Elastic and phonon properties of FeSi and CoSi in the B2 structure

First principles calculations of structural, electronic, elastic, and phonon properties of the intermetallic compounds FeSi and CoSi in the B2 (CsCl) structure are presented, using the pseudopotential plane-wave approach based on density functional theory, within the local density approximation. The optimized lattice constants, independent elastic constants, bulk modulus, and first-order pressure derivative of the bulk modulus are reported for the B2 structure and compared with earlier experimental and theoretical calculations. A linear-response approach to density functional theory is used to derive the phonon dispersion curves, and the vibrational partial and total density of states. Atomic displacement patterns for FeSi at the Γ, X, and R symmetry points are presented. The calculated zone-center optical phonon mode for FeSi is in good agreement with experimental and theoretical data.     

First-principles study of elastic and electronic properties of layered ternary nitride SrZrN2 under pressure

The structural, elastic, and electronic properties of SrZrN₂ under pressure up to 100?GPa have been carried out with first-principles calculations based on density functional theory. The calculated lattice parameters at 0?GPa and 0?K by using the GGA-PW91-ultrasoft method are in good agreement with the available experimental data and other previous theoretical calculations. The pressure dependence of the elastic constants and the elastic-dependent properties of SrZrN₂, such as bulk modulus B, shear modulus G, Young's modulus E, Debye temperature Θ, shear and longitudinal wave velocity V_S and V_L, are also successfully obtained. It is found that all elastic constants increase monotonically with pressure. When the pressure increases up to 140?GPa, the obtained elastic constants do not satisfy the mechanical stability criteria and a phase transition might has occurred. Moreover, the anisotropy of the directional-dependent Young's modulus and the linear compressibility under different pressures are analysed for the first time. Finally, the pressure dependence of the total and partial densities of states and the bonding property of SrZrN₂ are also investigated.     

纳米晶锐钛矿相TiO2的非简谐效应和声子局域

对晶粒尺寸为194,86和56nm的纳米晶锐钛矿相TiO₂,进行了从83到723K的变温拉曼散射测量,并对E_g(1)模式进行了详细研究.根据非简谐效应和声子局域模型,对E_g(1)拉曼峰进行了拟合与计算.结果表明,以上三种纳米晶粒的晶格振动机理,在本质上是相同的.三声子过程对频率蓝移起主要作用.为了得到很好的拟合,需要同时考虑三声子和四声子过程.随着温度的升高,四声子过程增强,并对三声子过程起抵消作用.与非简谐衰减相关的声子寿命随着晶粒 关键词： 2')" href="#">纳米晶TiO₂ 拉曼散射 非简谐耦合 声子局域     

Thermal annealing effects on the magnetic behavior of Ce2NiSi3

We have investigated the crystallographic, magnetic and thermodynamic properties of the as-cast and annealed Ce₂NiSi₃ alloys, crystallizing in the AlB₂-type hexagonal structure. The DC-magnetic susceptibility data show that the as-cast sample exhibits an antiferromagnetic (AFM) ordering below T_N= 3.8 K, whereas the annealed sample shows, at 4.2 K, a magnetic transition of AFM nature and, around 2.5 K, an additional anomaly. The specific heat shows a peak with at 3.8 K for the as cast sample, which shifts to lower temperatures when the magnetic field increases, consistent with the antiferromagnetic nature of the transition. On the other hand, in the annealed alloy, the maximum of the specific heat peak reaches at 4.2 K, and no additional anomalies were observed. The different magnetic behavior between the as-cast and annealed samples is attributed to thermal effects on the structural disorder of nickel and silicon atoms, as already observed in other isotypic R₂TSi₃ alloys, where R=U or Ce, and T= transition metal.     

The electronic properties tuned by the phase transition between the semiconducting and metallic phase of monolayer MoS2/WS2

Using first-principles methods, we systematically investigate the electronic properties and atomic mechanism of the monolayer MoS₂/WS₂ homo-junction structure, which contains different phase structures, either the semiconducting hexagonal (H) structure or metallic trigonal (T) structure. Through tuning the size of the lateral homo-junction structure of either MoS₂ or WS₂, it can produce different boundaries which induce different phase transferred styles. More interestingly, the electronic structures of homo-junction structures can also be tuned by changing the size of the armchair and zigzag shapes of nanoribbons. The homo-junction structure of either MoS₂ or WS₂ exhibits alterable band structure and band edge position with the changing of the size. The strong dependence of the band offset on the sizes of the homo-junction monolayer also implicates a possible way of patterning quantum structures with size engineering.     

First-principles study of structural,elastic, electronic and thermodynamic properties of topological insulator Bi2Se3 under pressure

The structural, elastic, electronic and thermodynamic properties of the rhombohedral topological insulator Bi₂Se₃ are investigated by the generalized gradient approximation (GGA) with the Wu–Cohen (WC) exchange-correlation functional. The calculated lattice constants agree well with the available experimental and other theoretical data. Our GGA calculations indicate that Bi₂Se₃ is a 3D topological insulator with a band gap of 0.287 eV, which are well consistent with the experimental value of 0.3 eV. The pressure dependence of the elastic constants C_ij, bulk modulus B, shear modulus G, Young’s modulus E, and Poisson’s ratio σ of Bi₂Se₃ are also obtained successfully. The bulk modulus obtained from elastic constants is 53.5 GPa, which agrees well with the experimental value of 53 GPa. We also investigate the shear sound velocity V_S, longitudinal sound velocity V_L, and Debye temperature Θ_E from our elastic constants, as well as the thermodynamic properties from quasi-harmonic Debye model. We obtain that the heat capacity C_v and the thermal expansion coefficient α at 0 GPa and 300 K are 120.78 J mol^?1 K^?1 and 4.70 × 10^?5 K^?1, respectively.     

Mechanical and electronic properties of endofullerene Ne@C60 studied via structure distortions

Elastic, strength, electronic properties and vibrational spectra of Ne@C₆₀ (I _h) in its ground electronic state (X ¹A_g) were investigated with density functional theory at B3PW91/6-31G level via structure distortions. The elastic properties were obtained from the potential energy curves (PECs) in all of the five independent distortional directions of the molecule with symmetries of 1. D _5d, 2. D _3d, 3. D _2h, 4. C _2h(1) and 5. C _2h(2). PECs were examined where the structure of Ne@C₆₀ was destroyed. The necessary energies to destroy the structure were thus obtained, which illuminated the stability of Ne@C₆₀. PECs were found to be anisotropic and were accurately fitted to polynomials. Elongations in the direction of D _5d and compression in D _2h encountered potential energy surface cross-linkages, which might be considered as a single electron pump for further application in the design of single electron devices. Time-dependent B3PW91/6-31G analysis predicted significant electronic spectra changes associated with structure distortions. Similarities and differences of the properties were compared with those in C₆₀ and He@C₆₀.     

First-principle study on the structural and electronic properties of H2S and SO2 adsorption on Pd-doped MoS2 monolayer

The partial discharge in SF₆-insulated equipment produces characteristic decomposition products: SO₂ and H₂S. The characteristic decomposition products vastly speed up the process of discharge faults. Based on density functional theory (DFT) calculation, single layer Pd-doped MoS₂ (Pd-MoS₂) is adopted as the adsorbent to adsorb SO₂ and H₂S to ensure the operational stability of SF₆-insulated equipment. The adsorption energy, charge transfer and structure parameters of SF₆, H₂S, and SO₂ adsorption on the Pd-MoS₂ monolayer are analysed to find the most stable adsorption structure. The molecular orbital theory, total density of states and partial density of states are studied to analyse the adsorption mechanism. The results show that Pd-MoS₂ adsorbent possesses high catalytic activity and excellent adsorption performance to H₂S and SO₂ by strong chemical adsorption. This study is of great significance to ensure the operational stability of SF6-insulated equipment by removing these characteristic decomposition products.     

UO2的电子结构及光学性质的第一性原理研究

采用局域自旋密度近似 (LSDA)和有效库仑相关能 (U) 方法研究了UO₂的晶格参数、能带结构和光学常数. 计算得到的UO₂晶体的晶格常数为5.40 ?,带隙宽度为1.82 eV,正确预测了UO₂的反铁磁性半导体基态性质. 能带结构和介电函数的分析结果表明,铀的6d电子在晶体场中发生劈裂形成两个能级,与实验结果较为符合.     

Yb:CaF2-SrF2激光晶体光谱性能以及热学性能的研究

采用坩埚下降法生长了Yb: CaF₂-SrF₂晶体,测试了该晶体的吸收和荧光光谱 以及在不同温度下晶体的热扩散系数和热膨胀系数,并且计算了晶体的热膨胀系数以及在常温下的热导率. 采用对比的方法,对晶体的吸收光谱,荧光光谱,热学性能进行了分析.从吸收和荧光光谱结果表明: 在掺杂相对较高浓度的SrF₂的混晶中, Yb³⁺吸收截面和发射截面比较大. Yb: CaF₂-SrF₂ (19%)晶体在1040 nm附近的发射截面比较大,光谱也比较宽. 这说明在掺杂相同浓度Yb时,混晶中CaF₂, SrF₂的比例不同,晶体的光谱性质不同, 主要原因是在混晶中晶体的无序度不同,晶体对称性降低,形成低对称光学中心. 从热扩散系数计算的热导率结果看出晶体具有比较好的热导率.     

Strain-tunable electronic,elastic, and optical properties of CaI2 monolayer: first-principles study

ABSTRACT

The effects of biaxial strain on the electronic structure and the elastic and optical properties of monolayer CaI₂ were studied using first-principles calculations. The two-dimensional (2D) equation of state for monolayer CaI₂ as fit in a relative area of 80–120% is more accurate. The band gap can be tuned under strain and reached a maximum at a tensile strain of 4%. Under compressive strains, the absorption spectrum showed a significant red shift at higher strains. The static reflectance and static refractive index decreased in the strain range of ?10% to 10%.     

Stability, structural, elastic, and electronic properties of polymorphs of the superconducting disilicide YIr2Si2

Using the ab initio approaches, the comparative stability, structural, elastic, and electronic properties of three polymorphs of the superconducting disilicide YIr₂Si₂, which differ in the atomic configurations of [Ir₂Si₂] (or [Si₂Ir₂]) blocks, were examined. For these YIr₂Si₂ polymorphs, the optimized structural data, elastic parameters, electronic bands, total and partial densities of states, Fermi surface topology, and chemical bonding were obtained and analyzed. Our studies showed that although ThCr₂Si₂- and CaBe₂Ge₂-type polymorphs are mechanically stable and relatively hard materials with low compressibility, they will behave as ductile systems. Among them, ThCr₂Si₂-type polymorph will show enhanced elastic anisotropy. In the vicinity of the Fermi energy, the topology of the electronic bands and the Fermi surface for various polymorphs are quite different. Besides, the CaBe₂Ge₂-type polymorph is expected to be anisotropic, i.e. happening mainly in the [Si₂Ir₂] blocks. The inter-atomic bonding for YIr₂Si₂ polymorph phases can be described as an anisotropic mixture of covalent, metallic, and ionic contributions, where inside the [Ir₂Si₂] (or [Si₂Ir₂]) blocks, Ir-Si and Ir-Ir bonds take place, whereas between the adjacent [Ir₂Si₂] (or [Si₂Ir₂]) blocks and Y atomic sheets, Si-Si and Ir-Y, Si-Ir and Si-Y, or mainly Ir-Ir bonds emerge for various polymorphs.     

XF2(X=B,N)分子基态的结构与势能函数

基于Gaussian03计算软件利用QCISD方法,选用不同基组对XF₂(X=B,N)分子基态结构进行了几何优化,在此基础上选出最优基组D95(df,pd)和D95+(df,pd)分别对BF₂和NF₂分子的谐振频率、力常数等进行了计算.推导出XF₂(X=B,N)分子基态的多体展式势能函数,同时根据势能函数绘制了XF₂(X< 关键词： 2')" href="#">BF₂ 2')" href="#">NF₂ 结构 势能函数     

First-principles studies of the structural,elastic, and lattice dynamical properties of ZrMo2 and HfMo2

A comprehensive investigation of the structural, elastic, and lattice dynamical properties for ZrMo₂ and HfMo₂ with C14, C15, C36, and CeCu₂ phases are conducted using density functional total energy calculations. The results have showed that C15 phase for both materials is energetically more stable than C14, C36 and CeCu₂ phases. We have also estimated the mechanical behaviours of these compounds, including mechanical stability, bulk modulus, Young's modulus, shear modulus, Poisson's ratio, ductility, and anisotropy. Additionally, the lattice dynamical properties are analyzed and discussed exhaustively for these phases. The calculated properties agree well with available experimental and theoretical data.     

Elastic constants and anisotropy of RuB2 under pressure

The structural, elastic constants and anisotropy of RuB2 under pressure are investigated by first-principles calcula-tions based on the plane wave pseudopotential density functional theory method within the local density approximation (LDA) as well as the generalized gradient approximation (GGA) for exchange and correlation. The results accord well with the available experimental and other theoretical data. The elastic constants, elastic anisotropy, and Debye temperature Θ as a function of pressure are presented. It is concluded that RuB2 is brittle in nature at low pressure, whereas it becomes ductile at higher pressures. An analysis for the calculated elastic constant has been made to reveal the mechanical stability of RuB2 up to 100 GPa.     

XPS investigation of diffusion of two-layer ZrO2/SiO2 and SiO2/ZrO2 sol–gel films

Two-layer ZrO₂/SiO₂ and SiO₂/ZrO₂ films were deposited on K9 glass substrates by sol–gel dip coating method. X-ray photoelectron spectroscopy (XPS) technique was used to investigate the diffusion of ZrO₂/SiO₂ and SiO₂/ZrO₂ films. To explain the difference of diffusion between ZrO₂/SiO₂ and SiO₂/ZrO₂ films, porous ratio and surface morphology of monolayer SiO₂ and ZrO₂ films were analyzed by using ellipsometry and atomic force microscopy (AFM). We found that for the ZrO₂/SiO₂ films there was a diffusion layer with a certain thickness and the atomic concentrations of Si and Zr changed rapidly; for the SiO₂/ZrO₂ films, the atomic concentrations of Si and Zr changed relatively slowly, and the ZrO₂ layer had diffused through the entire SiO₂ layer. The difference of diffusion between ZrO₂/SiO₂ and SiO₂/ZrO₂ films was influenced by the microstructure of SiO₂ and ZrO₂.     

Elastic,electronic and thermal properties of topological insulator SmB6: first-principles calculations

Abstract

The pressure dependence of the structural, elastic, electronic and thermal properties of Kondo insulator SmB₆ have been systematically studied by density functional theory combined with the quasi-harmonic Debye model. The calculated structure at zero pressure is in good agreement with the available experimental results at low temperature. The obtained elastic constants, bulk modulus and shear modulus indicate that SmB₆ is mechanically stable and behaves in a brittle manner under the applied pressure 0–20 GPa, consistent with available experimental data. In addition, the elastic-relevant properties, Young’s modulus and the Poisson ratio manifest that increasing pressure results in an enhancement in the stiffness of the compound. It is found that unlike temperature, pressure has little effect on the heat capacity of SmB_6. What more important is that we observed an insulator to metal phase transition at about 5.5 GPa through the disappearance of the band gap, well consistent with the experimental data. This transition has little effect on the physical properties of SmB₆.     

Structural, elastic, electronic and magnetic properties of ThCr2Si2 from first-principles calculations

The tetragonal (s.g. I4/mmm; #139) ThCr₂Si₂ is widely known as a structural type of the broad family of the so-called 122-like ternary phases which includes now more than 800 members. Among them the superconducting iron-pnictides (discovered in 2008, -earth metals) and the newest superconducting iron-chalcogenides (discovered in 2010, metals) have attracted recently enormous interest in this class of materials. Meanwhile, the data about the electronic, magnetic, and elastic properties of the ThCr₂Si₂ phase itself are still practically absent. Here, by means of first-principles calculations, the optimized structural parameters, spin ordering of the magnetic ground state, independent elastic constants, bulk, shear, and Young’s moduli, elastic anisotropy indexes, total and partial densities of states, and inter-atomic bonding picture for ThCr₂Si₂ were obtained for the first time and analyzed in comparison with the aforementioned most popular 122-like systems and .     

Preparation and Characterization of Poly(Methyl Methacrylate) Coated TiO2 Nanoparticles

Titanium dioxide (TiO₂) nanoparticles were modified with poly(methyl methacrylate) (PMMA) to improve the dispersion stability of the nanoparticles in a dielectric medium and to reduce the density mismatch between TiO₂ and a dielectric medium for a microcapsule‐type electrophoretic display application. Nanoparticles were coated with PMMA by in situ dispersion polymerization. The PMMA‐coated TiO₂ nanoparticles were characterized by fourier transform‐infrared spectrometrey (FT‐IR), electrophoretic light scattering (ELS), and scanning electron microscopy (SEM). Density of PMMA‐coated TiO₂ nanoparticles was found to be dependent on the thickness of the PMMA coating on the nanoparticles. An increase of thermal stability of the PMMA layer and the contents of PMMA on the surface of the nanoparticles were measured via thermogravimetric analysis (TGA).