Generalized stacking faults in model lattice of ordered Fe-Si alloys

The energies of the generalized stacking faults lying on {110} and {112} planes are calculated for Fe-Si alloys with the DO₃ long-range order for concentrations 10–25 at. % Si. In the used model, the interaction between atoms is described by the series of three central interatomic potentials fitted to experimental data. The lattice relaxations perpendicular to the stacking fault plane are considered. The displacement vectors corresponding to stable antiphase boundaries are found and some consequences for the structure of superdislocations in the DO₃ superlattice are mentioned.The author is indebted to Dr. F.Kroupa and Dr. L.Lejek for valuable discussion.     

Elastic constants of iron-silicon alloy single crystals

The adiabatic elastic constants of single crystals of Fe-Si alloys in the composition range from 6.3 at.% Si to 25 at.% Si have been determined by the ultrasonic pulse echo method at room temperature. The values of the elastic constants depend significantly on the silicon content, and their composition dependences are influenced by ordering.     

Empirical construction of interatomic potentials for studies of grain boundaries and other crystal defects: Pure metals and binary alloys

It is first argued that pair potentials are physically meaningful if they are regarded as describing interaction between atoms embedded in an environment of similar density as that for which their derivation was made. These potentials are capable of describing structural aspects of lattice defects but not the cohesive properties. Guidelines for determination of the shape of potentials and an empirical scheme for their construction in pure metals is then outlined. It is shown that this method when applied to simple metals leads to potentials similar to those obtained on the basis of a pseudopotential theory and extension of this scheme to other metals is discussed. An empirical scheme of construction of potentials for binary alloys is then briefly described. It is shown that concentration dependences of elastic constants and lattice stability evaluated using these potentials agree with experimental data and thus these potentials are suitable for studies of defects in binary alloys.     

Grüneisen parameter and thermal expansion of V3Si and V3Ge

The Grüneisen parameter and lattice thermal expansion of the A-15 compounds V₃Si and V₃Ge at room temperature are evaluated on the basis of the method due to Brugger and Fritz [1] from the third order elastic constants reported earlier [2]. The calculated values are compared with available experimental values and are found to fit satisfactorily.     

Structural, electronic, elastic and thermal properties of Mg2Si

First-principles calculations of the lattice parameter, electron density maps, density of states and elastic constants of Mg₂Si are reported. The lattice parameter is found to differ by less than 0.8% from the experimental data. Calculations of density of states and electron density maps are also performed to describe the orbital mixing and the nature of chemical bonding. Our results indicate that the bonding interactions in the Mg₂Si crystal are more covalent than ionic. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the plane-wave pseudopotential method, is applied to study the elastic, thermal and vibrational effects. The variations of bulk modulus, Grüneisen parameter, Debye temperature, heat capacity C_v, C_p and entropy with pressure P up to 7 GPa in the temperature interval 0-1300 K have been systemically investigated. Significant differences in properties are observed at high pressure and high temperature. When T<1300 K, the calculated entropy and heat capacity agree reasonably with available experimental data. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic Debye model is an efficient approach to simulate the behavior of Mg₂Si.     

碱土氟化物中离子间相互作用势经验参数的确定

描述离子间非Coulomb短程互作用势的函数形式确定后，即可据此讨论离子晶体的一系列物理性质，势函数中出现的参数通过与实验数据的拟合而确定．基于电子壳模型对CaF₂，SrF₂，BaF₂离子晶体的讨论中，强调电子壳电量应一律取负值的基本前提，以经验参数化方法拟合势参数时注意引用近期实验数据．将F离子电量调整为0.97e之后，重新获得的模型参数集与上述碱土氟化物晶格形成能、弹性常数、长光学横波频率、Raman频率，以及极化率实验数据，有较满意的对应关系．     

Anharmonic properties of rocksalt structure solids

An effort has been made for obtaining the anharmonic properties of rocksalt structure solids starting from primary physical parameters viz. nearest-neighbor distance and hardness parameter assuming long- and short-range potentials at elevated temperatures. The elastic energy density for a deformed crystal can be expanded as power series of strains for obtaining coefficients of quadratic, cubic and quartic terms which are known as the second-, third- and fourth-order elastic constants, respectively. When the values of the higher-order elastic constants are known for a crystal, many of the anharmonic properties of the crystal can be treated within the limit of the continuum approximation in a quantitative manner. In this study, higher-order elastic constants are computed up to their melting temperature for rocksalt structure solids, which are alkali cyanides, sodium and potassium halides. The first order pressure derivatives of second- and third-order elastic constants, the second-order pressure derivatives of second-order elastic constants and partial contractions are also evaluated at different temperatures for these substances. The results thus obtained are compared with experimental data and found in well agreement with present values.     

The influence of 3d-metal alloy additions on the elastic and thermodynamic properties of CuPd3

Embedded-atom method （EAM） potentials are used to investigate the effects of alloying （e.g. 3d-metals） on the trends of elastic and thermodynamic properties for CuPd3 alloy. Our calculated lattice parameter, cohesive energy, and elastic constants of CuPd3 are consistent with the available experimental and theoretical data. The results of elastic constants indicate that all these alloys are mechanically stable. Further mechanical behavior analysis shows that the additions of Cr, Fe, Co, and Ni could improve the hardness of CuPd3 while V could well increase its ductility. Moreover, in order to evaluate the thermodynamic contribution of 3d-metals, the Debye temperature, phonon density of states, and vibrational entropy for CuMPd6 alloy are also investigated.     

Third-order elastic constants of the alloy Fe72Pt28

The complete sets of second- and third-order elastic constants of the cubic Fe₇₂Pt₂₈ have been obtained using the strain energy density derived from interactions up to three nearest neighbours of each atom in the unit cell. The finite strain elasticity theory has been used to get the strain energy density of Fe₇₂Pt₂₈. The strain energy density is compared with the strain-dependent lattice energy density obtained from the continuum model approximation and the expressions for the second- and third-order elastic constants of Fe₇₂Pt₂₈ are given. The second-order potential parameter is deduced from the measured second-order elastic constants of Fe₇₂Pt₂₈ and the third-order potential parameter is estimated from the Lennard-Jones inter-atomic potential for Fe₇₂Pt₂₈. The inter-lattice displacements; the three independent second-order elastic constants and the six independent third-order elastic constants of Fe₇₂Pt₂₈ are also determined. The second-order elastic constants are compared with the experimental elastic constants of Fe₇₂Pt₂₈. We also study the effect of pressure on the second-order elastic constants of Fe₇₂Pt₂₈.     

A continuum thermal stress theory for crystals based on interatomic potentials

This paper presents a new continuum thermal stress theory for crystals based on interatomic potentials.The effect of finite temperature is taken into account via a harmonic model.An EAM potential for copper is adopted in this paper and verified by computing the effect of the temperature on the specific heat,coefficient of thermal expansion and lattice constant.Then we calculate the elastic constants of copper at finite temperature.The calculation results are in good agreement with experimental data.The thermal stress theory is applied to an anisotropic crystal graphite,in which the Brenner potential is employed.Temperature dependence of the thermodynamic properties,lattice constants and thermal strains for graphite is calculated.The calculation results are also in good agreement with experimental data.     

Internal strain parameter of silicon and GaAs and planar force constants

We show that the internal strain parameter ζ and elastic constants of the tetrahedral semiconductors can be determined from the force constants between planes of atoms. These have been computed recently for Si and GaAs from self-consistent electronic calculations using the Hellmann-Feynman theorem. The theoretical results are ζ = 0.72 for GaAs and ～0.57 for Si compared to experimental values of 0.76 and 0.73.     

Third-order elastic constants of ZnS and ZnSe

The complete set of non-vanishing third-order elastic constants of the semiconductors ZnS and ZnSe is obtained theoretically. The strain energy density is estimated using finite strain elasticity theory by considering the interactions up to two nearest neighbours of each atom in the unit cell of these compounds. This energy density is compared with the strain dependent lattice energy density from the continuum model approximation. The second-order parameter of the potential function φ is obtained from the measured principal axis C_ij. The third-order potential parameter is estimated by assuming a Lennard-Jones type of interatomic potential. The interlattice displacements as well as the second-order elastic constants are evaluated along with the six third-order elastic constants of ZnS and ZnSe. Using these second- and third-order elastic constants of ZnS, the pressure derivatives of second-order elastic constants are evaluated. The second- and third-order elastic constants of ZnSe are compared with the available experimental values. The third-order elastic constants show anisotropy in different directions.     

Configurational energies of different A7B types of the long-range order in Fe-Si alloys

In order to examine the existence of the long-range order of A₇B types in Fe-Si alloys with composition less than 12·5 at.% Si, the configurational energies of several superstructures in b.c.c. alloys with the cubic elementary cell having the lattice parameter 2a are calculated on the basis of a simple model with a pairwise interaction between atoms extending to the third neighbours. From the proposed model it follows that the superstructure of A₇B type suggested byFallot can be expected if the interaction parameter for the third neighboursv ₃>0. On the other hand, the two-phase structure (DO₃ order and pure Fe) will be favoured by the alloy crystal ifv ₃<0.The author is indebted to Dr. F.Kroupa and Dr. A.Gemperle for stimulating discussions.     

NaZn13型Fe基化合物的结构和热力学性质研究

利用Chen-Mbius晶格反演获得的原子间相互作用势,对NaZn13型Fe基金属间化合物进行原子级模拟研究.计算结果表明,Si原子和Co原子均优先占据96i晶位,Si原子和Co原子替代Fe原子后晶体平均结合能降低.随着Co含量的增加,LaFe13-x-yCoySix和NdFe13-x-yCoySix的晶格参数逐渐降低.声子态密度中,稀土原子主要激发低频模,Si原子主要激发高频模.LaFe11.5-yCoySi1.5化合物的德拜温度随Co含量的增加而增高.     

First-Principle Calculations for Elastic and Thermodynamic Properties of Diamond

The elastic constants and thermodynamic properties of diamond are investigated by using the CRYSTAL03 program. The lattice parameters, the bulk modulus, the heat capacity, the Grüneisen parameter, and the Debye temperature are obtained. The results are in good agreement with the available experimental and theoretical data. Moreover, the relationship between V/V₀ and pressure, the elastic constants underhigh pressure are successfully obtained. Especially, the elastic constants of diamond under high pressure are firstly obtained theoretically. At the same time, the variations of the thermal expansion α with pressure P and temperature T are obtained systematically in the ranges of 0-870 GPa and 0-1600 K.     

Third order elastic constants of V3Si and V3Ge

General expressions have been derived for the second order elastic constants and third order elastic constants of the A-15 structure compounds with the nearest neighbour central interaction. The second order elastic constants, the third order elastic constants and the pressure derivatives of the second order elastic constants of V₃Si and V₃Ge are reported and compared with the available experimental measurements.     

Investigating the temperature dependence of elastic constants by thermal fluctuation formula

The differences between the calculated values of elastic constants of materials and the experimental data are consistently restricting the application of thermal fluctuation formula to the mechanical properties of materials. In this work, the temperature dependence of elastic constants of many-body potentials is studied by thermal fluctuation formula. The differences between the calculated values and the experimental data are investigated in detail. Our studies show that the differences come from the thermal expansion of the materials: the calculated zero-stress states are bigger than the experimental zero-stress states of the materials, and this deviation makes Born terms of the thermal fluctuation formula decrease sharply as the temperature increases, while the fluctuation terms and the kinetic terms change little. As a result, the elastic constants, which are the sum of these three terms, decrease faster than the experimental data as the temperature increases. Our studies show that when the experimental zero-stress states are used as the reference states in constant volume and constant energy (NVE) simulations, the elastic constants calculated by thermal fluctuation formula are in good agreement with the experimental values.