure to rocksalt structure takes place at about 47 GPa for PtC. HfC-NaCl, ReC-CsCl, and HfC-ZnS have the smallest metallicity, leading to higher hardness. A valence electron number of 8/cell may be a stable valence shell configuration for 5d transition metal monocarbides in rocksalt and zinc blende structures.     

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     

纳米硫化锌球壳的高压相变研究

 采用同步辐射能量色散X射线衍射（EDEX）技术和金刚石对顶砧高压装置,对纳米硫化锌球壳进行了原位高压X射线衍射实验。最高压力达33.3 GPa。常压下纳米硫化锌球壳为纤锌矿结构和闪锌矿结构共存的混相结构。压力达到11.2 GPa时,纳米硫化锌空心球中的纤锌矿结构全部转变为闪锌矿结构。压力达到16.0 GPa时,发生了由闪锌矿结构向岩盐矿结构的相变,在17.5 GPa和21.0 GPa时分别出现未知峰,33.3 GPa时基本完全转变为岩盐矿结构。两个相变均为可逆相变。     

Volume compression of CuCl to 7 GPa

The unit cell volume of CuCl as a function of pressure has been measured up to 7 GPa (giga Pascals). The compression behaviour is quite normal. The analysis of the compression data gives 40·3±1·5 GPa for the bulk modulus of the zinc blende phase. The zinc blende phase transforms to a tetragonal phase at 5·5 GPa, the volume change associated with the transformation being 12%. A comparison of the bulk modulus of CuCl with those of CuBr and CuI indicates that an anomaly exists in this group.     

First principles study of the structural,electronic, mechanical and superconducting properties of WX (X=C,N)

The structural, electronic, mechanical and superconducting properties of tungsten carbide (WC) and tungsten nitride (WN) are investigated using first principles calculations based on density functional theory (DFT). The computed ground state properties, such as equilibrium lattice constant and cell volume, are in good agreement with the available experimental data. A pressure induced structural phase transition is observed in both tungsten carbide and nitride, from a tungsten carbide phase (WC) to a zinc blende phase (ZB), and from a zinc blende phase (ZB) to a wurtzite phase (WZ). The electronic structure reveals that these materials are metallic at ambient conditions. The calculated elastic constants obey the Born-Huang criteria, suggesting that they are mechanically stable at normal and high pressure. Also, the superconducting transition temperature is estimated for the WC and WN in stable structures at atmospheric pressure.     

High pressure structural phase transition and elastic properties of Ga<Subscript>1-x</Subscript>In<Subscript>x</Subscript>As semiconducting compounds

The present paper addresses the high-pressure phase transformation and mechanical properties of Ga_1-xIn_xAs (x = 0.25, 0.5 and 0.75) by formulating an effective interionic interaction potential. This potential consists of the long-range Coulomb and charge transfer caused by the deformation of the electron shells of the overlapping ions and the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbor ions and the van der Waals (vdW) interaction. The estimated values of phase transition pressure and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zinc blende (B3) to rock salt (B1). The equation of state curves plotted between V (P)/ V (0) and pressure are for both the zincblende (B3) and rocksalt (B1) structures. Further, the variations of the second and third order elastic constants with pressure have followed a systematic trend, which are almost identical to those exhibited by the observed data measured for other compounds of this family.     

Features of the semiconductor-metal transition in GaAs at ultrahigh pressures: New intermediate phases

Using the thermopower method (Seebeck effect), the semiconductor-metal transition that occurs in gallium arsenide single crystals of n and p types at ultrahigh pressures P above ～11–18 GPa has been studied. It has been found that the transition in n-type samples begins at lower pressures. In the region of the semiconductor-metal phase transition, features have been observed on the thermopower dependences S(P). These features indicate that lattices intermediate between the initial semiconductor structure of zinc blende and the Cmcm high-pressure orthorhombic metallic phase are formed. By analogy with ZnTe, one intermediate phase (semiconductor with hole conductivity) is suggested to have the cinnabar structure and the second intermediate phase (semimetallic with electron conductivity) possibly has the SC16 structure. A model of the semiconductor-metal transition is discussed. The behavior of the thermoelectric properties in GaAs under pressure is compared with the behavior of these properties in other A_NB_8?N semiconductors, which also undergo the transition to the metallic state.     

Electronic properties of cubic ScGaAs and ScGaN ternaries and superlattices

The electronic properties of both Sc_xGa_1−xAs and Sc_xGa_1−xN ternary alloy and superlattice systems are investigated within the first-principles full-potential linear muffin-tin orbitals method (FPLMTO) in its atomic sphere approximation (ASA) using the technique of empty spheres, which allows an accurate treatment of the interstitial regions. The phase transition from the rocksalt (B1) to the zinc blende (B3) structure is investigated and the possibility of zinc blende/zinc blende GaN/Sc_xGa_1−xN and GaAs/Sc_xGa_1−xAs superlattices is expected. Wide and direct gaps are found to be possible in these systems, predicting them as good candidates for optoelectronic applications.     

Pressure-induced structural phase transition in transition metal carbides TMC (TM = Ru,Rh, Pd,Os, Ir,Pt): a DFT study

First-principles calculations based on density functional theory was performed to analyse the structural stability of transition metal carbides TMC (TM = Ru, Rh, Pd, Os, Ir, Pt). It is observed that zinc-blende phase is the most stable one for these carbides. Pressure-induced structural phase transition from zinc blende to NiAs phase is predicted at the pressures of 248.5 GPa, 127 GPa and 142 GPa for OsC, IrC and PtC, respectively. The electronic structure reveals that RuC exhibits a semiconducting behaviour with an energy gap of 0.7056 eV. The high bulk modulus values of these carbides indicate that these metal carbides are super hard materials. The high B/G value predicts that the carbides are ductile in their most stable phase.     

Off‐resonance Raman analysis of wurtzite CdS ground to the nanoscale: structural and size‐related effects

Different techniques were used to follow the transformation of CdS platelets during grinding and under hydrostatic pressure. X‐ray diffraction and transmission electron microscopy revealed that the platelets included zinc blende (cubic) CdS nanodomains dispersed in a wurtzite (hexagonal) single‐crystalline matrix. Extended grinding led to a decrease of the grain size and to a progressive transformation of the hexagonal stacking into a cubic one. The same phase transition was observed up to 9 GPa under hydrostatic pressure. Off‐resonance Raman spectra collected at different stages of the transition led us to connect band groups that were usually overlooked (in resonance conditions) or considered separately. They all probe the stacking disorder and their intensity can be related to the density of stacking faults. Off‐resonance Raman spectroscopy offers a way of probing the optical properties of CdS (and, more generally, layered semiconductors) as a function of the structure and of the confinement of vibrations by structural defects. Copyright © 2011 John Wiley & Sons, Ltd.     

Structural phase transition and elastic properties of ZnSe at high pressure

We have evolved an effective interionic interaction potential to investigate the pressure-induced phase transitions from zinc blende (B3) to rock salt (B1) structure in II-VI [ZnSe] semiconductors. The elastic constants, including the long-range Coulomb and van der Waals (vdW) interactions and the short-range repulsive interaction of up to second-neighbor ions within the Hafemeister and Flygare approach, are deduced. Keeping in mind that both of the ions are polarisable, we employed the Slater-Kirkwood variational method to estimate the vdW coefficients. The estimated value of the phase transition pressure (P _t ) is higher than in the reported data, and the magnitude of the discontinuity in volume at the transition pressure is consistent with that data. The major volume discontinuity in the pressure-volume phase diagram identifies the structural phase transition from zinc blende to rock salt structure.

The variation of second-order elastic constants with pressure resembles that observed in some binary semiconductors. It is inferred that the vdW interaction is effective in obtaining the thermodynamic parameters such as the Debye temperature, the Gruneisen parameter, the thermal expansion coefficient and the compressibility. However, the inconsistency between the thermodynamic parameters as obtained from present model calculations and their experimental values is attributed to the fact that we have derived our expressions by assuming the overlap repulsion to be significant only up to the nearest second-neighbor ions, as well as neglecting thermal effects. It is thus argued that full analysis of the many physical interactions that are essential to binary semiconductors will lead to a consistent explanation of the structural and elastic properties of II–VI semiconductors.     

Study of structural stabilities and optical properties of HgTe under high pressure

A theoretical investigation on the structural stabilities and electronic properties of HgTe under high pressure was conducted using first principles based on density functional theory. Our results demonstrate that the sequence of the pressure-induced phase transitions of HgTe is from the zinc blende, to cinnabar, rocksalt, orthorhombic, and CsCl-type structures. The pressure effects on the optical properties were discussed and compared with previous calculations and experimental data whenever available.     

BeO高压相变和声子谱的第一性原理计算

采用第一性原理方法计算了BeO在零温时的高压相变和三种结构在零温零压时的声子谱.相变的计算表明,在122GPa左右的压力下BeO会发生从纤锌矿(B4)结构到氯化钠(B1)结构的相变,而闪锌矿(B3)结构在零温零压下是一种可能的亚稳态结构.采用冷声子方法计算了这三种结构的BeO在零温零压下的声子谱.计算结果表明:B1结构在零温零压下是一种不稳定的结构;尽管B4结构和B3结构具有明显的相似性,仍然可以通过声子谱来很好的区分.最后根据准简谐近似理论计算得到了BeO的高温高压相图.     

Structural properties of silver iodide and copper iodide

The structural changes within the Silver iodide (AgI) and Copper iodide (CuI) induced by pressure have been investigated using an effective interaction potential. CuI and AgI in their parent zinc blende (ZnS) to rock salt (NaCl) through an intermediate structure have been reported. The calculated values for the phase transition pressures and associate volume collapses are generally in good agreement with measured data.      

ZnTe结构相变、电子结构和光学性质的研究

运用密度泛函理论体系下的投影缀加波方法, 对闪锌矿和朱砂相结构的ZnTe在高压下的状态方程和结构相变进行了研究, 并分析了相变前后的原胞体积、电子结构和光学性质. 结果表明: 闪锌矿结构转变为朱砂相结构的相变压力为8.6 GPa, 并没有出现类似材料高压导致的金属化现象, 而是表现出间接带隙半导体特性. 相变后, 朱砂相结构Zn和Te原子态密度分布均向低能级方向移动, 带隙变小; 轨道杂化增强, 更有利于Te 5p与Zn 3d间的电子跃迁, 介电常数虚部主峰明显增强, 但宏观介电常数不受压力的影响.     

Effects of pressure on the carrier behavior of HgSe

In situ Hall effect measurement of HgSe was conducted up to 31?GPa. It was found that the carrier parameters changed discontinuously at each phase transition. The resistivity variation under compression was described by the carrier parameters. The decrease in cinnabar mobility indicates that the interaction between the helical chains becomes stronger under pressure. Combining the results of experiment and theory calculation, it can be concluded that in the phase transition process from zinc blende through cinnabar to rock salt, if Hg atom is mainly displaced , hole will be generated and its concentration will increase; on the contrary, if Se atom is mainly displaced, electrons will be generated and their concentration will increase.     

Multiple grains in nanocrystals: effect of initial shape and size on transformed structures under pressure

Pressure-induced structural transformations in spherical and faceted gallium arsenide nanocrystals of various shapes and sizes are investigated with a parallel molecular-dynamics approach. The results show that the pressure for zinc blende to rocksalt structural transformation depends on the nanocrystal size, and all nanocrystals undergo nonuniform deformation during the transformation. Spherical nanocrystals above a critical diameter >/=44 A transform with grain boundaries. Faceted nanocrystals of comparable size have grain boundaries in 60% of the cases, whereas the other 40% are free of grain boundaries. The structure of transformed nanocrystals shows that domain orientation and strain relative to the initial zinc blende lattice are not equivalent. These observations may have implications in interpreting the experimental x-ray line shapes from transformed nanocrystals.     

Mn effect on wurtzite-to-cubic phase transformation in ZnO

Mn doping effect on a wurtzite-to-cubic phase transformation in ZnO has been investigated by in situ high pressure X-ray powder diffraction using synchrotron radiation. Unit cell expansion is clearly observed in Mn-doped ZnO samples. Mn ions sit at Zn site in the wurtzite structure. The onset transition pressure for the wurtzite-to-cubic phase transformation decreases from about 9.5 GPa for pure ZnO to 6 GPa for sintered 2at.% Mn-doped ZnO while the compressibility and volume collapse at transition pressures are not sensitive to the Mn doping in the wurtzite phase. The doping of Mn ions in ZnO increases the onset transition pressure for the cubic-to-wurtzite phase transformation. The results could be explained by a reduction of phase transformation barriers for both transition paths by the Mn doping. The observation of reduction of the wurtzite-to-cubic phase transformation pressure might point out a new direction to synthesize cubic wurtzite phase of ZnO by doping transition element(s).     

The pressure influence on the incommensurate-commensurate ferroelectric phase transition in [4-NH2C5H4NH][SbCl4]

The dielectric properties of the [4-NH₂C₅H₄NH] SbCl₄ (abbreviated as 4-APCA) crystal were investigated under hydrostatic pressure up to 300 Mpa. The pressure-temperature phase diagram was given. The paraelectric-ferroelectric phase transition (II→III) temperature (T_c) increases linearly with increasing pressure with a slope dT_c/dp=21×10⁻² K/MPa. The pressure dependence of Curie-Weiss constants has been evaluated also. In the paraelectric phase (II) the Curie constant (C₊) was pressure dependent whereas the C₋ constant over the ferroelectric phase (III) was almost constant. The results are interpreted in terms of improper and displacive type phase transition model with a soft phonon at a zone boundary.