A ReaxFF potential for Al - ZnO systems

A reactive field force (ReaxFF) potential has been created in order to model the structural effects of low percentage dopant aluminium in a zinc oxide system. The potential’s parameters were fitted to configurations computed with Density Functional Theory (DFT): cohesive energies, binding energies and forces were all considered for bulk crystals, surface structures and ZnAl alloys. As a first application of the model, the energetic deposition (0.1 - 40 eV) of an aluminium atom onto the polar surface of a ZnO (000 ̄1) is considered. For low energies the Al atom attaches to two preferred sites on the surface but as the energy increases above ≈ 15 eV subplantation is preferred at near normal incidence, with high diffusion barriers between stable sites. At these energies, reflection of the Al atom occurs at incident angles above ≈ 55◦.

Controlling magnetic-semiconductor properties of the Si- and Al-doped blue phosphorene monolayer

Doping has been widely employed as an efficient method to diversify the materials properties. In this work, the structural, magnetic, and electronic properties of pristine, aluminum(Al)-, and silicon(Si)-doped blue phosphorene monolayer are investigated using first-principles calculations. Pristine monolayer is a non-magnetic wide gap semiconductor with a band gap of 1.81 eV. The 1Si-doped system is a ferromagnetic semiconductor. However, the magnetism is turned off when increasing the dopant composition with small Si-Si distance. Further separating the dopants recovers step by step the magnetic properties, and an antiferromagnetic(AFM)-ferromagnetic(FM) state transition will take place at large dopants separation. In contrast, Al doping retains the non-magnetic semiconductor behavior of blue phosphorene. However, significant energy gap reduction is achieved, where this parameter exhibits a strong dependence on the dopant concentration and doping configuration. Such control may also induce the indirect-direct gap transition. Our results introduce prospective two-dimensional (2D) materials for applications in spintronic and optoelectronic nano devices, which can be realized and stabilized in experiments as suggested by the calculated formation and cohesive energies.

Fast Orbital-Free Full-Potential Calculations for Large Nano Objects: C, Al and Ti

In the context of a full-potential orbital-free approach for the modeling of multi-atomic systems we investigated the dependence of the cohesive energies and bulk elastic modules of the large nanosystems Cn (n is up to 4096 atoms), Aln (n is up to 23,328 atoms) and tin (n is up to 2160 atoms). It was shown that the cohesive energies and elastic modules tend towards bulk crystal values at n ≈ 3000 for Cn systems, at n ≈ 1500 for Tin and at n ≈ 20,000 for Aln. The execution time for one energy iteration for Ti23328 was only 23 min.

Investigation of Mechanical, Thermodynamical, Dynamical and Electronic properties of RuYAs (Y = Cr and Fe) alloys

Investigation of structural, dynamical, mechanical, electronic and thermodynamic properties of RuYAs (Y = Cr and Fe) alloys have been performed from the first principle calculations. Among the three structural phases, ‘α’ phase is found to be energetically favorable for both the RuCrAs and RuFeAs compounds. The computed cohesive energies and phonon dispersion spectra indicate the structural and dynamical stabilities of both the compounds. Mechanical stability of these compounds are studied using elastic constants. The Pugh’s ratio predicts RuFeAs to be more ductile than RuCrAs. The RuCrAs alloy, on the other hand, is found to be a stiffer, harder and highly rigid crystal with stronger bonding forces than the RuFeAs. Furthermore, the thermodynamical properties have also been estimated with respect to the temperature under different pressures using the quasi-harmonic Debye model. In order to account for the effect of the highly correlated d transition elements in the system we incorporated the GGA+U approximations. Within the GGA+U approach, the electronic structure reveals the half-metallicity for both compounds, which follows the Slater-Pauling rule. The charge density and electron localized function reflect the covalent bonding among the constituent atoms. Bader analysis reveals that the charge transfer takes place from Cr/Fe to Ru and As atoms in both approximations. Both Raman and infrared active modes have been identified in the compounds.

Effects of the Rare Earth Y on the Structural and Tensile Properties of Mg-based Alloy: A First-Principles Study

In order to investigate the effect of the rare earth element Y on the strengthening potency of magnesium alloys and its strengthening mechanism under tension. In this paper, the solid solution structures with Y atom content of 1.8 at.% and 3.7 at.% were built, respectively, and their cohesive energies and stress-strain curve were calculated in the strain range of 0–20%. The calculation results of the cohesive energies showed that the structure of element Y is more stable with the increase of strains. The calculation results of stress and strain showed that Y element can improve the yield strength and tensile strength of the Mg-based alloy, and the strengthening effect is better when the Y content is 3.7 at.%.

Theoretical investigations on correlations between elastic behavior of Al-based alloys and their electronic structures

In this work, using the first-principles method, the alloying stability, electronic structure, and elastic properties of Al-based intermetallics were investigated. It was found that these alloys have a strong alloying ability and structural stability due to the negative formation energies and the cohesive energies. The valence bonds of these intermetallic compounds are attributed to the valence electrons of Cu 3δ states for AlCu3, Cu 3δ and Zr 4δ states for AlCu2Zr, and Al 3s, Zr 5s and 4δ states for AlZr3, respectively. Furthermore, the correlation between elastic properties of these intermetallic compounds and their electronic structures was revealed. The results show that structural parameters and elastic properties such as bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and anisotropy agreed well with experimental results.

Scrutinizing the Stability and Exploring the Dependence of Thermoelectric Properties on Band Structure of 3d Metal-Based Double Perovskites: An ab-initio Analysis

Through the conventional DFT computation, we have designed new oxide double perovskites Ba2BNiO6 (B = Fe and Co). The structural and thermodynamic stabilities are defined by optimizing the crystal energy and determination of tolerance factor and cohesive energies. Thereafter, at the optimized lattice constant, we have explored the different physical properties. The GGA+mBJ electronic band-structure depicts the semiconducting nature for Ba2CoNiO6 while half-metallic with 100% spin polarization for Ba2FeNiO6. The origin of such a diverse band profile upon changing Fe to Co is explained with the help of the orbital diagram and exchange interaction. The eg-eg interaction is strong in these perovskites compared to eg-t2g and t2g-t2g hybridization. The strong exchange interaction among eg states via O-p states happens because the B-O-Ni angle is strictly 180°. Furthermore, due to the narrow bandgaps, we have explored the transport properties to express the applicability of these materials towards thermoelectric technology. Also, herein we have investigated the dependency of transport properties on band profile. The semiconducting nature in Ba2CoNiO6 results in a high ZT~0.8 at room temperature makes it suitable for energy restoration.

Simulations of Crack Extensions in Arc-Shaped Tension Specimens of Uncharged and Tritium-Charged-and-Decayed Austenitic Stainless Steels Using Cohesive Zone Modeling

Crack extensions in arc-shaped tension specimens of uncharged and tritium-charged-and-decayed conventionally forged (CF) 21-6-9 austenitic stainless steels are simulated by two-dimensional finite element analyses using the cohesive zone modeling (CZM) approach with the smooth trapezoidal traction-separation law. The J integrals at the deviation loads of the arc-shaped tension specimens are taken as the reference cohesive energies and the maximum opening stresses ahead of the initial crack tips in the arc-shaped tension specimens are taken as the reference cohesive strengths. The cohesive strengths and cohesive energies are then adjusted to match the maximum loads of the experimental load-displacement curves of the arc-shaped tension specimens. The computational results showed that the computational load-displacement, load-crack extension, crack extension-displacement, and J-R curves of the uncharged and tritium-charged-and-decayed CF steel specimens are compared well with the experimental data.

Constructing Stable and Potentially High-Performance Hybrid Organic-Inorganic Perovskites with “Unstable” Cations

A new family of functional hybrid organic-inorganic perovskites (HOIPs) is theoretically designed based on the following chemical insights: when a proton is adhered to molecules like water or ethanol, the newly formed larger-sized cations (e.g., H5O2+, C2H5OH2+, and CH3SH+) entail low electron affinities mimicking superalkalis; they are conjugated acids of weak bases that cannot survive in solution, while their chemistry behavior in the HOIP frameworks, however, may be markedly different due to greatly enhanced cohesive energies of the proton, which facilitate the formation of new HOIPs. First-principles computations show that the putative formation reactions for these newly designed HOIPs typically release much more energy compared with the prevailing HOIP MAPbI3, suggesting the likelihood of facile solution-based fabrications, while the suppression of reverse formation suggests that the humidity stability may be markedly enhanced. During their formations, halide acids are unlikely to react with ethanol or methanethiol without the presence of metal halides, a condition further favoring their stability. The proposed structure of (H5O2)PbI3 may also clarify the origin of the long-speculated existence of HPbI3. Importantly, density functional theory computations suggest that many of these HOIPs possess not only direct bandgaps with values within the optimal range for solar light absorbing but also more desirable optical absorption spectra than that of MAPbI3, where their ferroelectric polarizations also benefit photovoltaics. The stability and photovoltaic efficiency may be even further improved for the newly designed two-dimensional (2D) HOIPs and 2D/3D hybrid HOIP structures.