Sıtkı Duman

Ab initio calculation of phonons in bulk HfC and the HfC(001)(1 × 1) surface

Structural, elastic and dynamical properties of the rock-salt HfC have been studied by employing an ab initio pseudopotential method and a linear response scheme, within the generalised gradient approximation. In addition to achieving good agreement with experimental measurements of the phonon dispersion results along the symmetry direction Γ-X, we have presented accurate dispersion results along several other directions in the Brillouin zone. Sharp features in the phonon density of states are ascertained and their origin discussed. This is followed by investigations of the structural, electronic and phonon properties of the HfC(001) surface. The agreement between our surface phonon spectrum and experimental measurements is very good. By analysing the atomic displacement patterns of the zone-centre and zone-edge phonon modes we have determined the energy locations and polarisation characteristics of Fuchs–Kliewer, Wallis, Lucas and Rayleigh modes.

Structural, electronic, optical, vibrational and transport properties of CuBX2 (X = S, Se, Te) chalcopyrites

The structural, electronic and optical properties of CuBX2 (X = S, Se, Te) chalcopyrite semiconductors have been studied using the full-potential (linearized) augmented plane-wave (FP(L)APW) method based on the density functional theory (DFT) within the Yukawa screened-PBE0 (YS-PBE0) hybrid functional as implemented in the WIEN2k package. We have found that our calculated structural and electronic parameters such as lattice parameter, tetragonal ratio, anion displacement and energy band gap are in very good agreement with previous experimental results. We have also presented the real and imaginary parts of the dielectric function, refractive index and absorption coefficients to describe optical properties of the investigated chalcopyrite semiconductors. Furthermore, the phonon dispersion curves and corresponding density of states have been studied by using a linear response approach based on the density functional perturbation theory implemented in the Quantum ESPRESSO code. Finally, transport properties such as the Seebeck coefficient, thermal and electrical conductivity and the figure of merit for these materials have been calculated using the semi-classical Boltzmann theory as implemented in the BoltzTraP code.

Investigation of structural, mechanical, electronic, optical, and dynamical properties of cubic BaLiF3, BaLiH3, and SrLiH3

The structural, mechanical, electronic, optical, and dynamical properties of BaLiF3, BaLiH3, and SrLiH3 cubic perovskite materials are theoretically investigated by using first principles calculations. Obtained results are in reasonable agreement with other available theoretical and experimental studies. The considered materials are found to be mechanically stable in the cubic structure. We found that all materials are brittle. The modified Becke–Johnson (mBJ) exchange potential has been used here to obtain an accurate band order. The calculated band-gap energy value of BaLiF3 (8.26 eV) within the mBJ potential agrees very well with the experimentally reported value of 8.41 eV. In order to have a deeper understanding of the bonding mechanism and the effect of atomic relaxation on the electronic band structure, the total and partial density of states have also been calculated. We have investigated the fundamental optical properties, such as the real e 1(ω) and imaginary e 2(ω) parts of the dielectric function, absorption coefficient α(ω), reflectivity R(ω), and refractive index n(ω) in the energy range from 0 to 40 eV within the mBJ potential. The band-gap energy obtained from the absorption spectrum is around 8.76, 3.99, and 3.31 eV for BaLiF3, BaLiH3, and SrLiH3 crystals, respectively. It should be noted that BaLiF3 could be a strong potential candidate as a laser material for the development of a vacuum-ultraviolet light emitting diode once direct transition is confirmed by experimental studies. Finally, we have calculated the lattice dynamical properties of BaLiF3, BaLiH3, SrLiH3, and SrLiF3 crystals. The full phonon dispersion curves of these materials are reported for the first time. Our results clearly indicate that the materials are dynamically stable, except for SrLiF3, in the cubic structure. The obtained zone-center phonon frequencies of BaLiF3, BaLiH3, and SrLiH3 accord very well with previous experimental measurements.

Investigation of the structural, mechanical, dynamical and thermal properties of CsCaF3 and CsCdF3

The structural, mechanical, dynamical and thermal properties of CsCaF3 and CsCdF3 are presented by using an ab initio pseudopotential method and a linear response scheme, within the generalized gradient approximation. The obtained structural and mechanical properties are in good agreement with other available theoretical and experimental studies. The calculated elastic constants of these materials obey the cubic stability conditions. It has been found that CsCaF3 is brittle whereas CsCdF3 has ductile manner. The full phonon dispersion curves of these materials are reported for the first time in the literature. We have found that calculated phonon modes are positive along the all symmetry directions, indicating that these materials are dynamically stable at the cubic structure. The obtained zone-center phonon modes for CsCaF3 (IR data) are found to be 83 (98) cm−1, 104 (115) cm−1, 120 cm−1, 180 (192) cm−1, 231 (250.5) cm−1, 361 (374) cm−1, 446 (449) cm−1. Also, we have calculated internal energy, Helmholtz free energy, constant-volume specific heat, entropy and Debye temperature as function of temperature. At the 300 K, specific heats are calculated to be 113.36 J mol−1 K−1 and 115.58 J mol−1 K−1 for CsCaF3 and CsCdF3 ,respectively, which are lower than Doulong–Petit limit (12 472 J mol−1 K−1).

Ab initio calculations of the electronic and phonon states on the HfC(001)-(1×1) surface

We have investigated the structural and electronic properties of the HfC(001)-(1×1) surface using the density functional theory and the pseudopotential method. The most important feature of the HfC(001) surface relaxation is that both in the surface and subsurface layers C atoms move out of the surface and Hf atoms move inward. In general, our structural results are in good agreement with previous experimental and theoretical works. We have identified surface electronic states on the HfC(001) surface in the bulk gap pocket around the Fermi energy. Using our calculated atomic and electronic structures, zone-centre and X point surface phonon modes are calculated by employing a linear response approach based on the density functional perturbation theory. Good agreement has been observed between our calculated surface phonons and available experimental results. A hitherto unidentified surface localized mode has been predicted and its polarization characterists has been examined.

First-principles study of structural, mechanical, lattice dynamical and thermal properties of nodal-line semimetals ZrXY (X=Si,Ge; Y=S,Se)

Abstract The nodal-line semimetals are new and very promising materials for technological applications. To understand their structural, mechanical, lattice dynamical and thermal properties in detail, we have investigated theoretical study of ZrXY (X = Si,Ge; Y = S,Se) using Density Functional Theory for the first time. Obtained lattice parameters are in excellent agreement with previous experimental data. These nodal-line semimetals obey the mechanical stability conditions for tetragonal structure. We obtain Bulk modulus, Shear modulus, Poisson’s ratio, Pugh ratio, sound velocities and thermal conductivity using elastic constant. All the materials behave in brittle manner. Poisson’s ratio data and Bader charge analysis results indicate that the ionic bonding characters are dominant. Next, the lattice dynamical properties are calculated. Phonon density of states shows that nodal-line semimetals ZrXY are also dynamically stable in the tetragonal structure. Raman and IR active phonon modes are determined. Highest optical mode at gamma point corresponds to A2u (IR active) and Eg (Raman active) modes for ZrXSe and ZrXS, respectively. Based on phonon density of states, thermal properties such as Helmholtz free energy, entropy, heat capacity at constant volume and Debye temperature are also presented and discussed.

Structural, electronic and lattice dynamical properties of the BeS(110) surface

We present results for the relaxed atomic geometry, electronic states, and lattice dynamics of the BeS(110) surface. Our investigations are based on first principles pseudopotential calculations within the local density approximation of the density functional theory. The relaxed geometry of this surface follows the general trend observed for traditional III-V(110) and II-VI(110) surfaces, with a surface tilt of 26.3°. Surface localised phonon modes below and above the bulk continuum, and within the acoustic-optical gap region have been dtermined.

Theoretical investigation of the structural, electronic, dynamical and thermal properties of YSn3 and YPb3

We have performed an ab initio study of the structural, electronic, dynamical and thermal properties of the cubic AuCu3-type YSn3 and YPb3 by using the density functional theory, plane-wave pseudopotential method and a linear response scheme, within the generalized gradient approximation. An analysis of the electronic density of states at the Fermi level is found to be governed by the p states of Sn and Pb atoms with some contributions from the d states of Y atoms. The obtained phonon figures indicate that these material are dynamically stable in the cubic structure. Due to the metallic behavior of the compounds, the calculated zone-center phonon modes are triply degenerate. Also the thermal properties have been examined.

The structural, electronic, dynamical and thermal properties of LaSn3 and LaPb3

The structural, electronic, dynamical and thermal properties of AuCu3-type LaSn3 and LaPb3 were investigated by using ab-initio pseudopotential method and a linear response scheme, within the generalized gradient approximation. The obtained atomic parameters are in good agreement with other available theoretical and experimental studies. We have found that calculated phonon modes are positive along the all symmetry directions, indicating that these materials are dynamically stable at the cubic structure. The obtained phonon energies of LaSn3 are in good agreement with Neutron scattering measurements. Obtained phonon modes are triply degenerated at the Γ point due to the metallic nature of these materials. However, at 300u2005K, specific heats of these compounds have been calculated 98.07u2005J/molK and 98.52u2005J/molK for LaSn3 and LaPb3 respectively.

Structural, elastic and vibrational properties of ScGa3

We have presented theoretical investigations of the atomic, elastic and lattice dynamical properties of ScGa3 using an ab-initio pseudopotential method and a linear response scheme, within the generalized gradient approximation. The calculated atomic parameters and elastic constants are in agreement with previous ab initio calculations and experimental measurements. The material shows ductile behavior. Full phonon dispersions for this material are reported.