Papers by Keyword: First-Principles Calculations

Abstract: In this paper, the mechanical, elastic, electronic and optical properties of thallium based-perovskites TlSnX₃ (X = F, Cl, Br and I) were investigated using the first-principles calculations. The elastic parameters calculations show that the perovskites are ductile, anisotropic, and mechanically stables. The cohesive energy calculations indicate that the evaluated perovskites are thermodynamically stable. Moreover, the band calculations with HSE06 method reveal that all perovskites TlSnX₃ (X = F, Cl, Br and I) present a semiconductor feature. Further, the optical properties such as reflectivity, refractive index, extinction and absorption coefficients have been calculated and compared for all perovskites compounds. Interestingly, the found results show that the absorption coefficient α(ω) in the visible and infrared regions reaches high values of 1.02, 1.19, 1.14 and 1.03 × 10⁶ cm^-1 for TlSnI₃, TlSnBr₃, TlSnCl₃ and TlSnF₃ , respectively. These results suggest that the heavy thallium perovskites TlSnX₃ (X = F, Cl, Br and I) have potential for optoelectronic applications.

Abstract: In this paper, we have investigated the electronic, optical and thermoelectric properties of the puckered Si₂SeTe monolayer when subjected to various levels of biaxial strain ranging from −10% to +10%. The structural stability, as determined by the cohesive energy, shows that the puckered Si₂SeTe structure is energetically stable. The results reveal that the unstrained Si₂SeTe monolayer is an indirect band gap semiconductor with an energy gap of 0.5 eV, which can be effectively adjusted with biaxial strain. The semiconductor–metal phase transition occurs when the monolayer is compressed by −4% biaxial strain. Moreover, the optical properties, including the real ε₁(ω) and imaginary ε₂(ω) components of the dielectric function, extinction coefficient K(ω), reflectivity R(ω), refractive index n (ω), and absorption coefficient α (ω), were evaluated as a function of the energy of light and under biaxial strain. We discovered that the puckered Si₂SeTe monolayer is capable of absorbing light in the visible region of 64.7×10⁴ cm⁻¹, 73.8×10⁴ cm⁻¹ for equilibrium state and under the compression strain (−8%), respectively. Lastly, the influence of biaxial strain on thermoelectric properties such as electrical conductivity (σ/τ), electronic thermal conductivity (k_e/τ), Seebeck coefficients, and electronic figure of merit (ZT_e) was studied. The calculated electronic figure of merit ZT_e presents an improvement in the p-type doping (μ<0) under the tensile biaxial strain. Taking into account the optical and thermoelectric properties, the puckered Si₂SeTe monolayer is a promising material for use in optoelectronic devices and energy conversion technologies.

Abstract: The intermediate band semiconductor of AgGa_1-xCr_xS₂ is investigated by the first principles calculations and further confirmed by the experimental results. The band structures of pure and Cr-doped crystals were calculated and it is shown that the crystal with a direct energy band gap of about 0.95 eV for AgGaS₂. Because of Cr dopant, a metallic intermediate band (IB) is successfully formed in the host. From the partial density of states (PDOS) of Cr-doped AgGaS₂, the IB mainly comes from the hybridization of the Cr-3d and S-3p states. Based on the theoretical predications, the Cr-doped AgGaS₂ is synthesized by the high-temperature solid state reaction. Two extra absorption responses are detected in the absorption spectra. The optical absorption coefficients are enhanced in the visible radiation range due to the formation of metallic and isolated IB. Therefore, Cr-doped AgGaS₂ with an intermediate band is suggested as a potential material to enhance the efficiency of solar cells.

Abstract: First-principles calculations were conducted to investigate the effects of Zn on the structure of β″ phase. The effects of Cu, which was often added in the alloy, were also taken into consideration. Firstly, single Zn or Cu atom was doped on different sites of the β″ phase. Then the formation enthalpies and lattice constants of doped β″ phases were calculated. The results showed that it was more energetically favorable for single Zn or Cu atom to occupy Si3/Al sites than other sites. Furthermore, different quantities of Zn or Cu atoms were doped on Si3/Al sites. With the amounts of doping atoms increasing, the formation enthalpies of β″ phases doped by Zn were lower than which doped by Cu, indicating that it was more preferential for Zn to enter the β″ phase when Zn content was higher than Cu. Additionally, the doping of Zn could reduce the formation enthalpies of the β″ phase, which promoted the formation of the β″ phases. As a result, the aging hardening response of the alloy was improved. High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization was also conducted on a peak-aging Zn added Al-Mg-Si-Cu alloy. The HAADF-STEM image of β″ phase showed that the occupancies of Zn atoms were just on the Si3/Al sites and substituted all the Al atoms, which was consistent with the results of first-principles calculations.

Abstract: Tungsten (W) is suitable for solid targets of spallation neutron source due to its high neutron yield. The prediction of radiation effects of W is, therefore, of importance; especially, the influence of solute elements are complex and are not clearly known to date. We discuss here the solute effects using the first principles and kinetic Monte Carlo (KMC) calculations and show that Re and Os, which are nuclear transmutation products of W, can largely change the stability and mobility of radiation defects. Such influences of the solute elements seem to explain the unsolved mechanism of the microstructural evolution of W-based materials under irradiation.

Abstract: The first-principles calculations were performed to research effects of elements X (Au, Be, Pd, Y, Ca, Cu, In and Zn) on mechanical and electronic properties of Ag with the density function theory (DFT). A supercell consisting of 107 atoms of Ag and one solute atom X was used. It was found that the bulk modulus of Ag dilute solutions were affected by the bulk modulus of pure alloying elements as well as their volume. The shear modulus G trend to decrease with increase of volume of Ag caused by alloying addition, but Ag-X covalent bond had positive correlation with shear modulus G. All of Ag₁₀₇X alloys were ductility since theirs B/G ratio, Poisson's ratios ν were larger than 1.75 and 0.33, respectively. Comparing to other calculated Ag₁₀₇X alloys, Ag₁₀₇Be and Ag₁₀₇Cu had the larger Vickers hardness, the value of which were 3.96GPa, 3.86GPa, respectively. There were not only metallic bonds (Ag-Ag) but also covalent bonds (Ag-X) in Ag₁₀₇X alloys. The strong covalent bonds between Y, Zn, Ca and Ag were mainly caused by orbital hybridization between Y-5p orbital, Zn-3d orbital, Ca-3d orbitals and Ag-4d, 5s and 5p orbitals.

Abstract: In recent time, interest to ferrite magnetic nanomaterials has considerably grown mainly due to their much promising medical and biological applications. The spinel ferrite powder samples having high heat generation ability in AC magnetic field was studied for application to hyperthermia treatment of cancer tumor. These properties of ferrites are strongly depending on their chemical composition, ion distribution, spin orientation and method of preparation in general and crystal structure in particular nature of the material. In this study, several samples of ferrite magnetic structures were investigated by neutron diffraction. The explanation of the mechanism to occurs the heat generation ability in the magnetic materials and the electronic and magnetic states of ferrite-spinel – type structures were theoretically defined by the first-principles calculations within the framework of DFT.

Abstract: First-principles calculations have been performed to investigate the phase stability, elastic, and thermodynamic properties of Co₃(Al,Mo,Ta) with the L1₂ structure. Calculated elastic constants showed that Co₃(Al,Mo,Ta) is mechanically stable and possesses intrinsic ductility. Young’s and shear moduli of polycrystalline Co₃(Al,Mo,Ta) were calculated using the Voigt-Reuss-Hill approach. It was found that the shear and Young’s moduli of Co₃(Al,Mo,Ta) were smaller than those of Co₃(Al,W). States density indicated the existence of covalent-like bonding in Co₃(Al,Mo,Ta). Temperature-dependent thermodynamic properties of Co₃(Al,Mo,Ta) could be described satisfactorily using the Debye-Grüneisen approach, including entropy, enthalpy, heat capacity and linear thermal expansion coefficient, showing their significant temperature dependences. Furthermore the obtained data could be employed in the modeling of thermodynamic and mechanical properties of Co-based alloys to enable the design of high temperature alloys.

Abstract: The structural, electronic and elastic properties of CeTl with CsCl-type B₂ structure have been investigated using full-potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT) within the generalized gradient approximation (GGA) for exchange and correlation potential. The ground state properties such as lattice constant, bulk modulus and pressure derivative of bulk modulus have been calculated which are in good agreement with available experimental data. The band structure and density of state depict that 4f electrons of Ce element have dominant character in electronic conduction and are responsible for metallic character of CeTl. The charge density plot reveals that the metallic as well as ionic bonding exist between Ce and Tl atoms. The calculated elastic constants indicate that CeTl is mechanically stable in cubic B₂ phase and found to be ductile in nature.

Abstract: To study the physical origin of the periodic arrangement of the quadrople solute-enriched layers in Mg-based LPSO structures, we carry out first-principles calculations of the formation energy of the L1₂ cluster and investigate effects of phonon on the inter-planer ordering of the solute-enriched layers using the 1-dimensional chain model with mass change. The formation energy of the L1₂ cluster increases as the period of the LPSO structure decreases. Thus, it is found that the electron-mediated interaction is short-range repulsive. On the other hand, in the case of considerably heavy mass change, the ordering of the mass changes is stabilized by phonons and the energy gain increases with the concentration of the mass changes, i.e., the short LPSO period is favorable. A promising mechanism of the inter-planer ordering of the LPSO structures is the phonon-mediated interaction of the quadrople layers where heavy solute atoms are enriched as the L1₂ clusters at SFs.