First-principles Structure Optimization Research Articles

English

Crystal lattice structure searching by Particle Swarm Optimization (PSO) and first-principles structural optimization have been used to explore polymorphs of BC2N, possessing sp3 hybridization, under a varying applied hydrostatic pressure. Two low Gibbs free energy structures were identified: one with a primitive orthorhombic structure and Space Group, Pmm2, and the other with a primitive tetragonal structure and Space Group, P m2. Dynamical and mechanical stabilities of the Pmm2, orthorhombic BC2N (o-BC2N) structure were established using its phonon dispersions and elastic constants. The bulk modulus of this predicted BC2N phase was 377.15 GPa, which indicates a super-hard compound. The material is brittle with a B/G ratio of 0.911 and a low degree of elastic anisotropy with a Universal Elastic Anisotropy Index of only 0.774%. Calculations of the electronic band structure demonstrated that the material is a direct band gap semiconductor with a band gap of 1.731 eV at zero applied pressure. The band gap increases monotonically with increased applied pressure and saturates to a value of about 1.756 eV above 1500 kbars; the hydrostatic pressure coefficients associated with this process were determined.  Key words: High pressure phase stability, elastic anisotropy, ultra-hard material.

High pressure phase stability, elastic anisotropy and electronic properties of GePd

Palladium germanide (GePd) is one of the most promising materials for use in nano-electronic ohmic contacts to active areas of germanium-based devices. It is important, for device integrity, to identify the various characteristics of this germinide as these can influence the stability of the device at various temperatures and pressures. We have used Particle Swarm Optimization (PSO) crystal lattice structure searching and first-principles structural optimization to explore three low Gibbs free energy polymorphs of GePd under a varying applied hydrostatic pressure from 0 to 1200 kbars. The first phase had a cubic zinc blende structure with Space Group, F4‾3m. The second had a cubic sodium chloride structure (Fm3‾m) and thirdly a structure with the primitive orthorhombic (Pnma). Dynamical and mechanical stabilities were only established for the orthorhombic GePd (o-GePd) structure using its phonon dispersions and elastic constants. This material was found to be ductile with a B/G value of 2.71. The Universal Elastic Anisotropy Index of o-GePd was 17.82% which indicates a notable susceptibility to micro-cracks under elevated temperature conditions. However, the material is essentially isotropic in terms of its response to hydrostatic compression without shearing. Our electronic structure calculations indicate that o-GePd presents metallicity with an electron-type (as opposed to hole-type) conductivity that is likely to increase with pressure.

Electronic properties, high pressure phase stability and elastic anisotropy of BC5

We have used Particle Swarm Optimization (PSO) crystal lattice structure searching and first-principles structural optimization to explore polymorphs of BC5, possessing sp3 hybridization, under a varying applied hydrostatic pressure from 0 to 1400 kbars. Three low Gibbs free energy structures were identified. The first had a body-centered orthorhombic structure with the Space Group, Imm2. The second had a body-centered tetragonal structure with the Space Group, I4¯m2 and thirdly a structure with a primitive trigonal Space Group, P3m1. Dynamical and mechanical stabilities were established for the Imm2, orthorhombic BC5 (o-BC5) structure using its phonon dispersions and elastic constants. The bulk modulus of this predicted BC5 phase was 374.30, which is high enough to indicate a super-hard compound. The material is brittle with a B/G ratio of 1.002 and a low degree of elastic anisotropy with a Universal Elastic Anisotropy Index of 2.946%. Our electronic structure study shows that o-BC5 presents metallicity with a likely increase in electron-type conductivity with increased applied pressure.

First-principles study of defects in amorphous-SiO2/Si interfaces

Defects in amorphous-SiO2/Si (a-SiO2/Si) interfaces influence greatly the performance and reliability of Si-based devices. The major interfacial defects that exhibit electronic activity are silicon dangling bonds (so-called Pb defects), which can trap charge carriers and contribute to interfacial charge build-up. In this study, the Pb0 and Pb1 defects at the most technically important a-SiO2/Si(100) interface are investigated quantitatively by using first-principles calculations with realistic interfacial models. Atomistic models of the interface are generated by classical molecular dynamics simulations of Si oxidation with a-SiO2 followed by first-principles structure optimization, and they can reproduce the experimental interfacial properties of the short-range structure parameters, Si oxidation state distribution, and band gap transition. The Pb0 and Pb1 defects are modeled atomistically within the interfacial models, and their g-values and hyperfine parameters are calculated quantitatively by the GIPAW (gauge including projector augmented waves) method. The Fermi contact and hyperfine axis (hf axis) calculated with the Si vacancy model for the Pb0 defect agree well with those obtained experimentally, and the hf axis is coincident with the dangling bond direction determined from the calculated g-tensor and spin density. The Fermi contact and hf axis are calculated for the Pb1 defect with three different models, namely dimer, bridge, and asymmetrically oxidized dimer (AOD), of which the AOD model leads to calculated results that agree well with the experimental ones. However, the AOD model is found to be sensitive to the local structure distortion associated with the softness of the oxygen bridge. What is known as the half AOD is then introduced by deoxidizing one of the two oxygen bridges of the AOD; this may serve as another atomistic model of the Pb1 defect according to the good agreement between the calculated Fermi contact, g-values, and hf axis and the experimental ones. The electronic structures are calculated with the atomistic defect models and show that both defects induce (i) two spin-asymmetric midgap levels in the Si band gap in the neutral state and (ii) spin-symmetric levels in the charged states. The charge transition levels of the defects are calculated with a correction method that is specific to surface/interfacial charge defects, and the calculated levels agree semi-quantitatively with those determined experimentally. The calculated +/0 and 0/− levels are in the Si band gap, with the 0/− level energy being higher than the +/0 one. This implies that both Pb0 and Pb1 defects can trap holes or electrons depending on the position of the Fermi level.

First-Principles Prediction of Thermodynamically Stable Two-Dimensional Electrides.

Two-dimensional (2D) electrides, emerging as a new type of layered material whose electrons are confined in interlayer spaces instead of at atomic proximities, are receiving interest for their high performance in various (opto)electronics and catalytic applications. Experimentally, however, 2D electrides have been only found in a couple of layered nitrides and carbides. Here, we report new thermodynamically stable alkaline-earth based 2D electrides by using a first-principles global structure optimization method, phonon spectrum analysis, and molecular dynamics simulation. The method was applied to binary compounds consisting of alkaline-earth elements as cations and group VA, VIA, or VIIA nonmetal elements as anions. We revealed that the stability of a layered 2D electride structure is closely related to the cation/anion size ratio; stable 2D electrides possess a sufficiently large cation/anion size ratio to minimize electrostatic energy among cations, anions, and anionic electrons. Our work demonstrates a new avenue to the discovery of thermodynamically stable 2D electrides beyond experimental material databases and provides new insight into the principles of electride design.

First-principles structure optimization of scandium and lutetium trihydrides

A group–subgroup family tree for the structure of stoichiometric scandium and lutetium trihydrides with an essentially hexagonal close-packed (hcp) metallic matrix is processed. The underlying symmetry reduction procedure starts from the full hcp symmetry (space group type P63/mmc) and leads to triclinic symmetry via point-symmetry reduction and unit-cell enlargement. It is shown that for both trihydrides the energetic and vibrational stability is reached for the space group P63. Further lowering of symmetry does not lead to further distortions.

Chlorine Adsorption on Graphene: Chlorographene

We perform first-principles structure optimization, phonon frequency, and finite temperature molecular dynamics calculations based on density functional theory to study the interaction of chlorine atoms with graphene predicting the existence of possible chlorinated graphene derivatives. The bonding of a single chlorine atom is ionic through the transfer of charge from graphene to chlorine adatom and induces negligible local distortion in the underlying planar graphene. Different from hydrogen and fluorine adatoms, the migration of a single chlorine adatom on the surface of perfect graphene takes place almost without barrier. However, the decoration of one surface of graphene with Cl adatoms leading to various conformations cannot be sustained due to strong Cl–Cl interaction resulting in the desorption through the formation of Cl2 molecules. On the contrary, the fully chlorinated graphene, chlorographene CCl, where single chlorine atoms are bonded alternatingly to each carbon atom from different sides of graphene with sp3-type covalent bonds, is buckled. We found that this structure is stable and is a direct band gap semiconductor, whose band gap can be tuned by applied uniform strain. Calculated phonon dispersion relation and four Raman-active modes of chlorographene are discussed.

Stable, Single-Layer MX2 Transition-Metal Oxides and Dichalcogenides in a Honeycomb-Like Structure

Recent studies have revealed that single-layer transition-metal oxides and dichalcogenides (MX2) might offer properties superior to those of graphene. So far, only very few MX2 compounds have been synthesized as suspended single layers, and some of them have been exfoliated as thin sheets. Using first-principles structure optimization and phonon calculations based on density functional theory, we predict that, out of 88 different combinations of MX2 compounds, several of them can be stable in free-standing, single-layer honeycomb-like structures. These materials have two-dimensional hexagonal lattices and have top-view appearances as if they consisted of either honeycombs or centered honeycombs. However, their bonding is different from that of graphene; they can be viewed as a positively charged plane of transition-metal atoms sandwiched between two planes of negatively charged oxygen or chalcogen atoms. Electron correlation in transition-metal oxides was treated by including Coulomb repulsion through LDA...

First-principles structural optimization and electronic structure of the superconductor picene for various potassium doping levels

We theoretically explore the crystal structures of K$_x$picene, for which a new aromatic superconductivity has recently been discovered for $x=3$, by systematically performing first-principles full structural optimization covering the concentration range $x=1$-4. The crystal symmetry (space group) of the pristine picene is shown to be preserved in all the optimized structures despite significant deformations of each picene molecule and vast rearrangements of herringbone array of molecules. For K$_x$picene ($x=1$-4) optimization indicates that (i) multiple structures exist in some cases, and (ii) dopants can enter not only in the interlayer region between the stack of herringbone structures, but also in the intralayer region. In the electronic structure obtained with the local density approximation for the optimized structures, the dopants are shown to affect the electronic properties not only through the rearrangement and distortion of molecules, but also molecule-metal atom hybridization. In other words, the rigid-band approximation is invalidated for multifold reasons. As a consequence the resultant Fermi surface exhibits a variety of multiband structures which take diverse topology for K$_1$picene and K$_3$picene.

Prediction of a superconductive superhard material: Diamond-like BC7

Through particle swarm optimization algorithm and first-principles structural optimizations, we have predicted two novel low-energy structures of BC7: graphite-like Amm2 structure and diamond-like P-4m2 structure. Structural stability of the proposed BC7 polymorphs was confirmed by calculating the elastic constants and phonon frequencies. Phase transition pressure from Amm2 to P-4m2 was determined to be at 2.2 GPa. Calculations for the electronic band structures demonstrated hole-type conductivity of the two novel phases. Ideal tensile strength along the 〈001〉 direction for the diamond-like BC7 was 155.2 GPa, which was approximately 52% higher than that of the recently predicted diamond-like BC5. Theoretical Vickers hardness of the diamond-like BC7 was 78.0 GPa, indicating that it is a superhard material. Electron-phonon coupling calculations revealed that diamond-like BC7 was superconducting with a critical temperature of ∼11.4 K.

Superhard and superconductive polymorphs of diamond-like BC3

We have explored the crystal structures of synthesized diamond-like BC3 (d-BC3) with particle swarm optimization (PSO) algorithm combined with first-principles structural optimizations. Three intriguing metallic low-energy structures are uncovered: (i) orthorhombic Pmma-a consisting of a sandwich-like “B-layer” between C layers, (ii) orthorhombic Pmma-b with novel BB bondings, and (iii) tetragonal P-4m2. The simulated Raman modes of Pmma-b phase are in agreement with experiments. Further hardness and electron–phonon calculations revealed that all the three candidate d-BC3 phases are superhard (>40 GPa) and superconductive materials with the superconducting critical temperature reaching at 16.6–23.4 K for Pmma-a phase.

Interactions, Behavior, And Stability of Fluorenone inside Zeolite Nanochannels

The development of functional materials based on the supramolecular organization of photoactive species in nanosized porous matrices requires a deep knowledge of host−guest interactions and of their influence on material properties and stability. Extensive first-principles investigations on the fluorescent dye fluorenone inside zeolite L, both at dry conditions and in the presence of water, have unraveled the molecular origin of the peculiar stability of this composite in humid environments, a fundamental prerequisite for practical applications. Results of first-principles molecular dynamics simulations, structural optimizations, and TDDFT calculations, validated by comparison with experimental data, provide a comprehensive picture of the structure, energetics, electronic excitation properties, and room-temperature behavior of the fluorenone/zeolite L composite and predict a substantial optical anisotropy for this material also maintained upon contact with water. The interaction of the fluorenone carbonyl group with the zeolite extraframework potassium cations is responsible for the dye stabilization in zeolite L nanochannels and features itself as a general leitmotiv regarding important properties of carbonyl functionalized photoactive species in hydrophilic matrices.

Crystal and electronic structure of superhardBC5: First-principles structural optimizations

A thermodynamically stable $I\overline{4}m2$ phase of superhard ${\text{BC}}_{5}$ was predicted from exhaustive structural searches combined with first-principles structural optimizations. From the computed bulk and shear moduli, this phase is expected to be harder than the recently synthesized ${\text{BC}}_{5}$ with the diamond structure. The stability and strength are attributed to multicenter B-C-B interactions when two B atoms are linked through a common C atom. The $I\overline{4}m2$ phase is metallic and superconducting with an estimated superconducting critical temperature $({T}_{c})$ of 47 K. Another candidate structure with $P\overline{1}$ symmetry was obtained and this fits well to the reported x-ray diffraction pattern. B atoms in the $P\overline{1}$ structure are not linked together by C atoms which yields higher energy. $P\overline{1}$ and $I\overline{4}m2$ structures have almost identical predicted hardness.

First-principles calculations of the geometry of a Zn–Sc cluster

A theoretical study is presented of the geometry of a Tsai-type cluster consisting of Zn and Sc. This cluster is a building block of both Zn–TM–Sc (TM = Mn, Fe, Co, Ni) quasicrystals and their approximants. First-principles structural optimizations are carried out and the stable structure of this cluster examined. Our results show that four Zn atoms inside the Tsai-type cluster form a distorted tetrahedron and the first shell of the cluster is also distorted.

First-principles studies on orientational ordering of atomic clusters in Cd6Ca

First-principles structural optimizations are carried out for 1/1 cubic approximant Cd6Ca. Newly proposed structure by Gómez and Lidin is used as the starting structure for the structural optimizations. The optimized structure is more stable than the non-optimized one by several eV. Two structures, which are almost degenerate before the structural optimizations, are split by about 100 meV per unit cell after the structural optimizations. Clusters are deformed to relax interatomic distances and most of the neighbouring Cd–Cd distances close to that of hcp-Cd. Relaxed structures of clusters depend on the structure of the innermost shells of adjacent clusters.

Ab initio Study of Strain-Induced Ferroelectricity in SrTiO3

Valley lines on total-energy surfaces for the zone-center distortions of free-standing and in-plane strained SrTiO3 are investigated with a newly developed first-principles structure optimization technique [Jpn. J. Appl. Phys. 43 (2004) 6785]. The results of numerical calculations confirmed that ferroelectricity is induced and the Curie temperature is increased by applying biaxial compressive or tensile strains. Along the distortion, strong nonlinear coupling between the soft- and hard-modes is demonstrated.

Magnetic orderings in Al nanowires suspended between electrodes

A theoretical analysis of a relation between atomic and spin-electronic structures for the ground state of single-row aluminum nanowires suspended between Al(001) electrodes is demonstrated using first-principles structural optimizations. We obtain an unusual result that a three-aluminum-atom nanowire sandwiched between the electrodes does not manifest magnetic ordering, although an isolated aluminum trimer molecule in a straight line is spin-polarized. On the other hand, a five-atom nanowire exhibits ferromagnetic ordering, where three central atoms form a spin-polarized trimer. Moreover, in the case of an eight-atom nanowire, the middle atoms in the nanowire form two spin-polarized trimers with antiferromagnetic ordering.

A first-principles study of the lattice distortion around a solute atom in BCC lithium

In this paper, we study the effect that the lattice distortion around a solute atom (Na, Mg, Al) has on the electronic and mechanical properties of BCC Li using the first-principles structural optimization method. In the present calculations, a 54-site supercell is used to get a better insight into the long-range behaviour of the lattice distortion around a solute atom. The results show that the behaviour of the first-nearest-neighbouring Li shell around a solute atom in all cases investigated is similar to that obtained in the corresponding 16-site supercell calculations. The distortion has, for instance, an effect on the average atomic volume, bulk modulus and heat of formation of Li alloys. From the present calculations, together with our previous studies, we deduce also the concentration dependence of these quantities. For example, the calculated average atomic volume and bulk modulus of dilute Li-Mg alloys, as a function of Mg concentration, agree qualitatively very well with experimental results. Furthermore, the pressure dependence of the lattice distortion is found to be rather important. Our results also predict that the solubility of Na in BCC Li under pressure above 50 kbar is possible.