High pressure phase transition and elastic properties of cerium chalcogenides and pnictides

The structural properties of Cerium mono-chalcogenides and mono-pnictides have been investigated for the first time by using a modified interionic potential theory. The calculated transition pressures are in good agreement with the experimental results. The ionic bonding is found to be more pronounced in Ce-mono-chalcogenides than mono-pnictides. The calculated values of elastic constants are also predicted for the first time.     

Pressure-induced structural phase transformation and elastic properties of transition metal mononitrides

The high-pressure structural phase transition in six transition metal mononitrides (TMNs) (M=Ti, Mo, V, Nb, Hf, and Zr), have been studied using a two-body interionic potential theory which includes the effect of Coulomb screening due to the semi-metallic nature of these compounds. The present theoretical results have been compared with the corresponding experimental and predictions of LDA theory. These TMN compounds have been found to undergo NaCl (B₁) to CsCl (B₂) phase transition, at a pressure quite high as compared to other binary systems. We have also predicted the elastic constants. It is shown that these binary materials are partially ionic in nature and the structural transformation is analogous to several other ionic binary systems.     

Pressure-induced phase transition and structural properties of CrO2

The structural properties and pressure-induced phase transitions of CrO₂ have been investigated using the pseudopotential plane-wave method based on the density functional theory (DFT). The rutile-type (P4₂/mnm), CaCl₂-type (Pnnm), pyrite-type (Pā3), and CaF₂-type (Fm-3m) phases of CrO₂ have been considered. The structural properties such as lattice parameters, bulk moduli and its pressure derivative are consistent with the available experimental data. The second-order phase-transition pressure of CrO₂ from the rutile phase to CaCl₂ phase is 10.9?GPa, which is in good agreement with the experimental result. The sequence of these phases is rutile-type?→?CaCl₂-type?→?pyrite-type?→?CaF₂-type with the phase-transition pressures 10.9, 23.9, and 144.5?GPa, respectively. The equation of state of different phases has also been presented. It is more difficult to compress with the increase of pressure for different phases of CrO₂.     

High pressure phase transition and elastic properties of thorium chalcogenides

The structural and elastic properties of thorium chalcogenides at high pressure, have been investigated using a suitable inter-ionic potential. The calculated equation of state, phase transition pressures for B1-B2 transition and bulk moduli for ThX (X=S,Se,Te) compounds agree well with the experimental results. ThTe, which crystallizes in the CsCl structure, does not show any structural transition up to 48 GPa. The present analysis does not show any anomalous features in elastic properties arising from ‘f’ electrons.     

Phonon properties of rare earth ytterbium pnictides

We report for the first time the complete phonon dispersion curves for the ytterbium pnictide compounds (YbN, YbP and YbAs) using a breathing shell model to establish their predominant ionic nature. The calculated results also show that this group of rare earth compounds does not show any elastic and phonon anomalies which are the characteristic features of other rare earth compounds. We emphasize the need for further Raman and neutron scattering measurements.     

Structural phase transition and elastic properties of thorium pnictides at high pressure

In the present paper we have pointed out the weaknesses of the approach by Aynyas et al [1] to study the structural phase transition and elastic properties of thorium pnictides. The calculated values of phase transition pressure and other elastic properties using the realistic and actual approach are also given and compared with the experimental and previous theoretical work.      

Structural phase transition and elastic properties of cerium chalcogenides at high pressure

The structural and elastic properties of cerium chalcogenides (CeZ, Z = S, Se, Te) under high pressure have been investigated by using the potential model considered up to third nearest neighbor interaction. The computed values of B1-B2 phase transition pressure, equation of state (compression curve), bulk modulus, its first order pressure derivative and elastic constants in the case of cerium chalcogenides agree well with the experimental results. The present study shows the anomalous behavior of cerium chalcogenides in comparison to the alkaline earth chalcogenides, due to the presence of Kondo effect and reentrant valence behavior of Ce in cerium chalcogenides.     

Phase transition,structural, mechanical and thermal properties of erbium pnictides under pressure

In this article, we have investigated the high-pressure structural phase transition of erbium pnictides (ErX; X?=?N, P and As). An extended interaction potential model has been developed (including the zero-point energy effect in three-body interaction potential model). Phase transition pressures are associated with a sudden collapse in volume. The phase transition pressures and associated volume collapses have been predicted successfully. The elastic constants, their combinations and pressure derivatives are also reported. The pressure behaviour of elastic constants, bulk modulus and shear modulus have been presented and discussed. Moreover, the thermophysical properties such as molecular force constant (f), infrared absorption frequency (υ ₀), Debye temperature (θ _D) and Grunneisen parameter (γ) have also been predicted.     

Pressure-induced structural transition and thermodynamic properties of RhN2 and the effect of metallic bonding on its hardness

The elastic constant, structural phase transition, and effect of metallic bonding on the hardness of RhN2 under high pressure are investigated through the first-principles calculation by means of the pseudopotential plane-wave method. Three structures are chosen to investigate for RhN2, namely, simple hexagonal P6/mmm （denoted as SH）, orthorhombic Pnnm （marcasite）, and simple tetragonal P4/mbm （denoted as ST）. Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure. On the basis of the third-order Birch Murnaghan equation of states, we find that the phase transition pressures from an SH to a marcasite structure and from a marcasite to an ST structure are 1.09 GPa and 354.57 GPa, respectively. Elastic constants, formation enthalpies, shear modulus, Young＇s modulus, and Debye temperature of RhN2 are derived. The calculated values are, generally speaking, in good agreement with the previous theoretical results. Meanwhile, it is found that the pressure has an important influence on physical properties. Moreover, the effect of metallic bonding on the hardness of RhN2 is investigated. This is a quantitative investigation on the structural properties of RhN2, and it still awaits experimental confirmation.     

Pressure-induced structural transition and thermodynamic properties of RhN2 and effect of metallic bonding on its hardness

The elastic constant, structural phase transition, and effect of metallic bonding on the hardness of RhN₂ under high pressure are investigated through the first principles calculation by means of the pseudopotential plane-waves method. Three structures are chosen to investigate for RhN₂, namely, simple hexagonal P6/mmm (denoted as SH), orthorhombic Pnnm (marcasite), and simple tetragonal P4/mbm (denoted as ST). Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure. On the basis of the third-order Birch-Murnaghan equation of states, we find that phase transition pressures from SH to marcasite structure and from marcasite to ST structure are 1.09 GPa and 354.57 GPa, respectively. Elastic constants, formation enthalpies, shear modulus, Young's modulus, and Debye temperature of RhN₂ are derived. The calculated values are, generally speaking, in good agreement with the previous theoretical results. Meanwhile, it is found that the pressure has an important influence on physical properties. Moreover, the effect of metallic bonding on the hardness of RhN₂ is investigated. This is a quantitative investigation on the structural properties of RhN₂, and it still awaits experimental confirmation.     

Elastic band structures of two-dimensional solid phononic crystal with negative Poisson's ratios

In this paper, the elastic band structures of two-dimensional solid phononic crystals (PCs) with both negative and positive Poisson's ratios are investigated based on the finite difference domain method. Systems with different combinations of mass density ratio and shear modulus ratio, filling fractions and lattices are considered. The numerical results show that for the PCs with both large mass density ratio and shear modulus ratio, the first bandgap becomes narrower with its upper edge becoming lower as Poisson's ratio of the scatterers decreases from −0.1 to −0.9. Generally, introducing the material with a negative Poisson's ratio for scatterers will make this bandgap lower and narrower. For the PCs with large mass density ratio and small shear modulus ratio, the first bandgap becomes wider with Poisson's ratio of the scatterers decreasing and that of the host increasing. It is easy to obtain a wide low-frequency bandgap by embedding scatterers with a negative Poisson's ratio into the host with a positive Poisson's ratio. The PCs with large filling fractions are more sensitive to the variations of Poisson's ratios. Use of negative Poisson's ratio provides us a way of tuning bandgaps.     

压力下氮化铝相转变及结构性质的第一性原理研究

利用密度泛函理论(DFT)研究了AlN的六角纤锌矿结构(B4),岩盐矿结构(B1),过渡态中间相六方结构(Hexa)和过渡态中间相四方结构(Tetra),计算了AlN在不同压力下B4和B1结构和过渡态中间相六方结构和四方结构的焓值,计算发现B4和B1相的转变压力是17.27 GPa,低压区中间相六方结构稳定,高压区中间相四方结构更稳定,AlN的常见的B4结构是直接带隙结构,带隙宽度是4.095 eV,带隙宽度与外压力之间关系符合二次函数方程,与其它理论研究结果一致.     

First principles study of structural,elastic and electronic structural properties of strontium chalcogenides

Structural, elastic and electronic properties of strontium chalcogenides SrX (X = O, S and Se) in the B1 (NaCl) and B2 (CsCl) phases were investigated in the present work. The calculations were performed using density functional theory (DFT) within generalized gradient approximation (GGA) using scalar relativistic Vanderbilt-type ultrasoft pseudopotentials. Results for structural properties of both phases, the pressure at which transition from B1 to B2 phase occurs and the volume compression ratio for each compound were reported. Elastic properties of the B1 phase of these compounds, such as elastic constants C₁₁, C₁₂, and C₄₄, shear modulus (G), Young's modulus (E), Poisson's ratio (σ), Kleinman parameter (ξ), and anisotropy factor (A) were also calculated at ambient conditions. The band gaps and density of states were studied too for the B1 structure of these compounds. The present results were compared with the available experimental and other theoretical results, and found to be in satisfactory agreement with them.     

Pressure-induced structural transition and thermodynamic properties of RhN₂ and the effect of metallic bonding on its hardness

The elastic constant,structural phase transition,and effect of metallic bonding on the hardness of RhN 2 under high pressure are investigated through the first-principles calculation by means of the pseudopotential plane-wave method.Three structures are chosen to investigate for RhN 2,namely,simple hexagonal P6/mmm(denoted as SH),orthorhombic Pnnm(marcasite),and simple tetragonal P4/mbm(denoted as ST).Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure.On the basis of the third-order Birch-Murnaghan equation of states,we find that the phase transition pressures from an SH to a marcasite structure and from a marcasite to an ST structure are 1.09 GPa and 354.57 GPa,respectively.Elastic constants,formation enthalpies,shear modulus,Young’s modulus,and Debye temperature of RhN 2 are derived.The calculated values are,generally speaking,in good agreement with the previous theoretical results.Meanwhile,it is found that the pressure has an important influence on physical properties.Moreover,the effect of metallic bonding on the hardness of RhN 2 is investigated.This is a quantitative investigation on the structural properties of RhN 2,and it still awaits experimental confirmation.     

稀土金属间化合物RFe2Zn20-xInx的结构属性研究

RFe2Zn20(R代表稀土元素)型稀土金属间化合物因其低稀土含量和良好的铁磁性,已成为铁磁材料的研究热点之一.添加第四组元对该系列化合物的晶体结构和材料性能会产生一定影响.利用晶格反演方法获得了一系列有效的原子间相互作用势,对三元RFe2Zn20和四元RFe2Zn20-xInx化合物进行原子级模拟计算.研究表明,随着稀土元素原子量的增加,三元体系的晶格参数和体积呈线性下降,第四组元引入与否对该线性关系无直接影响.第四组元In替代Zn时,择优占据16c晶位,占满16c后选择占据96g晶位,始终不占据48f晶位.择优占位的结论符合实验观测,并与晶格反演势分析的结果一致.     

Theoretical study of the structural phase transition and elastic properties of HfN under high pressures

The effect of hydrostatic pressure on the structures of HfN at 0 K was investigated by using the projector augmented wave (PAW) within the Perdew–Burke–Ernzerhof (PBE) form of the generalized gradient approximation (GGA). The transition pressure between NaCl (B1) and CsCl (B2) structures is predicted to be 277.3 GPa. This value is consistent with that reported by Kroll, while in contrast to the results obtained by Ojha et al. and Meenaatci et al. Moreover, the elastic properties of B1-HfN and B2-HfN under high pressures are successfully obtained. It is found that the elastic constants, bulk modulus B, shear modulus G, compressional and shear wave velocities increase monotonically with increasing pressure. The Debye temperature Θ calculated from the elastic constants of HfN is in good agreement with the experimental values. The anisotropies of B1-HfN and B2-HfN at zero pressure have also been discussed.     

氮化铂在高压下的相变及弹性性质的第一原理计算

用PBE形式下的广义梯度近似（GGA）赝势平面波方法研究了氮化铂的结构相变以及弹性性质，计算了氮化铂的氯化钠（B1）、氯化铯（B2）、闪锌矿（B3）、纤维矿（B4）等四种结构并应用高压下的焓与压强的关系，得出在常温常压下B4结构是最稳定的结构，这与Yu 等人得的结果一致，且 B4→B1及B1→B2的相变压强分别发生在36.7 GPa和 185.4 GPa，同时，研究了B4结构在高压的弹性性质，发现弹性常数、体模量、剪切模量、压缩波速、剪切波速以及德拜温度均随着压强的增大而单调增大     

氮化铂在高压下的相变及弹性性质的第一原理计算

用PBE形式下的广义梯度近似（GGA）赝势平面波方法研究了氮化铂的结构相变以及弹性性质,计算了氮化铂的氯化钠（B1）、氯化铯（B2）、闪锌矿（B3）、纤维矿（B4）等四种结构并应用高压下的焓与压强的关系,得出在常温常压下B4结构是最稳定的结构,这与Yu 等人得的结果一致,且 B4→B1及B1→B2的相变压强分别发生在36.7 GPa和 185.4 GPa,同时,研究了B4结构在高压的弹性性质,发现弹性常数、体模量、剪切模量、压缩波速、剪切波速以及德拜温度均随着压强的增大而单调增大     

Structural phase stability and elastic behavior of III-V scandium pnictides

A modified interaction potential (MIPM) model (including the covalency effect) has been developed and applied for the first time to investigate the high-pressure structural phase transition of scandium pnictides (ScAs and ScSb). Phase transition pressures are associated with a sudden collapse in volume indicating the occurrence of first order phase transition. The phase transition pressures and associated volume collapses obtained from present potential model show a generally better agreement with available experimental data than others. The elastic constants and their pressure derivatives are also reported. Moreover, the thermo physical properties have also been obtained successfully. Our results are in good agreement with available experimental and theoretical data.