Principles Total Energy Calculations Research Articles

First Principles Evaluation of Phase Stability in the In-Sn Binary System

The In-Sn binary alloy system exhibits several unusual features that challenge crystallographic and thermodynamic expectations. We combine first principles total energy calculation with simple thermodynamic modeling to address two key points. First, we evaluate energies along the Bain path to interpret the discontinuous transition between the phases α-In (Pearson type tI2) and β-In3Sn (also Pearson type tI2) that are identical in symmetry. Second, we demonstrate that the solid solution phases β-In3Sn and γ-InSn4 (Pearson type hP1) exist at high temperatures only, and they exhibit eutectoid decompositions at low temperatures.

Quasicrystal Structure Prediction: A Review

AbstractPredicting quasicrystal structures is a multifaceted problem that can involve predicting a previously unknown phase, predicting the structure of an experimentally observed phase, or predicting the thermodynamic stability of a given structure. We survey the history and current state of these prediction efforts with a focus on methods that have improved our understanding of the structure and stability of known metallic quasicrystal phases. Advances in the structural modeling of quasicrystals, along with first principles total energy calculation and statistical mechanical methods that enable the calculation of quasicrystal thermodynamic stability, are illustrated by means of cited examples of recent work.

Ni3InSb: Synthesis, Crystal Structure, Electronic Structure, and Magnetic Properties

The ternary phase with the composition Ni3InSb has been synthesized by high-temperature synthesis and structurally characterized by a combination of X-ray analysis, neutron diffraction analysis, and theoretical calculations. The structure of Ni3InSb crystallizes in the orthorhombic space group Pnma with lattice constants a = 7.111(3) Å, b = 5.193(3) Å, and c = 8.2113(2) Å. The crystal structure contains ∼20 atoms in its unit cell, which are distributed over four crystallographically independent positions (two Ni, one In, and one Sb). The crystal structure can be considered as a ternary substitutional variant of Ni3Sn2 (Pnma, no. 62), where a trivalent In and a pentavalent Sb orderly occupy two tetravalent Sn sites of Ni3Sn2. This site decoration pattern of two neighboring elements, In and Sb, is unique and confirmed by first principles total energy calculations. The crystal structure can be described by two building units: Ni2Sb (building unit of Ni2In) and NiIn (NiAs-type). They alternate in the crystal structure and form infinite ac-slabs (puckered), and the slabs are stacked along [010]. A triangular lattice formed by Ni atoms indicates the existence of a geometrically frustrated structure. The calculated density of states and crystal orbital Hamilton population enlighten the stability and bonding characteristics of the structure. The temperature-dependent neutron diffraction study down to 5 K reveals that the crystal structure remains in the same orthorhombic symmetry with a weak anomaly in the lattice parameters at ∼100 K. Detailed temperature- and magnetic field-dependent magnetic properties of the title phase Ni3InSb show spin-glass- or spin-disorder-like behaviors below ∼300 K with an unusual magnetic behavior below 100 K, where an enhancement of magnetization with a decrease of the coercive field has been found.

Modeling the Electronic structure and stability of three aluminum nitride phases

Phase transitions in aluminum nitride (AlN) were investigated by first principles total energy calculations. Three AlN crystal structures were considered: rock salt (NaCl), zinc blende and wurtzite. The cohesion energy was calculated within both GGA and LDA formalisms. According to the cohesion energy results, the ground state corresponds to the hexagonal wurtzite phase, in agreement with experimental evidence. However, the zinc blende and NaCl phases may be formed as metastable structures. To determine the energy gap the modified Becke-Johnson pseudopotential was applied, with results showing good agreement with the experimental data. The ground state structure exhibits direct electronic transitions. However, the zinc blende and NaCl phases show indirect band gap. Provided that external pressures may induce transitions from wurtzite to zinc blende or rock salt, these transitions were also investigated. Estimation of the pressure at the phase transition indicates that small pressures are needed to achieve such transitions.

Chemical substitution of Zn in the structure of ordered Cu6Zn2Sb2: A structural and theoretical study

A series of new ternary compounds with composition Cu6-xZn2+xSb2 (0.0 ≤ x ≤ 1.0) are synthesized by high-temperature solid-state method. Phase homogeneity that ranges from Cu5Zn3Sb2 to Cu6Zn2Sb2 [Cu6-xZn2+xSb2 (0.0 ≤ x ≤ 1.0)] is determined by X-ray diffraction technique and energy dispersive X-ray analysis. The structure and stability of ordered Cu6Zn2Sb2 have recently been reported. The atomic ordering of two neighboring elements Cu and Zn has been addressed in the previous report. In this study, two compounds, Cu5Zn3Sb2 and Cu5.5Zn2.5Sb2 that formed due to the specific chemical substitution of Cu by Zn have been structurally characterized. Like Cu6Zn2Sb2, both the compounds adopt TiAl3Ge structure type (P4/nmm (129)). Specific substitution pattern of Zn to Cu site has been confirmed by both neutron diffraction and first principle total energy calculations. The structure of Cu6Zn2Sb2 is viewed as alternating units of Cu2Sb and CsCl type CuZn. For the observed composition range - Cu6-xZn2+xSb2 (0 ≤ x ≤ 1.0), Cu site in CuZn unit is unaffected by chemical substitution, whereas the Cu site in Cu2Sb unit undergoes chemical substitution by Zn.

Alloying Element Segregation and Grain Boundary Reconstruction, Atomistic Modeling

Grain boundary (GB) segregation is an important phenomenon that affects many physical properties, as well as microstructure of polycrystals. The segregation of solute atoms on GBs and its effect on GB structure in Al were investigated using two approaches: First principles total energy calculations and the finite temperature large-scale atomistic modeling within hybrid MD/MC approach comprising molecular dynamics and Monte Carlo simulations. We show that the character of chemical bonding is essential in the solute–GB interaction, and that formation of directed quasi-covalent bonds between Si and Zn solutes and neighboring Al atoms causes a significant reconstruction of the GB structure involving a GB shear-migration coupling. For the solutes that are acceptors of electrons in the Al matrix and have a bigger atomic size (such as Mg), the preferred position is determined by the presence of extra volume at the GB and/or reduced number of the nearest neighbors; in this case, the symmetric GB keeps its structure. By using MD/MC approach, we found that GBs undergo significant structural reconstruction during segregation, which can involve the formation of single- or double-layer segregations, GB splitting, and coupled shear-migration, depending on the details of interatomic interactions.

Spin polarization effect on the ground state and electronic properties of ErCu using DFT and DFT+U approaches

Using first principles total energy calculation with the approach of density functional theory (DFT) and DFT + U calculations, the structural and electronic properties of erbium copper (ErCu) intermetallic compound were studied. In the present work, we used the local density approximation for the exchange and correlation potential for the spin polarized case. The structural optimization shows that ErCu is stable in the cubic structure. The obtained ground state properties viz; lattice constant ao (a.u), bulk modulus B (GPa) and magnetic moment (μB), which show good comparison with the available experimental data. We have also investigated the electronic properties such as electronic band structure, density of states, Fermi surfaces and charge density of states for spin-polarized case. In the band structure, majority spins energies contributions are higher than the minority spins energies contribution. The total density of states (TDOS) show that near the Fermi level, is mostly dominated by Er ‘f’ orbital electrons and below the Fermi level, are due to Cu ‘3d’ orbital electrons. The charge distributions of ErCu are ranging between +0.0003 e/Å3 to +0.0693 e/Å3 for majority channels and +0.0003 e/Å3 to +0.0715 e/Å3 for the minority spin channels.

High pressure structural stability of ThN: ab-initio study

The high pressure structural stability of thorium nitride (ThN) has been examined through first principle total energy calculations. The structural sequence predicted through present theoretical simulation is NaCl type structure (B1) to tetragonal phase (B10) to CsCl type (B2) phase with corresponding transition pressures ∼75 GPa and 120 GPa, respectively. The B1 → B10 transition with volume discontinuity of ∼10% is found to be of first order type, whereas, the B10 → B2 displays second order nature. The predicted high pressure structural sequence in ThN is different from those reported in earlier works [Gupta and Singh, Sol. State Sci., 2010 and Modak and Verma, Phys. Rev. B 2011]. The pressure induced phase transition sequence so obtained is further supported by examining the mechanical and phonon stability of the predicted structures in their structural stability regime. Also, the activation barrier for B1 to B10 transition determined from the calculated Bain path at the transition pressure turns out to be ∼0.75eV/f.u.(72.36 kJ/mol).

A Deeper Insight on the Stability of Water-Induced Reconstruction of Anatase (001) Surface

TiO 2 anatase (001) surface has been indicated for many years as a potential system for water dissociation and hydrogen production. Surface reconstruction periodicity of TiO 2 anatase (001) in water is revised on the basis of the new water induced reconstruction model that accounts for dissociative water adsorption in the first monolayer and self-assembling of surface hydroxyls. The study has been performed in the context of first principles total energy calculations on the basis of state of the art Density Functional Theory. Different surface periodical structures have been studied and compared in terms of residual surface stress and surface reactivity. While a preference seems to emerge for the (2 × 3) surface reconstruction, there are indications that this configuration might not occur spontaneously in bulk water.

Chromium nanostructure formation on the GaAs(1 1 1)–(2 × 2) surface: First principles studies

Structural, electronic and magnetic properties of the chromium (Cr) nanostructure formation on GaAs(1 1 1)–(2 × 2) surfaces are investigated. Studies are performed by first principles total energy calculations within the periodic spin polarized density functional theory plus Hubbard correction (GGA + U) with 1 < U < 5 eV, as developed in the PWscf code of the quantum ESPRESSO package. The chromium adsorption and incorporation into the GaAs(1 1 1)-(2 × 2) atomic structure are investigated on both gallium and arsenic terminated surfaces. Different coverage of Cr is considered from ¼ monolayer (ML) up to a full ML to determine the relaxed configurations and the most favorable structure. High symmetry sites are considered to explore the Cr adsorption and incorporation into the GaAs structure, these are: H3, T4, Top and Bridge. Surface atomic structures contain different number of atoms therefore to determine the most stable atomic geometry the surface formation energies are constructed. On the As terminated surface, under As rich conditions the CrAs bilayer formation is the most favorable structure. On the Ga terminated surface, under Ga rich conditions the Cr trimer formation is the most favorable configuration. Electronic properties of the most stable structures are investigated by calculating the total density of states (DOS) and projected DOS. Results indicate that the most favorable nanostructures exhibit metallic behavior with antiferromagnetic characteristics.

Intrinsic point defects in β-In2S3 studied by means of hybrid density-functional theory

We have employed first principles total energy calculations in the framework of density functional theory, with plane wave basis sets and screened exchange hybrid functionals to study the incorporation of intrinsic defects in bulk β-In2S3. The results are obtained for In-rich and S-rich experimental growth conditions. The charge transition level is discussed for all native defects, including VIn, VS, Ini, Si, SIn, and InS, and a comparison between the theoretically calculated charge transition levels and the available experimental findings is presented. The results imply that β-In2S3 shows n-type conductivity under both In-rich and S-rich growth conditions. The indium antiisite (InS), the indium interstitial (Ini), and the sulfur vacancy (VS′) are found to be the leading sources of sample's n-type conductivity. When going from the In-rich to the S-rich condition, the conductivity of the material decreases; however, the type of conductivity remains unchanged.

Mn induced 1 × 2 reconstruction in the τ-MnAl(0 0 1) surface

We report on first principles total energy calculations to describe the structural, electronic and magnetic properties of MnAl(0 0 1) surfaces. We have concentrated in structural models having 1 × 1 and 1 × 2 periodicities, since recent experiments of the similar MnGa(0 0 1) surface have found 1 × 1 and 1 × 2 reconstructions. Our calculations show the existence of two stable structures for different ranges of chemical potential. A 1 × 1 surface is stable for Al-rich conditions, whereas a Mn-induced 1 × 2 reconstruction appears after increasing the Mn chemical potential up to Mn-rich conditions. It is important to notice that experimentally, Mn rich conditions are important for improved magnetic properties. The Mn layers in both structures have ferromagnetic arrangements, but they are aligned antiferromagnetically with the almost no magnetic Al atoms. Moreover, the on top Mn atoms, which produce the 1 × 2 reconstruction, align antiferromagnetically with the second layer Mn atoms. These findings are similar to those obtained experimentally in MnGa thin films grown by molecular beam epitaxy. Therefore, this method could also be used to grow the proposed MnAl films.

A density functional study of the effect of hydrogen on electronic properties and band discontinuity at anatase TiO2/diamond interface

Tailoring the electronic states of the dielectric oxide/diamond interface is critical to the development of next generation semiconductor devices like high-power high-frequency field-effect transistors. In this work, we investigate the electronic states of the TiO2/diamond 2 × 1–(100) interface by using first principles total energy calculations. Based on the calculation of the chemical potentials for the TiO2/diamond interface, it is observed that the hetero-interfaces with the C-OTi configuration or with two O vacancies are the most energetically favorable structures under the O-rich condition and under Ti-rich condition, respectively. The band structure and density of states of both TiO2/diamond and TiO2/H-diamond hetero-structures are calculated. It is revealed that there are considerable interface states at the interface of the anatase TiO2/diamond hetero-structure. By introducing H on the diamond surface, the interface states are significantly suppressed. A type-II alignment band structure is disclosed at the interface of the TiO2/diamond hetero-structure. The valence band offset increases from 0.6 to 1.7 eV when H is introduced at the TiO2/diamond interface.

Systematic study of the elastic, optoelectronic, and thermoelectric behavior of MRh2O4 (M = Zn, Cd) based on first principles calculations

In the present study, we performed first principles total energy calculations to explore the electronic, elastic, optical, and thermoelectric behavior of MRh2O4(M = Zn, Cd) spinel oxides. We employed Perdew–Burke–Ernzerhof-sol as well as the modified Becke and Johnson potential to compute the elastic, optoelectronic, and thermoelectric behavior of MRh2O4(M = Zn, Cd). The optical behavior was investigated by calculating the complex dielectric constant, refractive index, optical reflectivity, absorption coefficient, and optical conductivity. All of the optical parameters indicated a shift to lower energies as the atomic size increased from Zn to Cd, thereby suggesting potential applications of the spinel oxides in optoelectronic device. Moreover, the thermoelectric properties of MRh2O4(M = Zn, Cd) spinel oxides were computed in terms of the electrical conductivity (σ), Seebeck coefficient (S), thermal conductivity (k), and power factor (σS2) using the BoltzTraP code.

Two-dimensional cadmium selenide electronic and optical properties: first principles studies

Structural, electronic and optical properties of two-dimensional (2D) cadmium selenide (CdSe) structures with $$2\times 2$$ periodicities are investigated. First principles total energy calculations are performed within the periodic density functional theory. Initially, the structural properties are determined using the local density approximation as implemented in the PWscf code of quantum ESPRESSO package. To investigate the electronic properties, the GW method is applied to determine the energy gap within the plasmon pole and the random phase approximations. Optical properties are investigated to determine the dielectric constant and the Bethe–Salpeter theory is used to calculate the exciton binding energies. Zinc blende and wurtzite phases are considered to calculate the bulk energy gaps, which are compared to the experimental values, finding good agreement. The 2D structure exhibits an energy gap larger than that of the bulk, indicating the effects of reduction in dimensionality; these changes can be attributed to the dangling bonds that are present in the 2D layer.

Magnetic properties of two-dimensional hydrocarbon networks of sp2 and sp3 C atoms

Based on first principles total energy calculations, we investigate geometric, electronic, and spin structures in two-dimensional polyacene-based hydrocarbon networks of $s{p}^{2}$ and $s{p}^{3}$ C atoms. Polyacenes connecting adjacent $s{p}^{3}$ atoms form kagome networks. The networks are stable and retain their covalent topologies up to 2000 K. They possess kagome flatbands at or near the Fermi level, depending on the size and structure of the polyacene adjoining the $s{p}^{3}$ C atoms. We find that the spin states of the two-dimensional covalent hydrocarbon network are changeable from antiferromagnetic to ferromagnetic by increasing the length of the $s{p}^{2}$ C region.

Density Functional Theory Study of Hydrogen Electroadsorption on the Pt(110) surfaces

The hydrogen adsorption on the Pt(110) and Pt(110)-(1x2) electrode surfaces has been investigated. To gain insight into detailed atomistic picture on the equilibrium coverage and structure, we have constructed a lattice gas model by determining the on-site energy and the interaction parameters using the first principles total-energy calculation. Therein atop, fcc, short bridge, long bridge and R, T, F, F’ sites for H/Pt(110) and H/Pt(110)-(1x2) are covered by hydrogen atoms under various coverage conditions 0 ML &lt; θ &lt; 1 ML and the total-energy calculations are done for the (1x1) and (1x2) cells. The surface of (1×2) and (1×1) lateral unit cells. The convergence property with respect to the number of Pt layers and the k-point mesh are found. The comparison between different surface types are done. By comparing the calculated results with two different theoretical simulated data, SIESTA and VASP, we found good agreement between them.

Magnetic properties and Mössbauer study of gallium doped M-type barium hexaferrites

Abstract Ga-substituted (0 12 O 19 ) were obtained via solid state reactions. The features of crystal structure were studied by XRD. It has been observed that with x increase the unit cell parameters linearly and slightly decrease. The very small reduction of the unit cell parameters and, of the resulting unit cell volume is caused by the very small difference of ionic radiuses of Ga 3+ (0.62 A) and Fe 3+ (0.64 A) cations. Raman spectroscopy investigations and Mossbauer studies were performed. The distribution of Ga 3+ in different non-equivalent oxygen coordination (octahedral, tetrahedral and bipiramidal position) is not statistical. It has been established that in BaFe 12−x Ga x O 19 (0 3+ cations distribution by different oxygen coordination. At a low level of diamagnetic substitution (x 3+ cations are preferably distributed in 2 b position. At a higher level of diamagnetic substitution (x 3+ cations are preferably distributed in 12 k position. Experimental data of Mossbauer studies are in good agreement with theoretical data which were obtained using the first principles total energy calculations (based on density functional theory).

Ab-initio study of ReCN in the bulk and as a new two dimensional material

First principles total energy calculations have been applied to describe the ReCN bulk structure and the formation of ReCN monolayers and bilayers. Results demonstrate a strong structural rearrangement in the monolayer due to a reduced dimension effect: an increase in the lattice parameter, accompanied with the contraction of the distance between the C and N planes. On the other hand, a ReCN bilayer has structural parameters similar to those of the bulk. Surface formation energies show that the monolayer is more stable than bilayer geometries. Although bulk ReCN shows a semiconductor behavior, the monolayer ReCN presents a metallic behavior. This metallic character of the ReCN monolayer is mainly due to the d-orbitals of Re atoms.

Structural, electronic and magnetic properties of the MnGa(111)-1 × 2 and 2 × 2 reconstructions: Spin polarized first principles total energy calculations

Using first principles total energy calculations within the periodic spin polarized density functional theory, we have investigated the structural, electronic, and magnetic properties of manganese gallium (MnGa) alloys. Specifically, we explore the MnGa(111)-1×2 and 2×2 reconstructions. The surface formation energies reveal that selected substitutions occur under Mn (Ga) rich growth conditions. Structures with top layers missing all Mn (Ga) atoms and two layers deep substitutions are also investigated. However, the formation energy shows that these structures are less favorable. For the stable structures, the magnetic properties per layer are proportional to the Mn:Ga ratio. Also, the density of states shows that the MnGa surfaces are metallic. The projected density of states shows that the electronic states in the vicinity of the Fermi level are due mainly to the manganese 3d orbitals. However charge density plots reveal that Mn 3d electrons are closer to the nucleus than Ga sp electrons. Consequently, experimental scanning tunneling microscopy images reveal periodically-arranged bright features, corresponding to the Ga atoms.