Papers by Keyword: Pseudopotential

Abstract: The effect of point defects on the shear rupture resistance in titanium aluminide is investigated by the density functional theory and pseudopotential methods. Vacancies, as well as substitution atoms – tungsten and chromium were considered as points defects. The shear was simulated in the(111) slip plane for two directions, namely [110] and [11-2]. It is shown that for a {111}<110> sliding system, vacancies significantly reduce the shear resistance. However, when alloying element occupies a titanium vacancy, it can partially compensate for this negative effect.

Abstract: Quantum-mechanical calculations were used to investigate shear rupture in intermetallic titanium aluminide (TiAl) alloys in the presence of vacancy or chromium dopant. The substitution of both Ti and Al atoms by Cr atoms in the γ-TiAl crystal lattice was considered. The simulation of shear was carried out in the (111) slip plane along two directions, namely the [110] and [11-2]. The decrease in the shear resistance of the defects present in the γ-TiAl lattice was estimated. It was shown that when chromium occupies a titanium vacancy, it can compensate for this defect by increasing the shear modulus for the {111} <110> slip system.

Abstract: The theoretical study of dynamical variables: velocity autocorrelation function (VACF), power spectrum (PS) and mean square displacement (MSD) of Bi₅₀Zn₅₀ bulk metallic glass at T= 793K, 873K, 1023K and 1123K have been calculated based on the static harmonic well approximation. The effective interatomic potential for bulk metallic glass is computed using our established model potential with the exchange correlation functions due to Farid et al. It is observed that negative dip in velocity auto correlation function decreases as the temperature increases. In the power spectrum as temperature increases, the peak of power spectrum shifts toward lower ω. The obtained result of MSD concludes that the vibrating component in the atomic motion is decreases as increases the temperatures.

Abstract: In the theoretical study of thermophysical properties of materials at high temperature and pressure anharmonic effects are important. In this regard, in the present communication, anharmonic effects are taken into account by coupling recently proposed mean field potential (MFP) in conjunction with local pseudopotential for fcc transition metals Ni, Pd and Pt. Using present approach, we have computed volume variation of binding energy at 0K, vibrational free energy of lattice ions (F_ion), Helmholtz free energy and relative volume thermal expansion up to melting temperature of Ni, Pd and Pt. Further, applicability of present coupling scheme has been tested by investigating equation of state (EOS) at 300K which explains the microscopic internal structure of material. Our results are in good agreement with experimental and other theoretical findings which demonstrate the capability of present coupling scheme of MFP with local pseudopotential alongwith its computational simplicity.

Abstract: Different elastic properties of Pd-based Pd₆₄Fe₁₆P₂₀ have been computed using two different forms of pseudopotential. Both the forms differ within the core region suggesting different degrees of orthogonalization and its r-dependence, while outside the core both preserves the Columbic nature. The only parameter, the core radius, separates these regions, and it is calculated using the realistic assumption based on the nearest neighbor distance. Further, five different forms of the local field correlated functions, namely, Hartree (H), Taylor (T), Ichimaru and Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are examined for better understanding regarding the screening effect on different elastic constants for Pd₆₄Fe₁₆P₂₀ bulk metallic glass. The calculated results for the various elastic moduli, Poisson's ratio, sound velocity and phonon dispersion curve are all found to be in general agreement with the available experimental data. This confirms the applicability of the model pseudopotential to bulk metallic systems.

Abstract: We present the calculation of structural properties for liquid Ga at different temperatures using pseudopotential theory. The temperature dependence of structure factor has been determined using the hard-sphere Percus-Yevick approximation which is characterized by single parameter hard sphere diameter or equivalently packing fraction. The temperature dependent hard-sphere diameter σ (T) is estimated using criterion from the calculated effective pair potential. The modified empty-core pseudopotential due to Hasegawa et al. (J. Non-Cryst. Solids. 117/118 (1990) 300), which is valid for all electrons and contains the repulsive delta function to achieve the necessary s-pseudisation is used for electron–ion interaction. The temperature effects have been studied via dimensionless damping term and potential parameter in the pair potential. Finally, the predicted results for structure factor, pair correlation function and coordination numbers have been compared with recent available data, and a good agreement has been achieved.

Abstract: The methodical expressions for the phonon frequencies of Zr57Ti5Cu20Ni8Al10 bulk metallic glass (BMG) both for longitudinal and transverse phonon modes are computed for the first time using Hubbard-Beeby (HB) approach and our well recognized model potential. The self-consistent phonon scheme given by Takeno-Goda, involving multiple scattering and phonon Eigen frequencies expressed in terms of many-body correlation functions of atoms as well as of interatomic potential in the solids, has been used to generate the collective modes in the Zr57Ti5Cu20Ni8Al10 BMG. In the present paper, the local field correction functions due to Hartree (H), Farid et al (F), Sarkar et al (S) and Hubbard Sham (HS) are used to examine the influence of screening effects on the vibrational properties. Further, from the long wavelength limit of phonon frequencies, various elastic constants have been calculated and are found to be in good agreement with experimental and other available data.

Abstract: Ternary alloys of group III-V semiconductors have important applications in fabrication of electro-optical devices. Their refractive index and related optical properties are of attractive interest in theoretical and experimental study. According to the Philips scale of iconicity, BP (fi =0.006) and BAs (fi =0.002) are the most covalent of the III-V semiconductors and there are interesting consequences of this property. We present a theoretical procedure for the study of refractive index of ternary alloy BAs1-xPx. The calculations are based on the pseudopotential formalism in which local potential coupled with the virtual crystal approximation (VCA) is applied to evaluate refractive index for the entire range of the alloy composition x of the ternary alloy BAs1-xPx. To incorporate screening effect, Nagy’s local field correction function has been employed. The screening functions of Hartree, Taylor, Ichimaru et al., Farid et al. and Sarkar et al. are also integrated for comparative study. Our results for parent compounds are compared to experiment and other available such theoretical findings and showed relatively good agreement. During present theoretical study it is concluded that refractive index fairly depends on the selection of the local field correction function.

Abstract: Near the melting point liquid alkali metals show positive dispersion, which can be described within generalized hydrodynamics as a visco-elastic reaction of the simple liquid. To understand an upward bending of the dispersion relation at small momentum transfer, treatment of pseudopotential theory on liquid potassium is performed at different temperatures in entire liquid regime. In the present study, we used the modified empty core potential due to Hasegawa et al. along with a local field correction due to Ichimaru-Utsumi (IU) to explain an electron-ion interaction. The potential used is composed of a full electron-ion interaction and a repulsive delta function to incorporate the orthogonalisation effect due to the s-core states. The temperature dependence of pair potential is calculated by using the damping term, exp(-πkBTr/2kF). While the expression for phonon dispersions are derived within the memory function formalism. Results for longitudinal phonon frequencies, and thus derived sound velocities and elastic modulii are compared with recent inelastic X-ray scattering experiment. It is found sufficient to carried temperature effect into the description only via the damping factor, keeping the volume of normal melting.

Abstract: A new proposal of a local pseudopotential is put forwarded here depending on the concept of extended core radius in which half of the nearest neighbour distance is treated as effective core radius. There is no input of any property for fitting this parameter in this formalism. This pseudopotential is found to satisfy all the necessary requirements for applications. With this model potential we have evaluated the form factors for several bcc, fcc and hcp metals and achieved excellent agreement with previous results. On the same footing, we have examined other 14 local pseudo potentials also and on the basis of the comparison, the presently proposed pseudopotential is found to be much better. As a test case study, we have evaluated phonon dispersion curves of some liquid metals, viz. Na (Z = 1), Mg (Z = 2), Al (Z= 3) and Pb (Z = 4) and obtained quite satisfactory results.