Structural, electronic, thermodynamic and optical properties of SrS1−xOx mixed crystals

The structural, electronic and thermodynamic properties of the SrS_1–xO_x ternary mixed crystals have been studied using the ab initio full potential linearized augmented plane wave (FP-LAPW) method within density functional theory. The effect of composition on lattice parameter, bulk modulus and band gap was investigated. The lattice constants from Vegard's law and the bulk modulus from linear concentration dependence were observed for the alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. The thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔH_m as well as the phase diagram. In addition to FP-LAPW method, the composition dependence of the refractive index and the dielectric constant was studied by different models.     

High-pressure and high-temperature bulk modulus of cubic fluorite-type MgF2 from quasi-harmonic Debye model

A detailed theoretical study of the isothermal and adiabatic bulk moduli of MgF₂ with a fluorite structure under high pressure and temperature has been carried out by means of first-principles density functional theory calculations combined with the quasi-harmonic Debye model in which the phononic effects are considered. Particular attention is paid to the prediction of the isothermal bulk modulus and its first and second pressure derivatives for the first time. The calculated ground state properties agree well with other theoretical values. At extended pressure and temperature ranges, the variation of the bulk modulus which plays a central role in the formulation of approximate equations of state has also been predicted. The properties of MgF₂ with a fluorite structure are summarized in the pressure range of 0–135 GPa and the temperature up to melting temperature 1500 K.     

Structural and thermodynamic properties of Os from first-principles calculations

We investigate the structural, thermodynamic and electronic properties of Os by plane-wave pseudopotential density functional theory method. The obtained lattice constants, bulk modulus and cell volumes per formula unit are well consistent with the available experimental data. Especially, from our calculated bulk modulus, we conclude that Os is more compressible than diamond. Moreover, the temperature induced phase transition of Os from HCP structure to FCC structure has been obtained. It is found that the transition temperature of Os at zero pressure is 2702 K. However no transition pressure is found in our calculations. The effect of bulk modulus B as well as other thermodynamic properties of Os (including the thermal expansion α and the Grüneisen constant γ) on temperatures have also been studied. Our calculated thermal expansion α=1.510×10⁻⁵ K⁻¹ and the Grüneisen constant γ=2.227 for HCP structure at room temperature agree very well with the experimental data. The density of states for HCP structure at 0 K and FCC structure at transition temperature 2702 K are also investigated in our work.     

The structural, elastic and thermodynamic properties of intermetallic compound CeGa2

The structural, elastic and thermodynamic characteristics of CeGa₂ compound in the AlB₂ (space group: P6/mmm) and the omega trigonal (space group: P-3m1) type structures are investigated using the methods of density functional theory within the generalized gradient approximation (GGA). The thermodynamic properties of the considered structures are obtained through the quasi-harmonic Debye model. The results on the basic physical parameters, such as the lattice constant, the bulk modulus, the pressure derivative of bulk modulus, the phase-transition pressure (P _t ) from P6/mmm to P-3m1 structure, the second-order elastic constants, Zener anisotropy factor, Poisson’s ratio, Young’s modulus, and the isotropic shear modulus are presented. In order to gain further information, the pressure and temperature-dependent behavior of the volume, the bulk modulus, the thermal expansion coefficient, the heat capacity, the entropy, Debye temperature and Grüneisen parameter are also evaluated over a pressure range of 0–6 GPa and a wide temperature range of 0–1800 K. The obtained results are in agreement with the available experimental and the other theoretical values.     

Cu_3N弹性和热力学性质的第一性原理研究

利用基于密度泛函理论的第一性原理全势线性缀加平面波方法,研究了立方反ReO3结构Cu3N在零温(0K)零压下的平衡晶格常数、体弹模量及其对压强的一阶导数,计算结果与其他实验及理论结果基本相符.同时得出Cu3N的弹性常数,Poisson比等,并分析出Cu3N在零温零压下是稳定的.通过准谐Debye模型计算Cu3N的热力学性质,得到了Cu3N的晶格常数、等压比热容、等容比热容、热胀系数与温度和压强之间的关系,同时计算出不同温度不同压强下其体弹模量及Debye温度的值。     

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.     

基于第一性原理的合金化掺杂MgH2的热力学稳定性研究

基于密度泛函的第一性原理,系统研究了合金化掺杂过渡金属（TM=Sc,Ti,Y）和IIA族元素（M=Ca,Sr,Ba）对MgH₂（金红石和萤石结构）的热力学稳定性的影响。结果表明,在低掺杂量（<20%） 时,MgH₂的萤石结构比金红石结构相对更稳定。掺杂Ti,Sr,Ba时,MgH₂的结构发生了失稳现象。MgH₂由金红石结构转变到萤石结构的掺杂TM和M的比例分别大约在20%和40%左右。Mg_0.5Ba_0.52萤石结构的形成焓比MgH₂萤石结构高约0.3 eV,表明其放氢温度在标准大气压下将远低于纯MgH₂。理论计算数据与实验数据有很好的一致性.     

Anomalous pressure-induced phase transformation in nano-crystalline erbium sesquioxide (Er2O3): partial amorphization under compression

Nanophase materials have novel physical and chemical properties, differing from bulk materials. It is of exceptional interest to investigate the size effect on structural stability in nanocrystals. Here, we investigated pressure-induced phase transitions in nanosized Er₂O₃ using angle-dispersive synchrotron X-ray diffraction up to 40.6 GPa. Nano-Er₂O₃ has enhanced transition pressure and higher bulk modulus (K₀) than its bulk counterparts. Amorphous Er₂O₃ nanoclusters with traces of monoclinic phase are obtained upon compression. This is the first time that partial amorphous structure under compression was observed in nano-Er₂O₃, indicating a kinetic trapping of partial amorphous Er₂O₃ on pressurizing.     

High-pressure in-situ X-ray diffraction and Raman spectroscopy of Ca2AlFeO5 brownmillerite

The behavior of Ca₂AlFeO₅ brownmillerite was studied by in situ synchrotron X-ray diffraction and Raman spectroscopy at 300?K with pressures up to 26.5 and 32.1 GPa, respectively. A reversible structural phase transition was observed. The P–V data were fitted by a third-order Birch–Murnaghan equation of state, and the isothermal bulk modulus was obtained as K₀?=?181.9(76) GPa with K₀′_?=?4.4(17). If K₀′ was fixed to 4, K₀ was obtained as 183.8(20) GPa. Ca₂AlFeO₅ brownmillerite shows an axial elastic anisotropy since the b-axis is more compressible than a- and c-axis. Combined with previous results, the isothermal bulk modulus and axial compressibility of Ca₂AlFeO₅ brownmillerite increase with more Al incorporated in the structure. The Raman spectra of Ca₂AlFeO₅ brownmillerite were analyzed and the pressure coefficients vary from 2.23 to 4.90?cm^?1/GPa. The isothermal mode Grüneisen parameters range from 0.83 to 1.77 and the thermal Grüneisen parameter is determined as 1.08(11).     

萤石结构TiO2的电子结构和光学性质

利用第一性原理计算了立方相萤石TiO₂的晶胞参数,能带结构和电子态密度.结果显示萤石TiO₂属于间接带隙半导体材料,其间接禁带宽度(Γ→X)E_g为2.07eV,比常见的金红石和锐钛矿TiO₂的禁带宽度窄.为了更清楚地了解萤石的光学性质,利用Kramers-Kronig色散关系,分别对萤石和金红石TiO₂的复介电常数、吸收率等参数进行了计算,并将二者结果做了 关键词： 2')" href="#">萤石结构TiO₂ 密度泛函理论 能带结构 光学性质     

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.     

Elastic,electronic and thermodynamic properties of fluoro-perovskite KZnF3 via first-principles calculations

The elastic, electronic and thermodynamic properties of fluoro-perovskite KZnF₃ have been calculated using the full-potential linearized augmented plane wave (FP-LAPW) method. The exchange-correlation potential is treated with the generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE). Also, we have used the Engel and Vosko GGA formalism (GGA-EV) to improve the electronic band structure calculations. The calculated structural properties are in good agreement with available experimental and theoretical data. The elastic constants C _ij are calculated using the total energy variation with strain technique. The shear modulus, Young’s modulus, Poisson’s ratio and the Lamé coefficients for polycrystalline KZnF₃ aggregates are estimated in the framework of the Voigt-Reuss-Hill approximations. The ductility behavior of this compound is interpreted via the calculated elastic constants C _ij . Electronic and bonding properties are discussed from the calculations of band structure, density of states and electron charge density. The thermodynamic properties are predicted through the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variation of bulk modulus, lattice constant, heat capacities and the Debye temperature with pressure and temperature are successfully obtained.     

First-principles prediction of crystal structure and physical properties of ScB3

The crystal structure of ScB₃ is seeked by combining the developed particle swarm optimisation algorithm for crystal structure prediction with first-principles calculations. A new monoclinic phase with the C2/m symmetry is predicted successfully, which is energetically more superior to the early reported R-3m-, P2₁/m-, P6₃/mmc-, P-6m2-, Pnma-, and Pm-3m-type structures in the pressure range from 0 to 100?GPa. The obtained elastic constants and phonon dispersion curve reveal that the C2/m-ScB₃ is mechanically and dynamically stable. The predicted large bulk module, high shear modulus, small Poisson’s ratio as well as the considerable hardness indicate that the C2/m-ScB₃ has outstanding mechanical property. Meanwhile, the dependences of the bulk modulus and Young’s modulus of ScB₃ on the crystal orientation are investigated theoretically. Through applying the strain–stress method, the ideal tensile and shear strengths along different crystal directions are also estimated, and the obtained results confirm that the shear mode dominates the failure mode in the C2/m-ScB₃ structure and it is intrinsically a hard material. The electronic structure calculation and chemical bonding analysis illustrate that the strong covalent B-B and Sc-B bonds are responsible for its structural stability and high hardness.     

First-principles investigations on elastic, phonon and thermodynamic properties of SrB6 under pressure

The elastic, phonon and thermodynamic properties of the divalent alkaline-earth hexaboride SrB₆ are investigated by using plane-wave pseudopotential density functional theory method. The calculated structure parameters and bulk modulus are well consistent with the available experiment and theoretical data. The pressure dependences of elastic constants C_ij, bulk modulus B₀, shear modulus G, Young's modulus E and Poisson's ratio σ are also presented. With these elastic parameters, we investigate the mechanical stability and compressibility of SrB₆. For the thermodynamic properties, both phonon and quasi-harmonic Debye model methods are adopted. Through the comparison with experimental and other theoretical results, we found the method of quasi-harmonic Debye model is a little better. Moreover, the phonon dispersion relations are also obtained. It is found that there are two LO/TO splitting around 5 THz and 26 THz, respectively.     

First-principles calculation of elastic and thermodynamic properties of Ni2MnGa Heusler alloy

The equilibrium lattice parameter, heat capacity, thermal expansion coefficient and bulk modulus of Ni 2 MnGa Heusler alloy are successfully obtained using the first-principles plane-wave pseudopotential (PW-PP) method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus B and temperature T up to 800 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure and decreases with increasing temperature. The pressure dependence of heat capacity C v and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperature of Ni 2 MnGa is determined from the non-equilibrium Gibbs function. Our calculated results are in excellent agreement with the experimental data.     

Thermodynamic properties of cubic ZrC under high pressure from first-principles calculations

The elastic and thermodynamic properties of Zirconium carbide (ZrC) are investigated by ab initio plane-wave pseudopotential density function theory method. The obtained lattice constant, elastic constant and bulk modulus B are consistent with the experimental and theoretical data. Through the quasi-harmonic Debye model, the dependences of the normalized volume V/V ₀ and the bulk modulus B on pressure P, as well as the specific heat C _V on the temperature T are obtained successfully. The relationships of the thermal expansion α with temperature and pressure are also investigated, which indicate the temperature hardly has any effect on the thermal expansion α at high pressure. Supported by the National Natural Science Foundation of China (Grant No. 10776022)     

Bi0.5Ba0.5Fe0.5Ti0.49Nb0.01O3热敏陶瓷的微结构和电学性能研究

采用固相合成工艺,制备了Bi_05Ba_05Fe_05Ti_049Nb_001O₃（BBFTN）热敏陶瓷,借助X射线衍射仪、扫描电子显微镜、阻温测试仪和交流阻抗谱考察其微结构、直流电阻、介电特性、阻抗和电学模量方面的电学性能. 结果表明：BBFTN材料依然为立方钙钛矿结构,平均晶粒尺寸约为10 μm,晶格常数相对于BaTiO₃的晶格常数有所变大;室温电 关键词： 05Ba_05Fe_05Ti_049Nb_001O₃')" href="#">Bi_05Ba_05Fe_05Ti_049Nb_001O₃ 微结构 电学性能     

Electronic,structural, and mechanical properties of possible metastable phases of beryllium hydride

Abstract

We have studied the electronic, structural, and mechanical properties of two possible metastable phases of beryllium dihydride, BeH₂, using total energy calculations based on the self-consistent, ob-initio pseudopotential method. The two investigated structures are the fluorite structure and the calcium carbide structure. The minimum-energy volumes per BeH₂ are found to be smaller for both structures than the volume per BeH₂ for the observed stable structure. As a result, the hydrogen density is greater for both investigated structures than for the observed BeH₂ phase. In addition, both phases are predicted to be semimetals with ionic bonding between the negatively-charged hydrogen ions and the positively-charged beryllium ions. The bulk modulus for the fluorite structure is three times larger than the one for the calcium carbide structure.     

First-principles calculations on elasticity of under pressure

First-principles calculations of the crystal structure and the elastic properties of OsN₂ have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The dependence of the elastic constants c_ij, the aggregate elastic moduli (B,G,E), Poisson’s ratio, and the elastic anisotropy on pressure has been investigated. Moreover, the variation of the Debye temperature and the compressional and shear elastic wave velocities with pressure P up to 60 GPa at 0 K have been investigated for the first time.     

Structural investigation of glasses with doped some transition metal oxides under the influence of gamma irradiation

The ultrasonic velocity of the alkali-borosilicate with different transition metal oxides (TMO) has been studied using the pulse echo technique. The elastic moduli such as elastic constant and bulk modulus have been obtained from the experimental data. Changes in the structure with different gamma irradiation doses have been investigated by using FTIR spectroscopy and ultrasonic studies. The results show that structural changes in the BO₃ to BO₄ due to TMO and irradiation are obtained.