Density-functional theory investigation of energy gaps and optical properties of Hg1-xCdxTe and In1-xGaxAs

We use a modified Becke-Johnson exchange plus a local density approximation correlation potential within the density functional theory to investigate the electronic structures of Hg_1-xCd_xTe and In_1-xGa_xAs with x being 0, 0.25, 0.5, 0.75, and 1. For both of the two series, our calculated energy gaps and dielectric functions (real part ε₁ and imaginary part ε₂) are in agreement with the corresponding experimental results with x being between 0 and 1. The calculated zero-frequency refractive index varies greatly with x for Hg_1-xCd_xTe, but changes little with x for In_1-xGa_xAs, which is consistent with the real parts of their dielectric functions. Therefore, this new approach is satisfactory to describe the electronic structures and the optical properties of the semiconductors.     

铌酸钾钠基无铅透明陶瓷制备及性能

采用传统固相烧结工艺, 结合特殊的气氛控制技术,制备了铌酸钾钠(KNN)基无铅透明陶瓷 xBa(Sc_0.5Nb_0.5)O₃-(1-x) (K_0.5N_0.5)NbO₃ (简写xBSN-(1-x)KNN). 并对其微观结构、介电、铁电性能和光学透光率进行了研究. 结果表明该体系陶瓷具有准立方钙钛矿结构, 没有其他杂相, 晶粒大小与可见光波长相当, 高度致密, 无明显的晶界存在. 在 x=0.05时, d₃₃最高可达到110 pC/N, 2P_r达到25.4 μC/cm². 同时该材料具有良好的透明性, 在可见光范围内,透过率达到54%左右, 近红外2500 nm处,透过率接近83%, 是一种有望取代铅基透明陶瓷的环境友好型无铅透明陶瓷.     

A novel Dy3+-doped Gd(PO3)3 white-light phosphors under VUV excitation for Hg-free lamps application

Novel Dy³⁺-doped Gd(PO₃)₃ white light phosphors each with an orthorhombic system are successfully synthesized by solid-state reaction. The luminescence properties of white-light Gd_1-x(PO₃)₃:xDy³⁺ (0<x≤ 0.25) under vacuum ultraviolet (VUV) excitation are investigated. The strong absorption at around 147 nm in excitation spectrum energy can be transferred to the energy levels of Dy³⁺ ion from the host absorption. Additionally, the white light phosphor is activated by a single Dy³⁺ ion. Therefore, the luminescence of Gd_1-x(PO₃)₃:xDy (0<x≤ 0.25) under VUV excitation is effective, and it has the promise of being applied to mercury-free lamp.     

纤锌矿BeO掺Cd的电子结构与能带特性研究

采用基于密度泛函理论平面波赝势方法, 对纤锌矿BeO掺Cd的Be_1-xCd_xO合金进行电子结构与能带特性研究. 结果表明: Be_1-xCd_xO的价带顶始终由O 2p电子态决定, 而导带底由Be 2s和Cd 5s的电子态决定.随着Be_1-xCd_xO合金的Cd掺杂量增加, Cd 4d与O 2p的排斥效应逐渐加强, 同时Be_1-xCd_xO的带隙逐渐变小, 出现"直接-间接-直接"的带隙转变. 为了使理论值与实验值相一致, 对Be_1-xCd_xO带隙进行修正, 并分析了纤锌矿BeO-ZnO-CdO三元合金的带隙和弯曲系数与晶格常数的关系.     

Si1-xGex合金半导体中CiCs和CiOi缺陷随Ge含量的变化

采用从头计算(ab initio)的方法对Si和Si_1-xGe_x合金半导体材料中C_iC_s缺陷的性质进行探讨，同时也对比调查了C_iO_i缺陷在Si和Si_1-xGe_x合金中的性质. 在不同Ge含量的Si_1-xGe_{x相似文献}   

Local-moment formation and metal-nonmetal transition in Ca1−x Y x VO3 and Ca1−x Y x TiO3

Electron-doped metallic states of Ca_1−x Y _x VO₃ and Ca_1−x Y _x TiO₃ change into non-metallic states around x∼0.4 and 0.6, respectively. The residual resistivity in the metallic states increases with increasing effective magnetic moment or coefficient of T ² term of resistivity. The effective moment reaches ∼ 0.5 μ_B/molecule in Ca_1−x Y _x VO₃ and also in Ca_1−x Y _x TiO₃ near the metal-nonmetal phase boundary. In these metallic states. ∼ 10% of 3d atoms seem to have large localized magnetic moments. In electron-doped metallic sample of Ca_1−x Y _x VO₃, the temperature dependence of resistance shows no resistance-minimum. However, weak negative magneto-resistance is observed for the sample with x=0.2 up to 50 Tesla at 4.2 K.     

锰位铁掺杂La0.67Sr0.33FexMn1-xO3薄膜的光诱导效应

分别采用溶胶-凝胶法和脉冲激光沉积的方法制备了La_067Sr_033Fe_xMn_1-xO₃(x=0, 005, 010, 015)系列块材和薄膜，研究了Fe部分替代对La_067Sr_033Fe_xMn_1-xO₃薄膜     

Sr和Ba替代对Eu掺杂Ca2.955Si2O7的结构和发光特性的影响研究

利用固相反应法制备了Sr和Ba替代的Ca_2.955-xM_xSi₂O₇: 0.045Eu²⁺ (M= Sr, Ba, x= 0.1-0.5)系列荧光粉, 利用较大离子半径的Sr和Ba元素替代Eu掺杂Ca_2.955-xM_xSi₂O₇ 中的Ca元素,研究Sr和Ba替代对样品结构和发光特性的影响. X射线衍射测试结果表明,少量Sr和Ba替代不会改变基质的晶体结构, 样品仍然为单斜晶系.未替代前, Ca_2.955Si₂O₇: 0.045Eu²⁺ 样品的发射峰在574 nm左右,随着Sr含量的增加,样品的发射峰发生蓝移; 而Ba含量在x= 0.1-0.4时不会引起发射峰位置的移动, 但x= 0.5样品的发射峰发生蓝移.同等含量的Sr和Ba部分替代样品中的Ca元素, Ba替代样品的光谱强度较强.     

Nd1-xSrxMnO3中掺杂浓度对电脉冲诱导电阻转变效应的影响

采用两线测量模式对固相烧结方法制备的Nd_1-xA_xMnO₃ (A= Ba, Ca, Sr,x= 0-0.9) 陶瓷样品电脉冲诱导电阻转变(EPIR)效应和I-V特性进行了测量. 结果表明, 与Nd_0.7Sr_0.3MnO₃一样, 相同浓度掺杂的Nd_0.7Ba_0.3MnO₃和Nd_0.7Ca_0.3MnO₃ 样品也能诱发稳定的室温EPIR效应. 进一步对Nd_1-xSr_xMnO₃系列样品的EPIR研究表明, 这种界面相关的EPIR效应与样品中电子或空穴掺杂浓度密切相关, 在半掺杂 (x= 0.5)附近, 样品与电极接触界面能诱发稳定的EPIR效应. 然而, 随掺杂浓度的进一步增大或降低, EPIR效应逐渐出现减弱、不明显到完全消失的过程. 产生这种现象的原因可能与锰氧化物中由于掺杂浓度差异所导致的界面缺陷在不同极性脉冲激励下重新分布而产生的内电场强弱有关.     

共轭低聚物中光生分子间电荷转移态的第一性原理研究

本文基于第一性原理中的Heyd-Scuseria-Ernzerh方法研究了单层In_1-xGa_xN的电子结构和光学性质.计算得到单层In_1-xGa_xN的能带结构和态密度（DOS）,发现随着掺杂比例的变化,体系带隙的变化范围是1.8~3.8 eV,表明通过Ga的掺杂可以实现体系带隙值的调节.并且还研究了单层In_1-xGa_xN的介电函数,折射率和吸收系数等光学性质,结果表明随着Ga掺杂浓度的增加,介电函数谱的主峰和吸收谱发生了显著的蓝移.此外,基于能带结构和态密度图谱,对单层In_1-xGa_xN的光学性质进行分析,预测这种材料独特的光学性质在纳米电子学和光学器件中会有广泛的应用.     

Ab initio study of Os0.5W0.5B2, Re0.5W0.5B2, and Os0.5Re0.5B2 with high shear modulus

By using first‐principles calculations, the authors investigate the structural, mechanical, and electronic properties of experimentally synthesized Os_0.5W_0.5B₂. The calculated structural parameters and elastic properties are in good agreement with the experimental results. In addition, two new 5d transition‐metal diborides (Re_0.5W_0.5B₂ and Os_0.5Re_0.5B₂) are predicted to have promising large shear moduli. The latter mainly come from the non‐uniform distribution of valence charge density, which raises the value of the shear moduli. We discuss potentially high hardness in these materials. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Supreme shear modulus predicted in the monocarbide system: the Rex W1–x C alloy

The energetic, mechanical and electronic properties as a function of composition for Re_x W_1–x C alloys in the WC structure have been investigated. It has been demonstrated that the shear modulus of WC can be enhanced by alloying with a small amount of Re, to a maximum shear modulus of 311 GPa at x = 0.23. The designed alloy is energetically stable and could be expected to be a potential extremely hard transition‐metal monocarbide, which is attributed to the strong metal–metalloid interaction with modulated valence electron concentration with respect to WC. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mechanical properties and electronic structure of TiC,Ti0.75W0.25C,Ti0.75W0.25C0.75N0.25, TiC0.75N0.25 and TiN

The first-principles calculations are performed to investigate the mechanical properties and electronic structure of TiC, Ti_0.75W_0.25C, Ti_0.75W_0.25C_0.75N_0.25, TiC_0.75N_0.25 and TiN. Density functional theory and ultrasoft pseudopotentials are used in this study. From the formation energy, it is found that nitrogen can increase the stability of TiC. The calculated elastic constants and elastic moduli of TiC compare favorably with other theoretical and experimental values. Tungsten and nitrogen are observed to significantly increase the bulk, shear and Young's modulus of TiC. Through the analysis of B/G and Cauchy pressure, tungsten can significantly improve the ductility of TiC. The electronic structure of TiC, TiN, Ti_0.75W_0.25C, Ti_0.75W_0.25C_0.75N_0.25, and TiC_0.75N_0.25 are used to describe nonmetal–metal and metal–metal bonds. Based on the Mulliken overlap population analysis, the hardness values of TiC, Ti_0.75W_0.25C, Ti_0.75W_0.25C_0.75N_0.25, TiC_0.75N_0.25 and TiN are estimated.     

Electronic structure,mechanical and optical properties of ternary semiconductors Si1-xGexC (X = 0, 0.25, 0.50, 0.75, 1)

The electronic structure of silicon carbide with increasing germanium content have been examined using first principles calculations based on density functional theory. The structural stability is analysed between two different phases, namely, cubic zinc blende and hexagonal phases. The zinc blende structure is found to be the stable one for all the Si_1-xGe_xC semiconducting carbides at normal pressure. Effect of substitution of Ge for Si in SiC on electronic and mechanical properties is studied. It is observed that cubic SiC is a semiconductor with the band gap value 1.243?eV. The band gap value of SiC is increased due to the substitution of Ge and the band gap values of Si _0.75 Ge _0.25 C, Si _0.50 Ge _0.50 C, Si _0.25 Ge _0.75 C and GeC are 1.322 eV, 1.413 eV, 1.574 eV and 1.657?eV respectively. As the pressure is increased, it is found that the energy gap gets decreased for Si_1-x Ge_xC (X?=?0, 0.25, 0.50, 0.75, 1). The elastic constants satisfy the Born – Huang elastic stability criteria. The bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are also calculated and compared with the other available results.     

Theoretical prediction of structural,elastic and electronic properties of Si-doped TiCuGe intermetallics

The structural, elastic and electronic properties of TiCuGe_1?xSi_x alloys (x = 0, 0.25, 0.5, 0.75 and 1) were investigated by means of first-principles calculations within the framework of density functional theory (DFT). The calculated results demonstrate that the partial substitution of Si with Ge in TiCuGe leads to a decrease of lattice constants, and the optimized structural parameters are in agreement with the available experimental values. The results of electron density are compared with the theoretical and experimental data from the literature. From energetic point of view, it is found that with increase of Si content the structural stability of TiCuGe_1?xSi_x compounds increases apparently. The single-crystal elastic constants are obtained by computing stress–strain function according to Hooke's law, and then the bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratio ν of polycrystalline aggregates are derived. The calculated results show that among the TiCuGe_1?xSi_x alloys, TiCuGe_0.75Si_0.25 exhibited the largest stiffness, while TiCuGe_0.25Si_0.75 showed the best ductility. Finally, the electronic density of states (DOSs) are further studied and discussed.     

Elastic constants and thermodynamic properties of Mg2SixSn1-x from first-principles calculations

<正>This paper stuides the elastic constants and some thermodynamic properties of Mg₂Si_xSn_n-1（x=0,0.25,0.5, 0.75,1） compounds by first-principles total energy calculations using the pseudo-potential plane-waves approach based on density functional theory,within the generalized gradient approximation for the exchange and correlation potential. The elastic constants of Mg₂Si_xSn_n-1 were calculated.It shows that,at 273 K,the elastic constants of Mg₂Si and Mg₂Sn are well consistent with previous experimental data.The isotropy decreases with increasing Sn content.The dependences of the elastic constants,the bulk modulus,the shear modulus and the Debye temperatures of Mg₂Si and Mg₂Si_0.5Sn_0.5 on pressure were discussed.Through the quasi-harmonic Debye model,in which phononic effects were considered,the specific heat capacities of Mg₂Si_xSn_1-x at constant volume and constant pressure were calculated.The calculated specific heat capacities are well consistent with the previous experimental data.     

Study on the stability of organic–inorganic perovskite solar cell materials based on first principle

ABSTRACT

In order to better understand and elucidate the structural stability of perovskite materials, the lattice parameters and tolerance factors of three crystal structures of perovskite materials are calculated based on the first principle of density functional theory. We find that the perovskite crystal structures are relatively stable and is consistent with the experimental facts as the tolerance factor 0.81?<?T?<?1.11. The elastic modulus of three crystal structures of MAPbI₃, FAPbI₃ and the elastic modulus of FA_0.75Cs_0.25Sn_0.5PB_0.5I₃ are studied. By Voigt-Reuss-Hill approximation, the elastic properties such as bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are obtained. From the elastic modulus C_ij, we can find that the other six kinds of crystal structures are relatively stable except for the orthogonal structure of MAPbI₃ (c). The ductility and brittle toughness of the material are also discussed by B/G and Poisson’s ratio. It is found that MAPbI₃ (a) is the hardest and FAPBI₃ (a) the weakest. Form the three-dimensional surface view of Young's modulus it is found that their dependence in three-dimensional direction is spherical for an isotropic system. The degree of deviation of the Young's modulus sphere reflects the anisotropy of crystal structures. The degree of elastic anisotropy of organic–inorganic perovskite materials follows the order of FAPbI₃(c)?>?MAPbI₃(a)?>?FA_0.75?Cs_0.25?Sn_0.5Pb_0.5I₃?>?FAPbI₃(a)?>?MAPbI₃(b)?>?MAPbI₃(c)?>FAPbI₃(b). Furthermore, by the adsorption energies and density of states (DOS) of these seven crystals for water molecules, the reasons why perovskite materials are easily denatured in high humidity environment were explored. The results show that perovskite materials are easy to denaturate in high humidity environment.     

钒基固溶体储氢材料弹性性质第一性原理研究

采用基于密度泛函理论的第一性原理赝势平面波方法,计算了（59Cr-41Ti）_100－xV_x（x=5,15,30,60,80,100）六种钒基储氢合金的晶格常数、弹性性质和电子态密度,计算结果与实验值符合较好.发现当x=60时的钒基合金具有较好弹性性质,杨氏模量为14930 GPa,切变模量为5442 GPa及体弹模量为19396 GPa.结合实验循环性能分析认为在吸放氢过程中合金已经发生塑性变形,弹性 关键词： 钒基固溶体 储氢合金材料 密度泛函理论 弹性性质     

Structural and electronic properties of GaN x As1−x alloys

The structural and electronic properties of cubic GaN _x As_1−x with N-concentration varying between 0.0 and 1.0 with step of 0.25 were investigated using the full potential–linearized augmented plane wave (FP-LAPW) method. We have used the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange and correlation potential. In addition the Engel-Vosko generalized gradient approximation (EVGGA) was used for the band-structure calculations. The structural properties of the binary and ternary alloys were investigated. The electronic band structure, total and partial density of states as well as the electron charge density were determined for both the binary and their related ternary alloys. The energy gap of the alloys decreases when we move from x=0.0 to 0.25; then it increases by a factor of about 1.8 when we move from 0.25 to 0.5, 0.75 and 1.0 using EVGGA. For both LDA and GGA moving from x=0.0 to 0.25 causes the band gap to close, showing the metallic nature of the GaN_0.25As_0.75 alloy. When the composition of N moves through x=0.25, 0.5, 0.75 and 1, the band gap increases.