By means of density-functional theory with the generalized gradient approximation and using the virtual-crystal approximation, we report first-principles calculation results on the structural and elastic properties of Ti_{1-x}Zr_{x}N alloy. In order to gain some further information on the mechanical properties of Ti_{0.5}Zr_{0.5}N compound, we also calculated the Young modulus, Poisson ratio, and anisotropy factor. The variation of calculated unit cell parameter for Ti_{1-x}Zr_{x}N structure increases with Zr content x. A linear dependence of the elastic constants and the bulk modulus over a range of composition x is found. All the C_{ij} of Ti_{0.5}Zr_{0.5}N increase linearly with increasing pressure. The same behaviour is observed for the other compounds with Zr compositions x.
Based on the density functional theory and the quasi-harmonic Debye model, the structural and thermodynamic properties of I-4m2-BCN have been studied in this paper. Some structural parameters are presented in this work. All of these results are in excellent agreement with the other available results. The anisotropy of elastic properties are also studied systematically in this paper. Finally, the thermodynamic properties of I-4m2-BCN are also researched through the quasi-harmonic Debye model. The relations among the thermal expansion α, the Debye temperature Θ_{D}, the heat capacity C_V and C_P, the Grüneisen parameter γ , entropy S, and the Gibbs free energy G with pressure P and temperature T are studied systematically.
There was employed the density functional theory plane-wave pseudopotential method with local density approximation and generalized gradient approximation to investigate the structural, elastic and mechanical properties of the intermetallic compound Ni_3Ga. The calculated equilibrium lattice constant and bulk modulus are in good agreement with the experimental values. The elastic constants were determined from a linear fit of the calculated stress-strain function according to Hooke's law. From the elastic constants, the bulk modulus B, anisotropy factor A, shear modulus G, Young's modulus E and Poisson's ratio υ for Ni_3Ga compound are obtained. Our results for the bulk modulus B, anisotropy factor A, shear modulus G, Young's modulus E and Poisson's ratio υ are consistent with the experimental values. The sound velocities and the Debye temperature are also predicted from elastic constants. The dependences of the elastic and mechanical properties of Ni_3Ga compound on pressure were investigated for the first time. It was found that the cubic Ni_3Ga compound is mechanically stable according to the elastic stability criteria and it is not elastically isotropic. By analyzing the ratio B/G, it was concluded that Ni_3Ga compound is ductile in nature.
Diamond anvil cell experiments suggest that upon compression above 26.5 GPa silane (SiH_4) forms a polymeric phase VI, whose crystal structure has not yet been solved. Here we present DFT calculations showing how phonon-guided optimization leads to a polymeric Fdd2 structure which is the lowest-enthalpy polymorph of SiH_4 above 26.8 GPa, and which most probably can be identified as the experimentally observed polymeric phase. The new algorithm of predicting the lowest-energy structures enables simultaneous inspection of the potential energy surface of a given system, calculation of its vibrational properties, and assessment of chances for obtaining a metastable ambient-pressure structure via decompression. Our calculations indicate that at room temperature the differences in the vibrational and entropy terms contributing to the Gibbs free energy of different polymorphs of silane are negligible in comparison with corresponding differences in the zero-point energy corrections, in contrast to earlier suggestions. We also show that the Fdd2 polymorph should be metastable upon decompression up to 5 GPa, which suggests the possibility of obtaining a polymeric ambient-pressure form of SiH_4. Polymeric silane should exhibit facile thermal decomposition with evolution of molecular hydrogen and thus constitute an efficient (12.5 wt%) material for hydrogen storage.
Our work focuses on the study of the electronic structure of undoped and K-doped ZnO using density functional theory as implemented in the Wien2k package. Generalized gradient approximation and GGA plus Tran-Blaha-modified Becke-Johnson (TB-mBJ) were used to calculate the exchange-correlation energy. From the electronic properties, ZnO has a direct band gap in (Γ-Γ) direction with a value of 0.76 eV within GGA and 2.63 eV within GGA + TB-mBJ. For the K-doped ZnO (12.5%) the gap was found to be 1.15 eV within GGA and 3.28 eV within GGA + TB-mBJ, we have observed that an emersion of a new narrow band exists in the valence band which is mainly caused by K 3p states with a little Zn 4s and Zn 3d effect.
Using first-principle method, we investigate the structural, electronic, optical, and thermodynamic properties of the CdS_{1-x}Te_x semiconductor alloys using generalized gradient approximation for the exchange-correlation potential calculation. The ground state properties are determined for the bulk materials (CdS and CdTe) in cubic phase. Quantities such as the lattice constants and bulk modulus of interest are calculated. Detailed comparisons are made with published experimental and theoretical data and show generally good agreement. The calculated lattice constants scale linearly with composition (Vegard's law). The microscopic origins of the bowing parameter were explained using the contributions from volume deformation, charge transfer and structural relaxation approach. The refractive index and optical dielectric constant for the alloys of interest were calculated by using different models. In addition, the thermodynamic stability of the alloys was investigated by calculating the critical temperatures of alloys.
In this paper 1D crystal lattice is analyzed within harmonic approximation, with one atom per elementary cell and nearest neighbor interaction included. For this type of crystal lattice dispersion relations are well known. Thermodynamic functions (specific heat and phonon thermal conductivity) are calculated via phonon density of states given in exact form. Thermodynamic variables are calculated for a whole temperature range. In limiting cases of low and high temperatures these thermodynamic variables can be found in analytic forms. For thermal conductivity the results of Callaway model for exact phonon density of states are compared with the results of Callaway model for Debye approximation of phonon density of states.
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