Ground-state Structure Research Articles

Characteristics of [formula omitted] screw dislocations and their slip on prismatic and pyramidal planes in pure [formula omitted] titanium from atomistic simulations

Molecular statics and dynamics simulations were carried out to characterize the core structure and calculate the critical resolved shear stress (CRSS) of an 〈a〉 screw dislocation in pure α-Ti using a recent modified embedded atom method spline like potential developed for Ti-Nb alloys. Firstly, it is shown that the generalized stacking fault energy (GSFE) curves calculated using this potential agree well with density functional theory (DFT) and nudged elastic band + DFT computations for the basal, prismatic-I and pyramidal-I planes. In particular, this potential predicts that the 〈a〉 prismatic stacking fault (SF) is lower in energy than the basal intrinsic I2 SF. Two stable core structures for the 〈a〉 screw dislocations were identified by this potential, one dissociates on the pyramidal-I plane, and the other on the prismatic-I plane. The pyramidal-I core is found to be the ground state core structure, while the prismatic-I core is metastable with an excess energy of 18.8 meV/b, which is in good agreement with published DFT calculations. Also, it is shown that the basal dissociation of the 〈a〉 screw dislocation is not stable and upon relaxation this core reconstructs directly to the pyramidal-I ground state structure. Finally, the CRSS for the slip of a prismatic 〈a〉 screw dislocation core on the prismatic-I plane is computed as a function of temperature between 0 and 300 K. It is found that screw dislocations glide by kink-pair mechanism on the prismatic-I planes, and the calculated CRSSs are in good agreement with experimental measurements.

Existence, structure, and robustness of ground states of a NLSE in 3D with a point defect

We study the ground states for the Schrödinger equation with a focusing nonlinearity and a point interaction in dimension three. We establish that ground states exist for every value of the mass; moreover, they are positive, radially symmetric, and decreasing along the radial direction and show a Coulombian singularity at the location of the point interaction. Remarkably, the existence of the ground states is independent of the attractive or repulsive character of the point interaction.

Van der Waals five-body size-energy universality

A universal relationship between scaled size and scaled energy is explored in five-body self-bound quantum systems. The ground-state binding energy and structure properties are obtained by means of the diffusion Monte Carlo method. We use pure estimators to eliminate any residual bias in the estimation of the cluster size. Strengthening the inter-particle interaction, we extend the exploration from the halo region to classical systems. Universal scaled size-scaled energy line, which does not depend on the short-range potential details and binding strength, is found for homogeneous pentamers with interaction potentials decaying at long range predominantly as r^{-6}. For mixed pentamers, we discuss under which conditions the universal line can approximately describe the size-energy ratio. Our data is compatible with generalized Tjon lines, which assume a linear dependence between the binding energy of the pentamers and the one of tetramers, when both are divided by the trimer energies.

Pressure-Dependent Structure of BaZrO3 Crystals as Determined by Raman Spectroscopy.

The structure of dielectric perovskite BaZrO3, long known to be cubic at room temperature without any structural phase transition with variation in temperature, has been recently disputed to have different ground state structures with lower symmetries involving octahedra rotation. Pressure-dependent Raman scattering measurements can identify the hierarchy of energetically-adjacent polymorphs, helping in turn to understand its ground state structure at atmospheric pressure. Here, the Raman scattering spectra of high-quality BaZrO3 single crystals grown by the optical floating zone method are investigated in a pressure range from 1 atm to 42 GPa. First, based on the analyses of the infrared and Raman spectra measured at atmospheric pressure, it was found that all the observed vibrational modes could be assigned according to the cubic structure. In addition, by applying pressure, two structural phase transitions were found at 8.4 and 19.2 GPa, one from the cubic to the rhombohedral Rc phase and the other from the rhombohedral to the tetragonal I4/mcm phase. Based on the two pressure-induced structural phase transitions, the true ground state structure of BaZrO3 at room temperature and ambient pressure was corroborated to be cubic while the rhombohedral phase was the closest second.

First-principles insights into thermoelectric properties of topological nontrivial semimetal LiAuTe material

Structural, electronic and thermoelectric properties of LiAuTe ternary compound are studied using density functional theory (DFT) and semi-classical Boltzmann transport theory. The cubic -phase (space group F3m) is predicted to be ground state structure with a significant energy difference compared to honeycomb structure (space group P63mmc). The mechanical and dynamical stability of the -phase is confirmed by calculating the elastic constants and phonon dispersion frequencies. At equilibrium lattice, with and without spin–orbit coupling, the LiAuTe compound band structure calculations show an s-p band inversion at Γ point, leading to a topological nontrivial semimetal phase. Thermoelectric parameters, such as Seebeck coefficient (S), electrical conductivity (σ), electronic (κ e ) and lattice (κ L ) thermal conductivities are computed. Electrons and holes relaxation times (τ) are also predicted. Hence, LiAuTe compound exhibits a low κ L value of 1.76 W mK−1 at room temperature which decreases with temperature increasing. At 900 K, κ L falls to 0.58 W mK−1 leading to a maximum ZT value of 0.52 at optimized n-doping concentration of 2.5 × 1020 cm−3. The present study reveals that LiAuTe compound is a suitable candidate for thermoelectric applications and will open new horizons for further researches on similar types of topological thermoelectric materials with better ZT.

Strain-tunable carrier mobility of Fe-doped GaN: A first-principles study

Using first-principles calculations together with acoustic deformation potential scattering theory, we have investigated the strain-tunable carrier mobility of Fe-doped GaN with a doping concentration of 3.125%. Our calculated results show that the electron mobility of Fe-doped GaN decreases with increasing compressive strain but increases with increasing tensile strain. Unlike that electron mobility, the hole mobility monotonically decreases as the compressive stress increases. The hole mobility reaches its maximum value in the ground state structure. From the calculated phonon dispersion we verified that the dynamic stability of Fe-doped GaN can be maintained in the strain range of ±1.5%. We also found that the tensile strain can effectively improve carrier mobility of Fe-doped GaN, especially for the electron mobility. The excellent electronic properties of Fe-doped GaN could have possible applications in photoconductive switches and optoelectronic devices.

Oxidation of two-dimensional electrides: Structural transition and the formation of half-metallic channels protected by oxide layers

Based on first-principles calculations, we performed a systematic study of the energetic stability, structural characterization, and electronic properties of the fully oxidized ${A}_{2}B$ electrenes, with the following combinations: (i) $A$ = Ca, Sr, and Ba for $B$ = N; (ii) $A$ = Sr and Ba for $B$ = P; and ${\mathrm{Y}}_{2}\mathrm{C}$ and ${\mathrm{Ba}}_{2}\mathrm{As}$. We have considered one side oxidation of single layer electrenes (O/${A}_{2}B$), and two side oxidation of bilayer electrenes (O/${({A}_{2}B)}_{2}$/O). We show that the hexagonal lattice of the pristine host is no longer the ground state structure in the (fully) oxidized systems. Our total energy results reveal an exothermic structural transition from hexagonal to tetragonal ($\mathrm{h}\ensuremath{\rightarrow}\mathrm{t}$) geometry, resulting in layered tetragonal structures [(${A\mathrm{O}AB)}^{\mathrm{t}}$ and ${(A\text{O}{(AB)}_{2}A\text{O})}^{\mathrm{t}}$]. Phonon spectra calculations and molecular dynamic simulations show that the O/${A}_{2}B$ and O/${({A}_{2}B)}_{2}$/O systems, with $A$ = Ba, Ca, Sr, and $B$ = N, become dynamically and structurally stable upon such a $\mathrm{h}\ensuremath{\rightarrow}\mathrm{t}$ transition. Further structural characterizations were performed based on simulations of the near edge x-ray absorption spectroscopy at the nitrogen $K$ edge. Finally, the electronic band structure and transport calculations reveal the formation of half-metallic bands spreading out through the $A\mathrm{N}$ layers, which in turn are shielded by oxide $A\mathrm{O}$ sheets. These findings indicate that ($A\mathrm{O}A\mathrm{N}$)${}^{\mathrm{t}}$ and ${(A\mathrm{O}{(A\mathrm{N})}_{2}A\mathrm{O})}^{\mathrm{t}}$ are quite interesting platforms for application in spintronics; since the half-metallic channels along the $A\mathrm{N}$ or ${(A\text{N})}_{2}$ layers (core) are protected against the environment conditions by the oxidized $A\text{O}$ sheets (cover shells).

Symmetry and strain analysis of combined electronic and structural instabilities in tungsten trioxide, WO3

Lattice parameter data from the literature have been used to provide a complete description of spontaneous strain variations across each of the six known phase transitions of WO3 in the temperature interval 5–1273 K. Analysis of strain/order parameter coupling reveals the character of each phase transition, a unified description of strain across the full temperature range, the relationship between strain and electronic effects, and new insights into the strain gradients likely to be present in each of the different domain walls that develop in four different ferroelastic phases. Tetragonal and orthorhombic shear strains have values of 4%–6% and 2%–3%, respectively, and are dominated by coupling with the order parameter for antiferroelectric-type displacements. Conversely, shear strains, e4, e5, and e6, of up to 2% are controlled by octahedral tilting. Changes in electronic structure and properties have been related back to the susceptibility of W6+ to develop cooperative second-order-Jahn–Teller distortions. Proximity to tilt instabilities along with group–subgroup relationships in the P4/nmm parent structure results in two overlapping sequences of structural phase transitions, which differ in the form of their electronic structure. The possibility of a ground state structure in space group P21/c can be rationalized in terms of the efficiency by which different combinations of shearing and tilting of the WO6 octahedra can reduce the unit cell volume and would imply that WO3 has a re-entrant phase transition. Gradients in up to three order parameters coupled with gradients in strain of up to 12% across ferroelastic domain walls indicate that the different ferroelastic phases of WO3 should have domain walls with varied and potentially exotic electronic properties for device applications such as in nanoelectronics and neuromorphic computing.

New explanation for the existence of B19′ phase in NiTi alloy from the perspective of twinning martensite

The ground state structure of NiTi martensite in experiments and existing theoretical calculations is the focus of ongoing disagreements. Based on Waitz et al.’s (2004, 2007) experiments, we use the first principle calculations to investigate the effect of (001) twin interface and thickness on the stability of NiTi martensite. Moreover, we determine the interface of the (001) martensitic twins observed in the experiment as the interface of twinned supercell-1 (TSC-1), in which the lattice parameters of individual unit cells are close to the experimental measurements of B19′. Furthermore, the energy per atom for TSC-1 is always between that of B19′ and B33 and does not vary with the twin thickness. Considering the structural similarity of TSC-1 and B33, an energy barrier between TSC-1 and B33 via the minimum enthalpy pathways was demonstrated. The influence of the (001) twin interface could proves the rationality of the existence of B19′ in experiments.

Synthesis of Donor–Acceptor Copolymers Derived from Diketopyrrolopyrrole and Fluorene via Eco-Friendly Direct Arylation: Nonlinear Optical Properties, Transient Absorption Spectroscopy, and Theoretical Modeling

A series of PFDPP copolymers based on fluorene (F) and diketopyrrolopyrrole (DPP) monomers were synthesized via direct arylation polycondensation using Fagnou conditions which involved palladium acetate as catalyst (a gradual catalyst addition of three different percentages were used), potassium carbonate as the base, and neodecanoic acid in N, N-dimethylacetamide. This synthesis provides a low cost compared with traditional methods of transition-metal-catalyzed polymerization. Among the different amounts of catalyst used in the present work, 12% was optimal because it gave the highest reaction yield (81.5%) and one of the highest molecular weights (Mn = 13.8 KDa). Copolymers’ chemical structures, molecular weight distributions, and optical and thermal properties were analyzed. The linear optical properties of PFDPP copolymers resulted very similarly independently to the catalyst amounts used in the synthesis of the PFDPP copolymers: two absorptions bands distinctive of donor–acceptor copolymers, Stokes shifts of 41 nm, a good quantum yield of fluorescence around 47%, and an optical bandgap of 1.7 eV were determined. Electronic nonlinearities were observed in these copolymers with a relatively high two-photon absorption cross-section of 621 GM at 950 nm. The dynamics of excited states and aggregation effects were studied in solutions, nanoparticles, and films of PFDPP. Theoretical calculations modeled the ground-state structures of the (PFDPP)n copolymers with n = 1 to 4 units, determining the charge distribution by the electrostatic potential and modeling the absorption spectra determining the orbital transitions responsible for the experimentally observed leading bands. Experimental and theoretical structure–properties analysis of these donor–acceptor copolymers allowed finding their best synthesis conditions to use them in optoelectronic applications.

Critical role of magnetic moments in heavy-fermion materials: Revisiting SmB6

Heavy-fermion family exhibits fascinating and often puzzling properties due to the presence of open-shell $f$ ions and the complexity of the associated charge, orbital, and spin degrees of freedom. SmB$_6 $ is a prototypical heavy-fermion compound that is electrically insulating but yet it displays quantum oscillations, which are a telltale signature of the metallic state. Adding to the enigma is the possibility that SmB$_6$ is a topological Kondo insulator. Here, by treating the spin degree of freedom on an equal footing with other degrees of freedom using the parameter-free strongly-constrained and appropriately-normed (SCAN) density functional, we explore the ground-state electronic structure of SmB$_{6}$. A number of competing magnetic phases lying very closely in energy are found, indicating the key role of spin fluctuations in the material. The computed band structure, crystal-field splittings in the $f$-electron complex, the heavy effective electron mass at the Fermi energy, and the large specific heat are all in good agreement with the corresponding experimental results. In particular, our predicted FS explains the experimentally observed bulk quantum oscillations as well as the low electrical conductivity of SmB$_{6}$. The topological Kondo state of SmB$_6$ is shown to be robust regardless of its magnetic configuration. The excellent performance of SCAN in heavy-fermion systems is explained in terms of its ability to treat self-interaction errors and symmetry breaking within the framework of the density functional theory. Our study provides a new approach for modeling heavy-fermion materials.

Recent Advances in Understanding the Structures of Translesion Synthesis DNA Polymerases.

DNA damage in the template strand causes replication forks to stall because replicative DNA polymerases are unable to efficiently incorporate nucleotides opposite template DNA lesions. To overcome these replication blocks, cells are equipped with multiple translesion synthesis polymerases that have evolved specifically to incorporate nucleotides opposite DNA lesions. Over the past two decades, X-ray crystallography has provided a wealth of information about the structures and mechanisms of translesion synthesis polymerases. This approach, however, has been limited to ground state structures of these polymerases bound to DNA and nucleotide substrates. Three recent methodological developments have extended our understanding of the structures and mechanisms of these polymerases. These include time-lapse X-ray crystallography, which allows one to identify novel reaction intermediates; full-ensemble hybrid methods, which allow one to examine the conformational flexibility of the intrinsically disordered regions of proteins; and cryo-electron microscopy, which allows one to determine the high-resolution structures of larger protein complexes. In this article, we will discuss how these three methodological developments have added to our understanding of the structures and mechanisms of translesion synthesis polymerases.

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Structural evolution, electronic properties and spectra of titanium clusters

The ground-state structures, electronic properties and spectra of Ti n c ( n = 3–16 and c = 0, ±1) clusters have been investigated based on density functional theory (DFT) and CALYPSO structure prediction. Geometry optimizations show that the neutral, cationic and anionic titanium clusters have the same growth pattern. The pentagonal bipyramid plays an important role in the growth process. Analysis of electronic properties shows that the thermal stability of cationic and anionic clusters is greater than that of neutral cluster. The pentagonal bipyramid and icosahedron clusters have relatively high stability. Chemical activity of titanium clusters decreases with the cluster size. The preferred dissociation pathway of Ti n + clusters is the loss of a single Ti atom to form Ti n − 1 + . Optical absorption of Ti n clusters and infrared and Raman spectra of Ti n + clusters have been simulated and can be used for their structural identification. The ground states of Ti n − clusters have been determined by comparing experimental and theoretical photoelectron spectra. • The pentagonal bipyramid plays an important role in the growth process of titanium clusters • The titanium cluster with pentagonal bipyramid or icosahedron geometry is more table than other clusters. • The preferred dissociation pathway of Ti n + clusters is the loss of a single Ti atom to form Ti n − 1 + . • The ground state of anionic Ti n − clusters are identified by photoelectron spectra.s

Ground-State Structures of the Ising Model on a Layered Triangular Lattice in a Magnetic Field

Ground-State Structures of the Ising Model on a Layered Triangular Lattice in a Magnetic Field

New Insights into Phase Separation of Cerium Hydrides under Pressure.

We report theoretical calculations of the static ground-state structures and pressure-induced phase transformations of four cerium hydrides: CeH, CeH2, CeH2.5, and CeH3. Under pressure, the experimental CaF2-type structure of CeH2 is likely to disproportionate to face-centered cubic (fcc) Ce and a cubic Pm3̅n (β-UH3 type) structure of CeH3 above 6 GPa. At further increasing pressures, fcc Ce will transform to a tetragonal I4/mmm structure above 12 GPa, while CeH3 moves through the following sequence of phases: Pm3®n (β-UH3 type) → Pm3̅n (A15 type) → R3®m; the corresponding transition pressures are calculated to be 10 and 70 GPa, respectively. The tetragonal I41/amd structure of CeH2.5 has the similar decomposition as that of CeH2. Finding this previously unreported pressure-induced decomposition of CeH2 will pave the way for investigations on the nature of hydrogen–metal interactions.

First-principles investigation of structural stability, electronic and optical properties of suboxide (Zr3O)

Zirconium oxides (Zr-O) has been widely used in energy storage system, catalyst, microelectronic, optoelectronics, and high-temperature ceramics etc. Unfortunately, the ground state structure, electronic and optical properties of Zr3O are unclear until now. Here, we apply the first-principles calculations to study the structure, electronic and optical properties of Zr3O. Three Zr3O oxides: rhombohedral (R32), rhombohedral (R-3c) and hexagonal (P6322) phase are studied. The results show that three Zr3O oxides are thermodynamic and dynamical stabilities. The calculated phonon density of state (PhDOS) further demonstrates the stable of Zr3O oxides. In particular, it is found that the rhombohedral (R-3c) Zr3O is slightly more stable than the other two Zr3O oxides. Unlike ZrO2, the Zr3O oxides exhibit better electronic properties due to the electronic jump between Zr- excitation band and Zr- conduction band near the Fermi level. Compared to the hexagonal Zr3O, the host peak of the rhombohedral Zr3O move into the low energy region, which leads to the red shift phenomenon occurs. In addition, it is found that the rhombohedral (R32, No.155) Zr3O oxide has better storage optical properties compared to the other two Zr3O oxides.

Contributions of bulk and surface energies in stabilizing metastable polymorphs: A comparative study of group 3 sesquioxides La2O3, Ga2O3 , and In2O3

Sesquioxides are an important class of compounds which are used in various applications such as optical glasses, high-power electronics, and gate dielectrics. Using density functional theory, we investigated bixbyite ${\mathrm{La}}_{2}{\mathrm{O}}_{3}, {\mathrm{In}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$; hexagonal ${\mathrm{La}}_{2}{\mathrm{O}}_{3}$; corundum ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$; and monoclinic ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. We compared the predicted structural properties for each material using the local density approximation (LDA) and the generalized-gradient approximation [GGA PBE (Perdew-Burke-Ernzerhof)]. We found that LDA predicts the correct ground state structure for all three sesquioxides; however, GGA PBE predicts the incorrect ground state structure for ${\mathrm{La}}_{2}{\mathrm{O}}_{3}$. To gain better insight on why this happens, we calculated the phonon properties and the free energy for all structures to determine phase transition temperatures. Additionally, we determined the transition pressure for ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$. We also studied several surface terminations for each compound and determined the lowest-energy surface for each structure as well as the interfacial energy for ${\mathrm{La}}_{2}{\mathrm{O}}_{3}$.

Plasma-Assisted Dinitrogen Activation via Dual Platinum Cluster Catalysis: A Strategy for Ammonia Synthesis under Mild Conditions

Plasma-Assisted Dinitrogen Activation via Dual Platinum Cluster Catalysis: A Strategy for Ammonia Synthesis under Mild Conditions

The structures and properties of Mon (n = 2[formula omitted]15) cluster

The stable isomers of Mon (n=2∼15) are determined by using genetic algorithm combined with DFT calculations. Some new ground-state structures are found. The 3D structure firstly appears for Mo4 and the distorted pyramid or prism isomers are energy favored for n = 4∼9, while icosahedron-like or cage-like structures are dominant for n = 10∼15. Mo3, Mo5, Mo7, Mo8, Mo11 and Mo13 are magic number clusters. There are small magnetic moments and vertical electron affinity but large vertical ionization potential for Mon. The electronic structure analysis indicates that there form strong Mo-Mo covalent bonds, which are also delocalization to a certain extent due to the 4d and 5s orbital hybridization. The IR and UV–vis spectra are acquired and the particularly large hyper polarizability of Mo9 also suggests that it can be used as a nonlinear optical material.