Structural,elastic, and lattice dynamical properties of YB2 compound

In this work, density functional theory calculations on the structural, mechanical, lattice dynamical, and thermodynamical properties of YB₂ in AlB₂-type and monoclinic (C2/m) structures are reported. The local density approximation has been used for modeling exchange–correlation effects. We have predicted the lattice constants, bulk modulus, elastic constants, shear modulus, Young’s modulus, Poison’s ratio, Debye temperature, and sound velocities. Furthermore, the phonon dispersion curves, corresponding phonon density of states, some thermodynamical quantities such as internal energy, entropy, heat capacity, and their temperature-dependent behaviors are presented. Our structural and some other results are in agreement with the available experimental and theoretical data.     

The effect of Sr addition on the electrical properties of BaZn2Sb2 Zintl compounds

Sr doped BaZn₂Sb₂ polycrystalline materials with nominal compositions of Ba_1?xSr_xZn₂Sb₂ (x = 0.0, 0.25, 0.5) were prepared by synthesizing single crystals using a Sn-flux method followed by vacuum melting. The materials were characterized by powder X-ray diffraction and scanning electron microscopy equipped with electron energy dispersive spectroscopy, respectively. The electrical conductivity and Seebeck coefficient of the materials from room temperature to 773 K were measured. The electrical conductivity of all the materials decreased with increasing temperature and turned to increase with increasing temperature when temperature is above ~600 K, while the Seebeck coefficient increased with increasing temperature and turned to decrease with increasing temperature when temperature is above ~580 K. The electrical conductivity and Seebeck coefficient both increased after doping Sr into BaZn₂Sb₂. The maximum power factor for the sample with nominal composition of Ba_0.5Sr_0.5Zn₂Sb₂ reached 10.67 μW cm^?1 K^?2, which was about five times as high as that of the pure BaZn₂Sb₂.     

First principles prediction of structural stability,elastic, lattice dynamical and thermal properties of osmium carbides

Abstract

First principles calculations are performed to investigate the structural stability, elastic, lattice dynamical and thermal properties of osmium carbides with various crystal structures. Our calculation indicates that the I₄Te type structure is energetically the most favourable for Os₄C. Based on stress–strain relationships, elastic constants are obtained, and the relevant mechanical properties are also discussed. The phonon dispersion relation and the dynamical stability are also predicted. We have found that the predicted structures are mechanically stable as well as dynamically stable except for cubic-Os₄C. Through the quasi-harmonic Debye model, the temperature and pressure effects on the bulk modulus, thermal expansion coefficient, heat capacity, Grüneisen parameter and Debye temperature are presented.     

Structural properties of Sb- and Te-based binary compounds: Spin-orbit effect

The band structure of AlSb, GaSb, ZnTe and CdTe is calculated using the empirical pseudopotential method (EPM) coupled with spin-orbit (SO) splitting. We applied our empirical model of bulk modulus with SO effect. It has been noticed that SO has a crucial effect on the band structure of these compounds but does not influence the structural phase transition. The calculated results are in good agreement with the experimental data.     

Structural and elastic properties of TiN and AlN compounds: first-principles study

First-principles calculations of the lattice constants, bulk modulus, pressure derivatives of the bulk modulus and elastic constants of AlN and TiN compounds in rock-salt (B1) and wurtzite (B4) structures are presented. We have used the fullpotential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) in the generalized gradient approximation (GGA) for the exchange-correlation functional. Moreover, the elastic properties of cubic TiN and hexagonal AlN, including elastic constants, bulk and shear moduli are determined and compared with previous experimental and theoretical data. Our results show that the structural transition at 0 K from wurtzite to rock-salt phase occurs at 10 GPa and ?26 GPa for AlN and TiN, respectively. These results are consistent with those of other studies found in the literature.     

Structural and dynamical characterization of Hele-Shaw viscous fingering

Viscous fingering occurs in the interfacial zone between two fluids confined between two plates with a narrow gap (Hele-Shaw geometry) when a highly viscous fluid is displaced by a fluid with relatively low viscosity. Using a mesoscopic approach--the lattice Boltzmann method--we investigate the dynamics of spatially extended Hele-Shaw flow under conditions corresponding to various experimental systems by tuning the 'surface tension' and the reactivity between the two fluids. We discuss the onset of the fingering instability (dispersion relation), analyse the structural properties (characterization of the interface) and the dynamical properties (growth of the mixing zone) of the Hele-Shaw systems, and show the effect of reactive processes on the structure of the interfacial zone.     

On the stability of periodic orbits in lattice dynamical systems

The variation of the norm of a matrix operator in the space l_q² when q varies is investigated. The linear part of a lattice dynamical system is a matrix operator. The consequences of this variation on the stability of periodic orbits in such systems is given.     

Pullback and forward attractors for dissipative lattice dynamical systems with additive noises

This work investigates the dissipative dynamical system in the infinite lattice ?. The dynamics of each node depends on itself and nearby nodes by a nonlinear function. When each node is perturbed with weighted Gaussian white noise, a unique pullback attractor and forward attractor exists whose domain of attraction are random tempered sets. Furthermore, we prove that the pullback and forward attractors are equivalent to a random equilibrium which is also tempered. Both convergence to the pullback and forward attractors are exponentially fast.     

Theoretical determination of the lattice dynamical properties of the mercury based superconductor HgBa2CuO4

A theoretical study of the lattice dynamical properties for the high temperature superconductorH _gBa₂CuO₄ is made within the framework of the shell model. A frozen-phonon first principles determination of frequencies and eingevectors of the Raman active modes is also made. We compare these results with the recent Raman and neutron scattering experimental work and use them to obtain insight for a possible clarification of mode assigment. The origen of the 570 cm^–1 shoulder peak that appears in the low frequency side of the oxygenA _1g peak is discussed althought an unambiguous distinction of thisT _1g like defect mode is not possible at present.     

Structural optimization of uncertain dynamical systems considering mixed-design variables

In this paper attention is directed to the reliability-based optimization of uncertain structural systems under stochastic excitation involving discrete-continuous sizing type of design variables. The reliability-based optimization problem is formulated as the minimization of an objective function subject to multiple reliability constraints. The probability that design conditions are satisfied within a given time interval is used as a measure of system reliability. The problem is solved by a sequential approximate optimization strategy cast into the framework of conservative convex and separable approximations. To this end, the objective function and the reliability constraints are approximated by using a hybrid form of linear, reciprocal and quadratic approximations. The approximations are combined with an effective sensitivity analysis of the reliability constraints in order to generate explicit expressions of the constraints in terms of the design variables. The explicit approximate sub-optimization problems are solved by an appropriate discrete optimization technique. The optimization scheme exhibits monotonic convergence properties. Two numerical examples showing the effectiveness of the approach reported herein are presented.     

Elastic properties and lattice dynamics of alkali chalcogenide compounds Na2S,Na2Se and Na2Te

We report on first principles study of the elastic, vibrational and dielectric properties of the alkali chalcogenide compounds Na₂S, Na₂Se and Na₂Te using the pseudopotential method within the local density approximation and linear response theory. The calculated lattice constants for the studied compounds are in good agreement with available experimental data as well as with other theoretical results. The phonon dispersion curves and phonon density of states are calculated by using density functional perturbation theory. For Na₂S the experimental features of lattice dynamics data are well reproduced by our calculations. The width of the acoustic frequency range decreases with increasing chalcogen atomic number. The phonon densities of states show that Na⁺ ions are involved in the lower frequency modes in Na₂S. However, the inverse occurs in the case of Na₂Se and Na₂Te. The mean-square ionic displacements show that Na⁺ ions perform large thermal vibrations even at temperatures well below the melting point. The Born effective charge increases, while the IR oscillator strength decreases with anion atomic number.     

Densities,lattice parameters and defect chemistry of pure non-stoichiometric compounds

General relations are derived between the quantities describing the defect chemistry of pure, non-stoichiometric compounds and the experimental values of lattice parameters and densities of these materials. These relations show how to analyse the experimental data without the need for arbitrary assumptions about the defect situation. The theory is applied to published data of non-stoichiometric TiO, TiS₂, Ti₂S₃, and Bi₂Te₃. The models assumed in the literature, in order to interpret the experimental data, are shown to be only the most simple among a series of possible defect situations which are consistent with the data.     

Structural studies of glassy and crystalline selenium-sulphur compounds

X-ray diffraction studies revealed that the compositional dependence of the glassy structure of the binary selenium-sulphur system conforms to and can be explained by structure variations in corresponding crystalline compounds. For up to 28% S, sulphur atoms are accommodated in the structure based on the selenium matrix and distorting it. As the percentage of sulphur reaches 50%, complete phase transitions take place. The discontinuity observed in the lattice parameters of crystalline selenium-sulphur compounds is interpreted in terms of the interatomic and inter-molecular forces. The crystallization process of the sample SSe₂₀ was studied, where sulphur clusters were likely to be formed during the growth of selenium crystals, but sulphur atoms were accommodated in the fully crystalline selenium matrix so that no sulphur phase was formed.     

Structural studies of amorphous and crystalline Tl-As-Se compounds

X-ray diffraction and microstructure studies for amorphous samples of composition (As₂Se₃)_1–x Tl _x , wherex varies between 0 and 0.7, show that up tox=0.25 the short-range order is similar to that of As₂Se₃. A change in the structure is observed asx increases. Diffraction from samples crystallized by annealing confirms that a thallium content up tox=0.25 is not enough to alter a characteristic matrix similar to crystalline As₂Se₃; however, it results in a decrease of crystallization tendency. Forx>0.25 up to 0.7 the structural matrix features are almost similar to those of crystalline TlAsSe₂. The observed structural changes could rationalize reported changes in the electrical and thermal transport properties of the amorphous ternary system compounds.     

Spin susceptibility and dynamical properties of conduction electrons in cesium

Using the de Haas-van Alphen (dHvA) effect, we have made a detailed investigation of the effective mass and g-factor of conduction electrons in cesium. A mass anisotropy of 21.6% is observed about a Fermi surface (FS) average value ofm*/m _o =1.44₇±0.02. The g-factor, derived in a variety of ways, has a mean value 2.96±0.03, with little if any orientational variation. These results yield a spin susceptibility 2.14±0.01 times the free-electron value, and a Fermi liquid parameterB _o =?0.322±0.007. When all factors including FS topography, temperature, electron scattering, spin splitting, and effective mass are taken into account, the marked amplitude variation of the dHvA effect with orientation is described rather exactly. The results are compared with recent many-body calculations.     

Structural instabilities and superconductivity in La-214 compounds

In La_2–x Ba _x CuO₄ (LBCO) the transition to a low-temperature tetragonal phase and the suppression of superconductivity occur at the carrier concentration p 1/8 per copper. We will discuss the roles of various material parameters that control this instability. An unusual lattice softening has been found by ultrasonic measurement on La_2–x Sr _x CuO₄ (LSCO). This softening is present only in an in-plane shearing mode and is ascribed to the growth of structural fluctuations in the normal state.These phenomena are closely related because both the structural change in LBCO and the applied strain in LSCO lift the degeneracy of in-plane oxygen sites. They clarify the importance of strong coupling between the normal-state electronic system and the lattice by a Peierls-type mechanism.     

Structural characteristics and non-stoichiometry of β-alumina type compounds

Structural characteristics of β-alumina type compounds are investigated by comparing them with those of isomorphous compounds in the BaOFe₂O₃MeO system (where Me represents a spinel-forming divalent ion) and by observing the lattice images of a series of β-alumina type compounds by transmission electron microscopy. The results suggest that the stoichiometry of β-alumina is fixed but is far from the idealized formula Na₂O·11Al₂O₃ and that the existence of the solubility range is due to its structural characteristics to accommodate mixed periods in the structure rather than to a change in the Na composition in the “defect” layer. Important consequences derived from these conclusions are discussed.     

材料热力学性质的第一原理晶格振动计算研究

对计算固体物质晶格振动、研究材料的热力学性质的方法做了简要的探讨.以氧化锆为例,介绍了第一原理线性响应理论计算声子谱和声子态密度的方法,推导出热容、自由能、德拜温度等相关的热力学性质随温度的变化,并计算了氧化锆相变温度.结果表明通过这种方法元需任何实验数据完全从原子、电子层次出发进行材料热力学设计,为设计和研究全新材料体系提供了一个新的途径.