Ab initio studies of structural,electronic, optical,elastic and thermal properties of Ag-chalcopyrites (AgAlX2: X=S,Se)

The ab-initio calculations for the structural, electronic, optical, elastic and thermal properties of Ag-chalcopyrites (AgAlX₂: X=S and Se) have been reported using the full potential linearized augmented plane wave (FP-LAPW) method. In this paper, the recently developed Tran–Blaha modified Becke–Johnson potential is used along with the Wu-Cohen generalized gradient approximation (WC-GGA) for the exchange-correlation potential. Results are presented for lattice constants, bulk modulus and its pressure derivative, band structures, dielectric constants and refractive indices. We have also computed the six elastic constants (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, C₆₆). The thermodynamical properties such as thermal expansion, heat capacity, Debye temperature, entropy, bulk modulus are calculated employing the quasi-harmonic Debye model at different temperatures (0–900 K) and pressures (0–8 GPa) and the silent results are interpreted. Hardness of the materials is calculated for the first time at different temperatures and pressures.     

Elastic and optoelectronic properties of novel Ag3AuSe2 and Ag3AuTe2 semiconductors

Ag₃AuSe₂ and Ag₃AuTe₂ are interesting class of semiconducting materials. Here, elastic and opto-electronic properties of Ag₃AuSe₂ and Ag₃AuTe₂ semiconductors are studied in detail using density functional theory. Different schemes are selected to treat the exchange-correlation effects. The unit cell of the compounds is fully optimized and calculated cell constants are found in agreement to the existing experimental data. The calculated elastic constants and elastic moduli reveal that the compounds possess ductile nature and are elastically stable. It is found that both compounds are direct bandgap semiconductor with bandgap value of 1.009 eV for Ag₃AuSe₂ and 0.551 eV for Ag₃AuTe₂. As the compounds have narrow and direct bandgaps, therefore optically active. The optical properties like reflectivity, absorption coefficient, energy loss function, refractive index including and complex dielectric function are studied in detail. The direct band gap nature of these compounds make them useful candidate for different devices applications.     

Theoretical investigation of the electronic structure,optical, elastic,hardness and thermodynamics properties of jadeite

A detailed theoretical study of the electronic structure, optical, elastic and thermodynamics properties of jadeite have been performed by means of the first principles based on the state-of-the-art of density functional theory within the generalized gradient approximation. The optimized lattice constants and the atomic positions are in good agreement with experimental data. The total density of states and partial density of states of jadeite have been discussed. The energy gap has been calculated along the Γ direction found to be 5.338 eV, which shows that jadeite has wide direct band gap. The optical properties, such as the dielectric function, refractive index, extinction coefficient, reflectivity coefficient, loss function and absorption coefficient for [100] and [001] directions have been described for the first time in the energy range 0–40 eV. The elastic constants, bulk modulus, Young׳s modulus, anisotropic factor and Poisson׳s ratio have been calculated. Furthermore, the Vickers hardness and Debye temperature of jadeite have been predicted. The calculated values of all above parameters are compared with the available experimental values.     

Structural,elastic, electronic and thermodynamic properties of uranium filled skutterudites UFe4P12: First principle method

We present a theoretical study of structural, elastic, thermodynamic, and electronic properties of the uranium filled skutterudite UFe₄P₁₂. We use the full-potential linear muffin–tin orbital (FP-LMTO) method in which the local density approximation (LDA) is used for the exchange-correlation (XC) potential. The lattice parameter at equilibrium, the bulk modulus, its pressure derivative, the elastic constants and the band structure energy of the filled skutterudite UFe₄P₁₂ are calculated and systematically compared to available theoretical and experimental data. Herein, we use the total energy variation as function of strain technique to determine independent elastic constants and their pressure dependence. Furthermore, using quasi-harmonic Debye model with phonon effects, the effect of pressure P and temperature T on the lattice parameter, bulk modulus, thermal expansion coefficient, Debye temperature and the heat capacity of UFe₄P₁₂ are investigated for the first time. Band structure of UFe₄P₁₂ indicates a tendency of forming a pseudo-gap that appears above the Fermi level at Γ point. This is a unique characteristic of skutterudite, especially when a single phosphorous p-band crosses the Fermi level. The crossing band is, indeed, pushed down by the repulsion of U f-resonance states.     

Density functional theory study of tin and titanium dioxides: Structural and mechanical properties in the tetragonal rutile phase

Structural and mechanical properties in rutile (tetragonal) phases of SnO₂ and TiO₂ are investigated by performing first-principle density functional theory (DFT) calculations. Generalized Gradient Approximation (GGA) potentials of electronic exchange and correlation part parameterized by Perdew–Burke–Ernzerhof (PBE) are used. Second order elastic stiffness constants, bulk modulus, first-derivative of bulk modulus, and pressure behavior of these mechanical properties are studied up to pressure of 10 GPa. Structural properties and elastic constants of SnO₂ and TiO₂ calculated in this study are compatible with experimental and other available theoretical studies. Electronic band gap energies of these semiconductors are also calculated. As expected, the calculated values by standard DFT calculations are underestimated in comparison to experimental values.     

Structural,elastic, electronic and thermodynamic properties of the filled skutterudite CeOs4Sb12 determined by density functional theory

Structural, elastic, electronic and thermodynamic properties of the ternary cubic filled skutterudite CeOs₄Sb₁₂ compound were calculated using the full-potential linear muffin-tin orbital implementation of density functional theory. The exchange-correlation potential was treated with the local density approximation. The calculated ground state quantities such as the lattice parameter, atomic position parameters of Sb atoms, bulk modulus and its pressure derivative are compared to the available experimental data. We have computed the elastic moduli and their pressure dependence, which have not been calculated or measured yet. The Debye temperature is estimated from the average sound velocity. From the elastic parameter behavior, it is inferred that this compound is elastically stable and brittle in nature. The electronic band structure calculations revealed metallic behavior for the herein studied compound at zero pressure, but under pressure effect, the metallic character disappears and the compound becomes a narrow indirect band gap semiconductor. Through the quasi-harmonic Debye model, in which phononic effects are considered, the effect of pressure P and temperature T on the lattice constant, bulk modulus, heat capacity, thermal expansion coefficient and Debye temperature are investigated.     

Novel dipolar 5,5,10,10-tetraphenyl-5,10-dihydroindeno[2,1-a]-indene derivatives for SM-OPV: A combined theoretical and experimental study

In order to investigate the photo-physical, electrochemical, and optoelectronic properties of dipolar 5,5,10,10-tetraphenyl-5,10-dihydroindeno[2,1-a]-indene (TDI) derivatives, a facile synthesis has been developed to integrate arylamine (electron donor fragment, D) and aryl-2-methylenemalononitrile (electron acceptor fragment, A) into the TDI bridge. According to calculation results using the DFT/B3LYP/6-31G(d) method, the HOMO and LUMO energies of TDI derivatives are relevant to the extent of corresponding electron donating and accepting abilities, and influence the open-circuit voltage (V_oc) and driving force (ΔE) in organic photovoltaics (OPV). The projected density of state (pDOS) analysis shows that the electron density distribution from the D fragment to TDI bridge in the HOMO is attributing to the electron-donating ability, whereas the electrons are mainly localized on A fragment in the LUMO. Calculations of the reorganization energy by the DFT/B3LYP/6-31G(d) method suggest these D-TDI-A derivatives are hole-transporting type materials. On the other hand, the calculated absorption spectra for these molecules in CH₂Cl₂ are simulated by using the TD-DFT/BH and HLYP/6-31G(d) method within the Polarizable Continuum Model (PCM) and provide the maximum absorption wavelength (λ_max), which can be assigned to the HOMO to LUMO transition. HOMO is found to be the π orbital which is delocalized between the D fragment and the π-linker and LUMO is the π¹ orbital which is concentrated on the A fragment. The optical properties of D-TDI-A derivatives can be influenced by the D fragment and π-conjugated length. Calculated results of D-TDI-A derivatives also exhibited a large light harvesting efficiency related to the maximum absorption wavelength (RLHE)) and, according to these results, the D-TDI-A derivatives containing the A_b and A_d fragments would be useful electron donor materials for further development of new small molecular organic photovoltaic solar cell (SM-OPV) devices.     

First principles study of electronic and optical properties of the ternary SnSb4S7 using modified Becke-Johnson potential

The electronic and optical properties of SnSb₄S₇ compound are calculated by the full-potential linearized augmented plane-wave (FP-LAPW) method. The density of states (DOS) is carried out by the modified Becke-Johnson (mBJ) exchange potential approximation based on density functional theory (DFT). The compound SnSb₄S₇ has a monoclinic structure with the space group P2₁/m with lattice parameters of a=11.331 Å, b=3.865 Å and c=13.940 Å. The band gap is calculated to be 0.8 eV. The optical parameters, like dielectric constant, refractive index, reflectivity and energy loss function were also calculated and analyzed. The present work provides information about variation of the electronic and optical properties which reveals that SnSb₄S₇ is suitable for optoelectronic devices.     

First principles study of structural,electronic, elastic and magnetic properties of gadolinium hydride system GdHx (x=1, 2, 3)

The structural, electronic, elastic and magnetic properties of gadolinium and its hydrides GdH_x (x=1, 2, 3) are investigated by using Vienna ab-initio simulation package with the generalized gradient approximation parameterized by Perdew, Burke and Ernzerhof (GGA-PBE) plus a Hubbard parameter (GGA-PBE+U) in order to include the strong Coulomb correlation between localized Gd 4f electrons. At ambient pressure all the hydrides are stable in the ferromagnetic state. The calculated lattice parameters are in good agreement with the experimental results. The bulk modulus is found to decrease with the increase in the hydrogen content for the gadolinium hydrides. A pressure-induced structural phase transition is predicted to occur from cubic to hexagonal phase in GdH and GdH₂ and from hexagonal to cubic phase in GdH₃. The electronic structure reveals that mono and di-hydrides are metallic, whereas trihydride is half-metallic at normal pressure. On further increasing the pressure, a half-metallic to metallic transition is also observed in GdH₃. The calculated magnetic moment values of GdH_x (x=1, 2, 3) are in accord with the experimental values.     

Selenium alloying of indium sulfide: Ab-initio study of structural,electronic and optical features

In the framework of density functional calculations and using the Linear Augmented Plane Waves with local orbital method (LAPW+lo), we have investigated the structural, electronic and optical properties of indium sulfoselenide (InS_1−xSe_x). The present study confirms that InS_1−xSe_x are indirect band gap materials. The non-linear dependence concentration x of the theoretical forbidden energy band is clearly visible and the microscopic origins of gap bowing are identified and calculated. In order to identify the angular momentum characteristics of the bands structure, the total and partial densities of states (DOS and PDOS) are analyzed. The feature bonding is also investigated from charge density study. Using the projected total densities of states (DOS) and the bands structure, we have calculated the linear optical properties, namely, the complex dielectric function ε(ω), refractive index n(ω), absorption coefficient α(ω) and the electron energy loss L(ω). In particular, we take into account the effect of Se composition on anisotropy, real part of the dielectric function and refractive index. The present alloy is found to be a challenging material in optoelectronic, medical and photovoltaic devices. Good agreements are found with the available experimental and theoretical results.     

掺杂钛酸钡薄膜的合成及性能研究

通过化学溶液沉积法制备的BiFeO3-BaTiO3薄膜在室温下能够同时显现铁电性和铁磁性。在600℃至700℃的条件下,以Pt/TiOx/SiO2/Si为载体,能够成功得到钙钛矿单相0.7BiFeO3-0.3BaTiO3薄膜。随着结晶温度上升,晶粒持续增长,最终在700℃时到达更高的结晶度。由于0.7BiFeO3-0.3BaTiO3薄膜的绝缘电阻较低,它所显现的极化(P)-电场(E)磁滞回线较弱。尽管如此,在0.7BiFeO3-0.3BaTiO3薄膜铁的位置上添加锰,高作用场的漏电流有效地减少,最终铁电性质得到了提高。在室温下,添加了摩尔分数5%的锰的0.7BiFeO3-0.3BaTiO3薄膜同时显现铁电极化和铁磁磁化磁滞回线。     

巨型计算机热设计技术现状及趋势

介绍了巨型计算机热设计技术现状和发展趋势。强迫空气冷却是当前巨型机主流冷却技术,但随着器件表面功率密度和系统体积功率密度的不断提高,需要研究和应用更加高效的冷却手段。为应对未来十年巨型机热设计所面临的挑战,讨论了几种可行冷却技术及其今后的努力方向。     

Studies on the Reorganization of Extended Defects with Increasing n in the Perovskite‐Based La4Srn–4TinO3n+2 Series

Perovskite titanates with nominal stoichiometry ABO_3+δ often exhibit quite interesting properties, but their structural characterization is not always rigorous. Herein, we demonstrate how excess oxygen can be incorporated in a titanate perovskite‐based lattice. A new family of layered perovskites La₄Sr_n–4Ti_nO_3n+2 has been investigated by means of X‐ray diffraction, neutron diffraction, transmission electron microscopy, thermogravimetric analysis, and density and magnetic measurements. Such layered perovskites are known to be able to accommodate extra oxygen beyond the parental ABO₃ perovskite in crystallographic shears. The structure evolves with increasing n. Firstly, the perovskite blocks become more extensive and the oxygen intergrowth layers move further apart; then the spacing between the intergrowth layers increases further and their repetition becomes more sporadic. Finally, the layered structure is lost for the n = 12 member (La₂Sr₄Ti₆O_19–δ). In this structure, excess oxygen is accommodated within the perovskite framework in randomly distributed short‐range linear defects. These defects become more dilute as the cubic perovskite, that is, n = ∞, composition is approached.