Narrow Results

In this paper, we have studied theoretically the effects of gold adsorption on the Al(001) surface, using ab initio pseudo-potential method in the framework of the density functional theory. Having found the hollow sites at the Al(001) surface as the most preferred adsorption sites, we have investigated the effects of the Au adsorption with different coverages (Θ =0.11, 0.25, 0.50, 0.75, 1.00 ML) on the geometry, adsorption energy, surface dipole moment, and the work function of the Al(001) surface. The results show that even though the work function of the Au substrate increases with the Au coverage, the surface dipole moment decreases with the changes in coverage from Θ =0.11 to 0.25 ML. We have explained this behavior by analyzing the electronic and ionic charge distributions. Furthermore, by studying the diffusion of Au atoms into the substrate, we have shown that at room temperature the diffusion rate of Au atoms into the substrate is negligible but increasing the temperature to about 200°C the Au atoms significantly diffuse into the substrate, in agreement with the experiment.

The bulk modulus of hard sphere solids has been computed directly by constant pressure Monte-Carlo simulations, using the histogram of the volume fluctuations. In considering first the one-component system, we show that the method is accurate in a large range of pressures, including high-pressure regime. The method is then applied to a polydisperse solid with relatively low polydispersity index. For illustrative purpose, we took a three-component mixture with symmetric size-distribution, and we studied the solid phase (fcc crystal) of this system. Our results show that the equation of state is very sensitive to the polydispersity. Furthermore, in the high-pressure region, where no (accurate) analytical fit for the equation of state exists, our simulations are able to predict the bulk modulus of such systems.

The mechanical property of alloys plays a vital role in applications. In order to probe the mechanical property of the Zr–Ti alloy, we have successfully generated a 16-atom special quasi-random structure (SQS) at concentration 0.5 for the Zr–Ti alloy (${\mathrm{\text{Zr}}}_8{\mathrm{\text{Ti}}}_8$) with supercell ($2\times 2\times 2$) based on the hcp Bravais cell, which has the same shape with the hcp Bravais cell. Such an SQS can be used as a good template at the 0.5 concentration of all binary hcp random alloys as the initial step for the further description of their alloy properties. Our calculated bulk modulus of ${\mathrm{\text{Zr}}}_8{\mathrm{\text{Ti}}}_8$ is 101.7GPa, falling within the region for the extreme two constituents $({\mathrm{\text{B}}}_0^{\mathrm{\text{Zr}}}<$ B${}_0^{\mathrm{\text{Zr}}{}_8\mathrm{\text{Ti}}{}_8}<$ B${}_0^{\mathrm{\text{Ti}}}$). Obtained $B∕G$ of ${\mathrm{\text{Zr}}}_8{\mathrm{\text{Ti}}}_8$ (3.60) is significantly larger than those of Zr (2.75) and Ti (2.64), which means that the ductility of ${\mathrm{\text{Zr}}}_8{\mathrm{\text{Ti}}}_8$ alloy is better than those of its constituents. The maximum Cauchy stress of ${\mathrm{\text{Zr}}}_8{\mathrm{\text{Ti}}}_8$ is 20.53GPa at strain ($\epsilon )=0.43$ in [0001] direction. In addition, this SQS template for ${\mathrm{\text{Zr}}}_8{\mathrm{\text{Ti}}}_8$ has been proved to be mechanically stable.

The elastic constants are paramount to determine the strength of alloys. The elastic constants of $M$–Ir, $M_3$–Ir, and $M$–Ir₃ where $M$ represents $\mathrm{\text{Cu}}$, $\mathrm{\text{Au}}$, $\mathrm{\text{Ni}}$, $\mathrm{\text{Ag}}$, $\mathrm{\text{Pt}}$, $\mathrm{\text{Al}}$, $\mathrm{\text{Pd}}$ and $\mathrm{\text{Rh}}$ were computed at room temperature using the embedded atom method (EAM) and the alloy mixing potentials. The potential parameters of the selected pure metals were fitted to the experimental values to compute some properties of Ni–Al, Ni₃–Al, Ni–Al₃, Cu–Au, Cu₃–Au and Cu–Au₃, and by comparing the experimental data with our predictions, the employed potential predicted some results in reasonable agreement to available experimental data with discrepancies in some cases, and these discrepancies linked to the dependence of the computed elastic constants on the fitting parameters. The potential with the metallic parameters was used as alloy parameters in computing the elastic constants, bulk modulus, and the shear modulus of the iridium binary alloy. It was generally observed that, the selected metals improve the ductility of Iridium with the highest value, recorded for Pd–Ir, and consequently the minimum value for Al–Ir, and Rh–Ir which characterized them to be ductile. The balance orders for the binary alloys were provided through the formation enthalpy.

The electronic structure of the Iridium based L1₂ intermetallic compounds (A₃B) such as Ir₃Ti, Ir₃Zr, Ir₃Hf, Ir₃V, Ir₃Nb and Ir₃Ta, which have wide applications as high temperature structural materials are studied by means of Self-Consistent Tight Binding Linear Muffin Tin Orbital (TB-LMTO) method. These compounds are found to crystallize in the Cu₃Au type structure. The total energies are calculated as a function of volume and fitted to Birch equation of state to find the equilibrium lattice parameter and the bulk modulus. They are tabulated and compared with the available experimental and other theoretical data. The partial number of electrons at A and B sites of these compounds are calculated as a function of volume. We find that there is a continuous transfer of d-electrons from A-site to B-site. The band structure and density of states histograms are plotted. From the DOS histograms, we find that the plots are similar for all the compounds except for Ir₃V. In the case of Ir₃V, we find that there are hybridization between Ir-d like and V-d like states at the Fermi level. Hence, it is predicted that under compression there may be a Lifshitz type of transition in Ir₃V. The cohesive energy, heat of formation and the electronic specific heat coefficient of the compounds are also computed.

An analysis has been presented using the Roy-Roy equation of state (EOS), which represents an extended and modified version of the Murnaghan EOS. It is found that the value of for various types of solids remain between 5/3 and 2. The Roy-Roy EOS has been found to become applicable for the core of the Earth, predicting the values of the pressure and bulk modulus in the range of seismological data. Modifications suggested recently to remove the shortcomings of the Murnaghan equation have been discussed. The maximum values of obtained from the Roy-Roy EOS for different inorganic solids and organic solids are also discussed.

In the present paper, a new relationship for the pressure dependences of elastic constants is developed by using a new expression for the pressure dependence of bulk modulus and a method developed by Grover et al. The proposed relationship is applied to study elastic constants of MgO, NaCl, CaF₂, and CaO. The results obtained for elastic constants are found in good accordance with the experimental and first-principle results.

The elastic and thermodynamic characteristics of NiAl and Ni₃Al crystals have been investigated by using a method of density functional theory within the generalized gradient approximation. The three independent elastic constants are C₁₁ = 229.8 GPa, C₁₂ = 124.7 GPa, C₄₄ = 115.7 GPa for NiAl and C₁₁ = 239.6 GPa, C₁₂ = 151.7 GPa, C₄₄ = 123.4 GPa for Ni₃Al. The bulk moduli, shear moduli, Young's moduli, Poisson's ratios and ratios of B/G of NiAl and Ni₃Al are also calculated. In addition, the dependences of the bulk moduli on temperatures and pressures as well as the linear thermal expansion coefficients (α_L) versus temperatures are evaluated and discussed. Both NiAl and Ni₃Al crystals are elastically anisotropic. Especially for NiAl crystal, which should be much brittler than Ni₃Al, lies at the critical point of brittle behavior. In addition, the present results are well in line with experimental and other theoretical results.

We have investigated the elastic and thermal properties of Sr_1-xCa_xRuO₃(0≤x ≤1) perovskite using a modified rigid ion model (MRIM). The trend of variation of our computed specific heat in the temperature range 1 K ≤ T ≤ 1000 K are in good agreement with corresponding experimental data for almost all the compositions (x). The specific heat found to increase with temperature from 1 K to 300 K, while they decrease with concentration (x) for these perovskite ruthenates. Besides, we have reported the thermal properties, like thermal expansion (α), molecular force constant (f), Reststrahlen frequency (υ), cohesive energy (ϕ), Debye temperature (θ_D) and Gruneisen parameter (γ).

The S phase (Al₂CuMg) is an important strengthening phase for the Al–Cu–Mg alloys, which are widely used in the aerospace and transportation industries. The commonly added alloying elements (Mn, Ti, Zr) and the impurity elements (Fe and Si) in the Al–Cu–Mg alloys are always found in the S phase. First-principles calculations based on the density functional theory (DFT) were used to investigate the influence of doping Mn, Ti, Zr, Fe and Si elements on the S phase. Key findings demonstrated that these elements prefer to occupy different atomic sites in the S phase. Ti and Zr improved the structural stability of the S phase. The bulk modulus of the Fe, Si, Ti and Zr doped S phases becomes larger than that of the pure S phase. Both the crystal and electronic structures of the S phase are affected by the dopants. The results of this study provide a better theoretical understanding of the S phase, providing guidance for improved composition design and performance optimization of Al–Cu–Mg alloys.

The melting temperatures of alkali halides (LiCl, LiF, NaBr, NaCl, NaF, NaI, KBr, KCl, KF, KI, RbBr, RbCl, RbI and CsI) have been evaluated over a wide range of pressures. The solid–liquid transition of alkali halides is of considerable significance due to their huge industrial applications. Our formalism requires a priori knowledge of the bulk modulus and the Grüneisen parameter at ambient conditions to compute $T_m$ at high pressures. The computed values are in very good agreement with the available experimental results. The formalism can satisfactorily be used to compute $T_m$ at high pressures where the experimental data are scanty. Most of the melting curves ($T_m$ versus $P$) exhibit nonlinear variation with increasing pressure having curvatures downward and exhibit a maximum in some cases like NaCl, RbBr, RbCl and RbI. The values of $T_m^{max}$ and $P_{max}$ corresponding to the maxima of the curves are given.

Molecular dynamics (MD) simulations are employed to study the elastic properties of gallium nitride (GaN) nanosheets. Young’s and bulk moduli of GaN nanosheets with different side lengths and height/width ratio are obtained. Besides, the configuration of the nanosheet at different strains is represented until the fracture initiation and final fracture are observed. It is seen that the zigzag nanosheets have larger elastic moduli than armchair ones with the same sizes. Moreover, increasing the length size of the nanosheets results in decreasing Young’s modulus. Bulk moduli of GaN nanosheets are also obtained by applying biaxial loading on all edges. It is seen that under the biaxial tensile force, the fracture is initiated at the nanosheet corners and is continued toward the nanosheet center. A nonlinear relation between the bulk modulus and nanosheet size is observed.

In the present paper, we have investigated SmAlO₃ for their thermodynamic properties under effect of pressure and temperature by employing density functional theory (DFT) and quasi-harmonic Debye model. The various thermodynamic properties like Bulk Modulus, entropy, internal energy, Helmholtz free energy, Debye temperature, coefficient of thermal expansion, Grüneisen parameter and heat capacities of the ternary alloy are calculated. We found that Bulk Modulus, Debye temperature and Helmholtz free energy have decreasing trend with rise of temperature while their values have increasing behavior with rise of pressure. The internal energy of the system almost remains same with variation in pressure but temperature effectively increasing it. Our results are in good agreement with available data at low-temperature limit.

The expressions for the pressure dependences of bulk modulus and the Grüneisen parameter used by Arafin and Singh [Int. J. Mod. Phys. B 30, 1750031 (2016)] in the Lindemann law for determining melting curves of 14 alkali halides have been found to yield no results which are consistent with the recent studies on equations of state. It is demonstrated here that Arafin–Singh’s expressions are physically not acceptable as they are shown here to give unrealistic values of bulk modulus and Grüneisen parameter for the materials at high pressures.

The results of ab initio calculations of the bulk moduli (B₀) and related structural and electronic properties of selected transition metals and their nitrides are presented. There is a correlation between B₀ and valence charge density. B₀ does not vary monotonically with the addition of d electrons. Charge density and density of states (DOS) plots enable us to explain it.

Wide band gap semiconductors such as group-IV carbides (SiC, GeC) and group-III nitrides (AlN, GaN and BN) are known to be important materials for novel semiconductor applications. They also have interesting mechanical properties such as having a particularly high value for their bulk modulus and are therefore potential candidates for hard coatings. In this paper we report the theoretical calculations for the bulk modulus for zincblende and wurzite polytypes of these materials. The Density Functional and Total-energy Pseudopotential Techniques in the Generalized Gradient approximation, an ab initio quantum mechanical method, is used to obtain the theoretical structure, from which equilibrium lattice parameters and volume of the cell versus pressure may be extracted. The Murnaghan's equation of state is then used to calculate bulk modulus under elastic deformation, which is related to the hardness of a material under certain conditions. The results for bulk modulus are compared with other theoretical and experimental values reported in the literature.

The letter casts some light on the structural, elastic and electronic properties of C49- and C54-TiSi₂, using an ab initio plane-wave ultrasoft pseudopotential method based on generalized gradient approximation (GGA). An intrinsic advantage in the growth stage for C49 phase might explain its kinetically favored phenomena in a solid-state reaction.

The structural and electronic properties of selected noble metal nitrides and carbides are studied using the local density approximation and the generalized gradient approximation. The zinc-blende and rock-salt structures are employed. The nitride and carbide of palladium have high bulk modulus. From the DOS, the MX (M = Pd, Ag, Au; X = N, C) compound display metallic nature.

Through the quasi-harmonic Debye model, the pressure and temperature dependences of linear expansion coefficient, bulk modulus, Debye temperature and heat capacity have been investigated. The calculated thermodynamic properties were compared with experimental data and satisfactory agreement is reached.

On the basis of the thermal equation-of-state a simple theoretical model is developed to study the pressure dependence of melting temperature. The model is then applied to compute the high pressure melting curve of 10 metals (Cu, Mg, Pb, Al, In, Cd, Zn, Au, Ag and Mn). It is found that the melting temperature is not linear with pressure and the slope $d{T}_m/dP$ of the melting curve decreases continuously with the increase in pressure. The results obtained with the present model are also compared with the previous theoretical and experimental data. A good agreement between theoretical and experimental result supports the validity of the present model.