Unusual thermodynamic properties and nonergodicity in ferroelectric superlattices

The properties of [Pb(Zr(1-x(1))Ti(x(1)))O(3)](n)/[Pb(Zr(1-x(2))Ti(x(2)))O(3)](n) superlattices, with a 2n period, are simulated using an ab initio based approach. The x(1) and x(2) compositions are chosen to be located across the morphotropic phase boundary of the corresponding disordered alloys, while the (x(1)+x(2))/2 average composition lies inside this boundary. These superlattices exhibit an unusual thermodynamic phase transition sequence, including a triclinic ground state. They also have the kind of peculiar free-energy landscape yielding nonergodicity. The effects responsible for these anomalies are discussed.     

Theoretical investigation of V2xFe2(1-x)Zr and ScxY1-xFe2 (0

《Physics letters. A》2020,384(26):126658

Concentration-dependent crystal structure,elastic constants and electronic structure of Zr x Ti1−x alloys under high pressure

The physical properties of ZrxTi1-x（x=0.0, 0.33, 0.5, 0.67, 0.75 and 1.00） alloys were sinmlated by virtual crystal approximation （VCA） methods which is generally used for disordered solid solutions modeling. The elastic constant, electronic structure and thermal Equation of state （EOS） of disor- dered ZrxTi1-x alloys under pressure are investigated by plane-wave pseudo-potentia1 method. Our simulations reveal increasement of variations of the calculated equilibrium volumes and decrease- ment of Bulk modulus as a function of the alloy compositions. Lattice parameters a and c of alloys with differentZr concentrations decrease linearly with pressure increasing, but the c/avalues are increasing as pressure increases, indicating no phase transitions under pressure from 0 GPa to 100 GPa. The elastic constants and the Bulk modulus to the Shear modulus ratios （B/G） indicate good ductility of Zr, Zr0.33 Ti0.67 Zr0.5Ti0.5, Zr0.75Ti0.25 and Ti, but the Zr0.67Ti0.33 alloy is brittle under 0 K and 0 GPa. The metallic behavior of these alloys was also proved by analyzing partial and total DOS.     

L12型铝合金的结构、弹性和电子性质的第一性原理研究

运用第一性原理方法研究了L12型铝合金相Al3Sc和Al3Zr的晶体结构、电子结构和弹性.结合能和形成能的计算表明,两种合金具有较强的合金化能力,且Al3Zr较Al3Sc具有更强的结构稳定性.电子结构分析表明,费米能级以下较多的价电子数决定了Al3Zr具有较强的结构稳定性.计算并分析比较了两种合金相的单晶弹性常数(C11,C12和C44)以及多晶弹性模量(体弹性模量B、剪切模量G、杨氏模量Y、泊松比ν和各向异性因子A).通过对比实验和其他理论计算结果,进一步分析和解释了两种合金相的力学性质.     

CdSexS1-x电子学结构及光学性质的第一性原理研究

CdSe是Ⅱ-Ⅵ族中重要的半导体材料,一定条件下可与CdS形成无限固溶体CdSexS1-x(0≤x≤1)。CdSexS1-x在薄膜太阳电池及光电器件等领域具有重要的应用,对CdSexS1-x的电子学结构和光学性质进行研究有助于进一步提高其在光电器件等方面的应用。基于第一性原理,采用平面波超软赝势方法,计算了CdSexS1-x的电子学结构及光学性质,并将计算结果与实验进行了对比。结果表明,CdSexS1-x的晶格常数随着Se组分的增加呈线性增大趋势,态密度向低能级方向移动,禁带宽度减小,光吸收边发生一定程度的蓝移。当Se含量为0.5时,CdSexS1-x的光折射、反射和能量损失最大。除了Se和S的比例为1∶1时CdSexS1-x所属晶系为三斜晶系,其他比例下均为立方晶系。理论计算结果与实验符合。     

First-principles investigation of the structural,optoelectronic and thermodynamic properties of InAsxP1-x ternary alloys under hydrostatic pressure effect

The main objective of our work is the study of structural, optoelectronic and thermodynamic properties of InAs_xP_1-x alloys in the zinc-blende structure using the full potential linearized augmented plane wave method (FP-LAPW) based on density functional theory (DFT). Different exchange correlation potentials were used, as well as the local density approximation (LDA) and the generalized gradient approximation (GGA) parameterized by Perdew–Burke–Ernzerhof (PBE-GGA) and PBE sol-GGA of Perdew, to estimate structural properties such as lattice parameters, the bulk modulus and its first pressure derivative. For electronic properties, the Tran-Blaha modified Becke–Johnson potential (TB-mBJ) was used for density of states (DOS) and band structure calculations. The results show that the compounds of interest are semiconductors with direct band gaps for the full range of x compositions and that the optical band gap decreases from 1.58 to 0.41 eV with increasing As concentrations. The obtained results show a good agreement with experimental and theoretical data found in the literature. In addition, we have investigated the dielectric function as well as the refractive index and the reflectivity. The electronic and optical properties were studied under hydrostatic pressure (P = 0, 5, 10, 15, 20, and 25 GPa), and it was found that the band gaps of the binary compounds change from a direct to an indirect harmonic Debye model was used, which takes into account the effect of pressure P and temperature T on the lattice parameter, to explore the heat capacity, the Debye temperature and the entropy under pressures ranging from 0 to 20 GPa and temperatures ranging from 0 to 1200 K.     

Density functional theory study of structural,electronic,and thermal properties of Pt,Pd, Rh,Ir, Os and PtPdX(X= Ir,Os, and Rh) alloys

The structural, electronic, mechanical, and thermal properties of Pt, Pd, Rh, Ir, Os metals and their alloys Pt Pd X(X= Ir, Os and Rh) are studied systematically using ab initio density functional theory. The groundstate properties such as lattice constant and bulk modulus are calculated to find the equilibrium atomic position for stable alloys. The electronic band structure and density of states are calculated to study the electronic behavior of metals on making their alloys. The electronic properties substantiate the metallic behavior for all studied materials. The firstprinciples density functional perturbation theory as implemented in quasi-harmonic approximation is used for the calculations of thermal properties.We have calculated the thermal properties such as the Debye temperature, vibrational energy, entropy and constant-volume specific heat. The calculated properties are compared with the previously reported experimental and theoretical data for metals and are found to be in good agreement. Calculated results for alloys could not be compared because there is no data available in the literature with such alloy composition.     

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     

含Nb或Ge的锆合金表面氧吸附行为的第一性原理研究

采用基于密度泛函理论的第一性原理方法, 研究了纯锆表面和含Nb或Ge锆合金表面上氧的吸附性质. 结果表明, Nb和Ge对Zr(0001), (1120)和(1010)表面吸附性质的影响各不相同. 根据计算得到的偏聚能结果, Nb和Ge迁移到Zr(0001)表面比迁移到其他两个表面更容易, 而Nb和Ge 都可以降低Zr(0001)表面对氧原子的吸附能力, 因此这两种元素都能抑制锆合金的初始氧化. 进一步的电子结构分析发现, Nb和Ge改变表面对氧原子的吸附能力是通过改变表面d能带的分布来实现的.     

The structural, elastic and thermodynamic properties of thorium tetraboride

The structural, elastic and thermodynamic properties of thorium tetraboride (ThB₄) have been investigated by using first-principles plane-wave pseudopotential density functional theory with generalized gradient approximation. The behaviors of structural parameters under 0-70 GPa hydrostatic pressure are studied by means of Broyden, Fletcher, Goldfarb, and Shanno (BFGS) geometry optimization scheme. By using the stress-strain method, single crystal elastic constants are calculated to test the mechanical stability of the crystal structure and to determine mechanical properties such as bulk modulus at each pressure. However, in order to study the thermodynamic properties of ThB₄, the quasi-harmonic Debye model is used. Then, the dependencies of bulk modulus, heat capacities, thermal expansions, Grüneisen parameters and Debye temperatures on the temperature and pressure are obtained in the whole pressure range 0-70 GPa and temperature range 0-1500 K.     

Effects of atomic short-range order on the properties of perovskite alloys in their morphotropic phase boundary

The effects of atomic short-range order on the properties of Pb(Zr(1-x)Ti(x))O3 alloy in its morphotropic phase boundary (MPB) are predicted by combining first-principles-based methods and annealing techniques. Clustering is found to lead to a compositional expansion of this boundary, while the association of unlike atoms yields a contraction of this region. Atomic short-range order can thus drastically affect properties of perovskite alloys in their MPB, by inducing phase transitions. Microscopic mechanisms responsible for these effects are revealed and discussed.     

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.     

Structural,electronic,vibrational,and thermodynamic properties of Zr_(1-x)Hf_xCo:A first-principles-based study

The physical properties including structural,electronic,vibrational and thermodynamic properties of Zr1-xHfx Co(x is the concentration of constituent element Hf,and changes from 0 to 1) are investigated in terms of the ABINIT program.The results reveal that all of Zr_(1-x)Hf_x Co have similar physical properties.When Hf concentration x gradually increases from 0.0 to 1.0,the lattice constant decreases from 3.217 ?A to 3.195 ?A very slowly.The calculated density of states(DOS)indicates that the metallic nature is enhanced and the electrical conductivity turns better with the increase of Hf.Moreover,as Hf concentration increases from 0 to 1,the Fermi energy gradually increases from-6.96 e V to-6.21 e V,and the electronic density of states at the Fermi level(N(E_f)) decreases from 2.795 electrons/e V f.u.down to 2.594 electrons/e V f.u.,both of which imply the decrease of chemical stability.The calculated vibrational properties show that the increase of Hf concentration from 0 to 1 causes the maximum vibrational frequency to decrease gradually from about 223 cm~(-1) to 186 cm~(-1),which suggests a lower dispersion gradient and lower phonon group velocities for these modes.Finally,the phonon related thermodynamic properties are obtained and discussed.     

Structural,elastic, thermodynamic and electronic properties of LuX (X = N,Bi and Sb) compounds: first principles calculations

ABSTRACT

The structural, electronic, elastic and thermodynamic properties of LuX (X = N, Bi and Sb) based on rare earth into phases, Rocksalt (B1) and CsCl (B2) have been investigated using full-potential linearized muffin-tin orbital method (FP-LMTO) within density functional theory. Local density approximation (LDA) for exchange-correlation potential and local spin density approximation (LSDA) are employed. The structural parameters as lattice parameters a₀, bulk modulus B, its pressure derivate B’ and cut-off energy (E_c) within LDA and LSDA are presented. The elastic constants were derived from the stress–strain relation at 0 K. The thermodynamic properties for LuX using the quasi-harmonic Debye model are studied. The temperature and pressure variation of volume, bulk modulus, thermal expansion coefficient, heat capacities, Debye temperature and Gibbs free energy at different pressures (0–50 GPa) and temperatures (0–1600 K) are predicted. The calculated results are in accordance with other data.     

Site preference of Ti and Nb in L12-ordered Co-Al-W phase and their effect on the properties of the alloy: first-principles study

In this work,the equilibrium structure,electronic and elastic properties of L1₂-ordered Co-Al-W and Co-Al-W-X(X=Ti and Nb)phase were calculated,using first-principles calculations.Among six nonequivalent sites(Al₁,Al₂,Co₃,Co₄,W₅,W₆),Ti and Nb prefer to occupy the W₆site,since the formation enthalpy of the system is lowest when Ti and Nb occupy the W。site.Both Ti and Nb most affect the density of states of Al atoms.Compared with the Al₂ site,which is the sub-preference site of Ti and Nb,the density of states of Al atoms is higher with the addition of Ti and Nb in the W₆site,which means that the latter system is more stable.According to the bulk modulus B,shear modulus G,Young's modulus E,hardness Hv and Poisson's ratioσ,for Co₃(AI,W)alloy,the addition of Ti and Nb in the W₆site decreases its hardness hut increases its duetility.This work cnnfirms that Ti and Nh can stahilize the Cag(Al,W)alloy and have a positive effect in solving the relatively poor ductility of this alloy,which has important implications for the development of cobalt.based alloys.     

Mg-Al-Ca合金中三元Laves相的稳定性和电子结构

采用基于密度泛函理论的第一性原理方法,应用VASP (Vienna Ab-initio Simulation Package) 计算软件,研究了Mg-Al-Ca合金中三元Laves相,即Ca(Mg1-x,Alx)2和Al2(Ca1-x,Mgx) (x=0, 0.25, 0.50, 0.75, 1)在不同形态结构(C14, C15和C36)下的相稳定性及电子结构。计算所得的晶格常数和实验值吻合很好,形成能和相关能的计算用来研究三元Laves相的合金化能力和稳定性,结果表明：C14-Ca(Mg0.25,Al0.75)2具有很好的合金化能力,而C15- CaAl2具有很好的结构稳定性。态密度和电荷密度的计算用来研究Mg-Al-Ca合金中三元Laves相稳定性的内在微观机制。     

Plastic deformation properties of Zr–Nb–Ti–Ta–Hf high-entropy alloys

We have investigated the plastic deformation properties of single-phase Zr–Nb–Ti–Ta–Hf high-entropy alloys from room temperature (RT) up to 300 °C. Uniaxial deformation tests at a constant strain rate of 10^?4?s^?1 were performed, including incremental tests such as stress relaxations, strain-rate changes, and temperature changes in order to determine the thermodynamic activation parameters of the deformation process. The microstructure of deformed samples was characterized by transmission electron microscopy. The strength of the investigated Zr–Nb–Ti–Ta–Hf phase is not as high as the values frequently reported for high-entropy alloys in other systems. At RT we measure a flow stress of about 850 °C. We find an activation enthalpy of about 1 eV and a stress dependent activation volume between 0.5 and 2 nm³. The measurement of the activation parameters at higher temperatures is affected by structural changes evolving in the material during plastic deformation.     

Effects of alloying element on stabilities,electronic structures,and mechanical properties of Pd-based superalloys

The thermodynamic stabilities, electronic structures, and mechanical properties of the Pd-based superalloys are studied by first principles calculations. In this work, we discuss the effect of Pd-based superalloys made from Al, Si, Sc, Ti,V, Cr, Mn, Fe, Cu, Zn, Y, Zr, Nb, Mo, Tc, Hf, Ta, W, Re, Os, Ir and Pt, and we also calculate a face centered cubic(fcc)structure 2 × 2 × 2 superalloy including 31 Pd atoms and one alloying element T M(Pd_(31)TM). The mixing energies of these Pd-Based superalloys are negative, indicating that all Pd-based superalloys are thermodynamically stable. The Pd_(31)Mn has the lowest mixing energy with a value of-0.97 eV/atom. The electronic structures of the Pd-based superalloys are also studied, the densities of states, elastic constants and moduli of the mechanical properties of the Pd-based superalloys are determined by the stress-strain method and Voigt–Reuss–Hill approximation. It is found that Pd_(31)TM is mechanically stable, and Pd_(31)Tc has the largest C_(11), with a value 279.7 GPa. The Pd_(31)Cr has the highest bulk modulus with a value of299.8 GPa. The Pd_(31)Fe has the largest shear modulus and Young's modulus with the values of 73.8 GPa and 195.2 GPa,respectively. By using the anisotropic index, the anisotropic mechanical properties of the Pd_(31)T M are discussed, and threedimensional(3 D) surface contours and the planar projections on(001) and(110) planes are also investigated by the Young modulus.     

First-principles calculations of structural stability,elastic, dynamical and thermodynamic properties of SiGe,SiSn, GeSn

A first-principles calculations, based on the norm-conserving pseudopotentials and the density functional theory (DFT) and the density functional perturbation theory (DFPT) as implemented in the ABINIT code, have been performed to investigate the structural stability, elastic, lattice dynamic and thermodynamic properties of the ordered SiGe, SiSn and GeSn cubic alloy in zinc-blende (B3) structure. The calculated lattice parameters and bulk modulus agree with the previous results. The second-order elastic constants have been calculated and other related quantities such as the Young’s modulus, shear modulus, anisotropy factor are also estimated. We also obtain the data of lattice dynamics and the temperature dependent properties currently lacking for SiGe, SiSn and GeSn. Findings are also presented for the temperature-dependent behaviors of some thermodynamic properties such as the internal energy, Helmholtz free energy, entropy and heat capacity.     

Pressure-dependence of mechanical and thermodynamic properties of Al3Zr in Al–Li alloys from first-principles calculations

The pressure-dependence of mechanical, electronic and thermodynamic properties of metastable (L1₂ type) and stable (D0₂₃ type) Al₃Zr precipitations in Al–Li alloys were investigated by employing the first-principle calculations. The calculated equilibrium parameters are in good agreement with experimental and previous calculation results available. Elastic properties including bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and universal anisotropic index are determined by Voigt–Reuss–Hill approximation. It is found that for both phases, external pressure can improve the mechanical stability, ductility and plasticity. The electronic structures are determined to reveal the bonding characteristics of both phases. In addition, both phonon method and Gibbs program have been proposed to predict thermodynamic properties of two phases. All of these results can help to have a better understanding of the physical and chemical properties of Al₃Zr precipitations in Al–Li alloy. And can offer theoretical guidance for the weight lighting, energy conservation and emissions reduction in the design of new aluminium alloys.