First-principles Total Energy Research Articles

A first-principles study of the phase transitions in ultrahigh temperature shape memory alloy RuNb

Ultrahigh temperature shape memory alloys (UHT-SMAs) have transition temperatures above 600 °C. They are needed for sensing and actuating devices in aerospace applications. However, very few such UHT-SMAs have been found. Among them are Ru-based alloys such as RuNb and RuTa, whose martensite structures and phase transitions are totally different from those of NiTi-based SMAs and were poorly understood. In this work, we carried out a systematical study of RuNb using first-principles total energy calculations and molecular dynamics (MD) simulations. We revealed the transition paths and mechanisms in cubic → tetragonal → monoclinic transitions. We determined the transition sequence and martensitic transition temperatures (MTTs) by evaluating the Gibbs free energies using thermodynamic integration. The calculated MTTs are in very good agreement with the experimental data. We also found that the monoclinic phase at the second transition has the P21/m symmetry instead of experimentally identified P2/m. The insights gained by this study and the verified ab initio methods for accurate MTT calculations can be applied to fast screen and quantitatively design novel UHT-SMAs having similar properties with desirable MTTs and much reduced cost.

Site-selective cobalt substitution in La–Co co-substituted magnetoplumbite-type ferrites: 59Co-NMR and DFT calculation study

The La–Co co-substituted magnetoplumbite-type (M-type) ferrites AFe12O19 (A = Ca, Sr and Ba, ion sizes Ca2+ < Sr2+ < Ba2+) with Co compositions around 0.2 have been subjected to 59Co-NMR. The results show that Co occupies the 4f1, 2a and 12k sites, and that the smaller the A ion, the more Co tends to occupy the 4f1 minority spin site, which is effective in enhancing both uniaxial anisotropy and magnetisation. First-principles total energy calculations based on density functional theory (DFT) of undoped AFe12O19 and a supercell ( 2×2×1 of the unit cell) in which 1/96 of Fe3+ is replaced by Co2+ were performed to predict the stable structure and Co occupancy sites. The results show that regardless of A, Co is most stable when it occupies the 4f1 site, followed by the 2a and 12k sites with energy differences on the order of 100 meV, and Co practically does not occupy the 2b and 4f2 sites. As the A ion becomes smaller, the energy difference when Co occupies each Fe site tends to increase, and the Co occupancy of the 4f1 site also increases. The site selectivity of Co can be roughly explained as a result of the difference in uniaxial strain along the c-axis associated with the difference in A. However, the influence of the A ion differs between the R and S blocks and the local strain also has a secondary effect on the Co distribution. Based on these results, the guidelines for improving the performance (anisotropy and magnetisation) of La–Co co-substituted M-type ferrite magnets with a limited amount of Co can be summarised as follows: It is effective to select as small A ions as possible and to post-anneal at low temperature or cool slowly to concentrate Co at the 4f1 site in tetrahedral coordination.

Study of lithium transport in Li2O component of the solid electrolyte interphase in lithium-ion batteries

In lithium-ion batteries (LIBs), as a promising energy storage device, materials with fast lithium (Li) transport are required for high-power applications such as electric vehicles. Thus, a deeper understanding of Li transport in components of LIBs is crucial for improving their rate capability. In this study, the Li transport in lithium oxide (Li2O), as one of the key components of the solid electrolyte interphase (SEI) layer, was examined by a multiscale computational approach ranging from density functional theory (DFT) to Monte Carlo simulations. The DFT calculations were used to investigate the recombination of Frenkel pairs, their first-principles total energies, and the Li diffusion mechanisms. The effect of atomic configurations on both first-principles total energies and diffusion barrier energies was formulated by periodic and local cluster expansions. These formalisms were then incorporated into the Monte Carlo and Kinetic Monte Carlo (KMC) simulations to calculate the diffusion coefficient of Li. Our calculations revealed that the vacancy-mediated jump along the 〈100〉 direction in the antifluorite structure of Li2O possesses the lowest barrier energy compared to other diffusion mechanisms. The KMC simulations indicated that the diffusion coefficient of Li better converged with the direct experimental measurement when the recombination of Frenkel pairs was integrated into the simulations. At a temperature of 300 K, the KMC simulation yielded a Li diffusion coefficient of 3.8×10-12cm2/s in Li2O. This is only one order of magnitude larger than indirect experimental measurement, suggesting the accuracy of our formalism. Thus, our formalism for studying Li transport in Li2O will pave the path to a rational design of inorganic SEI in the future development of LIBs.

Stoichiometric and Nonstoichiometric Surface Structures of Pyrochlore Y2Zr2O7 and Their Relative Stabilities: A First-Principles Investigation

First-principle total energy calculations were performed to investigate the atomic structures and relative stabilities of two low miller-index surfaces of pyrochlore Y2Zr2O7. The stoichiometric Y2Zr2O7 (110) and (100) surfaces were predicted, with lowest formation energies of 1.20 and 1.47 J/m2, respectively. Based on a thermodynamic defect model, non-stoichiometric Y2Zr2O7 surface energies were further evaluated as a function of environmental oxygen partial pressure (pO2) and temperature (T). With all of the results, we were able to construct the surface phase diagrams for T = 300 and 1400 K. The strong correlation between the structural stabilities and the surface stoichiometry was revealed as varying T and pO2. At a given T, the most stable termination of the (110) surfaces would change from a (Y,Zr)−rich (ns−2Y2Zr6O) to O−rich ones (ns−4O_2 and ns−4O_1) as increasing pO2, while that of the (100) surfaces would change from the stoichiometric (stoi−1Y1Zr_1) to the O−rich one (ns−5O). The critical pO2 value for termination transition moves to its higher end as increasing T.

The effect of solute segregation on stability and strength of Cu symmetrical tilt grain boundaries from the first-principles study

Grain boundary (GB) stability and strength are two essential points for nanocrystalline materials. In this study, the effects and underlying mechanisms of segregation-induced GB stabilization and GB strengthening in various Cu GBs for selected 8 alloying elements (Mg, Ca, Cr, Ni, Zn, Zr, Ag and Sn) were investigated through first-principles total energy calculations. The segregation energy results show that all other elements excluding Ni can segregate to GB thus improving the GB stability. The segregation driving force is associated with the volume of Voronoi cell of solute, in the trend that increases along with the volume of Voronoi cell growth. The stabilizing mechanism should attribute to the decrease in the number of antibonding electrons. As to GB strength, the embrittling potency results show that Mg, Cr, Zr and Ag are enhancers in Σ5 [001](310) GB, while for Σ5 [001](210) and Σ11 [110](113) GBs, only Cr and Zr are enhancers. The embrittling potency is concerned with segregation energy and the volume of Voronoi cell of solute, in a trend that increases with the segregation energy decline and the volume of Voronoi cell growth. The enhancing mechanism lies in the formation of covalent-like bonds between solute and matrix atoms. Furthermore, by comparing the stabilizing and strengthening effectiveness of alloying elements in various Cu GBs. The trend that the worse the stability and strength of GBs, the better the stabilizing and strengthening effects of alloying elements were revealed.

Electronic and elastic properties of BN, AlN and GaN

We have carried out first-principles total energy calculations to investigate electronic and elastic properties of both zinc-blende and wurtzite BN, AlN, and GaN. We have calculated lattice parameters, elastic constants, deformation potential constants, phonon frequencies at r point, Born effective charges, and piezoelectric constants. Lattice parameters are fully relaxed by using the first-principles molecular dynamics method with variable cell shape. The internal strain in a strained crystal is also relaxed by the first-principles molecular dynamics method. The internal strain influences the elastic constants, the deformation potential constants, and the piezoelectric constants effectively. We have calculated the wurtzite deformation potential constants D1 – D5 considering the internal strain correction. The piezoelectric constants of wurtzite and also zinc-blende crystals have been calculated using the Berry phase approach and we have found from first principles that those of BN have an inverse sign in contrast to AlN and GaN. Discussions will be given in comparison with results obtained herein with the previous ones. © 1998 American Institute of Physics. [S0021-8979(98)07921-3]

Phase diagrams and critical temperatures for coherent and incoherent mixtures of InAs1−xSbx alloys using first-principles calculations

Phase diagram calculations are performed for incoherent and coherent mixtures of an InAs1−xSbx (InAsSb) ternary alloy, which is an important material for the applications to infrared detector technology. Our calculations are based on the cluster expansion approach and Monte Carlo simulations combined with first-principles total energy calculations in the framework of density functional theory with Perdew–Burke–Ernzerhof (PBE) and Heyd–Scuseria–Ernzerhof (HSE) exchange-correlation functionals. Because of a lattice mismatch (∼7%) between InAs and InSb, coherency strain plays an important role for the phase stability of the InAsSb alloys. The alloys without the coherency strain (incoherent mixtures) show a miscibility gap with the critical temperature at ∼700 K with 42% (45%) Sb concentration in PBE (HSE), which is in good agreement with the experimentally determined equilibrium miscibility gap temperature. The alloys with the coherency strain (coherent mixtures) show several ground states whose structures are short period superlattices along the [201] direction. The critical temperature is ∼200 K with 50% Sb concentration in both PBE and HSE, which is reduced by ∼500 K compared to that of incoherent mixtures. This reduction of the critical temperature is consistent with the experimental observation where the homogeneous InAsSb alloy continues to grow inside the empirical miscibility gap.

Electronic and elastic properties of BN, AlN and GaN

We have carried out first-principles total energy calculations to investigate electronic and elastic properties of both zinc-blende and wurtzite BN, AlN, and GaN. We have calculated lattice parameters, elastic constants, deformation potential constants, phonon frequencies at r point, Born effective charges, and piezoelectric constants. Lattice parameters are fully relaxed by using the first-principles molecular dynamics method with variable cell shape. The internal strain in a strained crystal is also relaxed by the first-principles molecular dynamics method. The internal strain influences the elastic constants, the deformation potential constants, and the piezoelectric constants effectively. We have calculated the wurtzite deformation potential constants D1 – D5 considering the internal strain correction. The piezoelectric constants of wurtzite and also zinc-blende crystals have been calculated using the Berry phase approach and we have found from first principles that those of BN have an inverse sign in contrast to AlN and GaN. Discussions will be given in comparison with results obtained herein with the previous ones. © 1998 American Institute of Physics. [S0021-8979(98)07921-3]

Microstructure and properties of NbVZr refractory complex concentrated alloys

The microstructure and mechanical properties of the equiatomic NbVZr alloy are reported. The laser-processed and as-cast samples both exhibit a dendritic BCC solid solution with hexagonal C14 and cubic C15 Laves phases in the interdendritic regions. The stability of the Laves phases is discussed based on first-principles total energy calculations. Nb is found to increase the stability of C14 and C15 phases. The Laves phases strengthen the material by acting as obstacles to dislocation motion, as evidenced by nanoindentation and TEM observations of deformed samples. A high concentration of stacking faults was observed in the Laves region, which confirms the low stacking fault energy predicted by first-principles calculations, and are expected to result in additional plasticity. This work serves as an example of a systematic study of the stability of intermetallic phases in refractory complex concentrated alloys using a high-throughput synthesis technique. Such phases play an important role in the overall mechanical properties of the alloy.

Surface structures of magnetostrictive D03-Fe3Ga(0 0 1)

First-principles total energy calculations and scanning tunneling microscopy experiments were performed to study the surface reconstruction of the magnetostrictive Fe3Ga alloy. The inverse magnetostrictive behavior was evaluated in the bulk by compressing and stretching its lattice parameter, showing an increase in magnetic moments as strain increases. Surface analysis demonstrates two thermodynamically stable surfaces, the (1 × 1) and (3 × 1). The (1 × 1) is an ideal FeGa terminated surface, whereas the (3 × 1) is also FeGa terminated but it has a first-layer Fe atom substituted by a Ga atom every three unit-cells, forming a row-like surface structure. Tersoff–Hamann scanning tunneling microscopy simulations were obtained and compared with experimental results. We found good agreement between theory and experiment, in which the distance between rows is ~12.3 Å. Theoretical findings suggest that the substrate-induced strain may increase the stability of the (3 × 1) reconstruction. Analysis of the magnetic moments in the reconstructions showed that their behavior is affected by a surface effect, as well as by the inverse magnetostriction of the structure. A good understanding of the FeGa surface reconstructions is an important step towards further improvements in magnetic storage devices and sensors.

Phase Transition in a Memristive Suspended MoS2 Monolayer Probed by Opto- and Electro-Mechanics.

Semiconducting monolayers of a 2D material are able to concatenate multiple interesting properties into a single component. Here, by combining opto-mechanical and electronic measurements, we demonstrate the presence of a partial 2H-1T' phase transition in a suspended 2D monolayer membrane of MoS2. Electronic transport shows unexpected memristive properties in the MoS2 membrane, in the absence of any external dopants. A strong mechanical softening of the membrane is measured concurrently and may only be related to the 2H-1T' phase transition, which imposes a 3% directional elongation of the topological 1T' phase with respect to the semiconducting 2H. We note that only a few percent 2H-1T' phase switching is sufficient to observe measurable memristive effects. Our experimental results combined with first-principles total energy calculations indicate that sulfur vacancy diffusion plays a key role in the initial nucleation of the phase transition. Our study clearly shows that nanomechanics represents an ultrasensitive technique to probe the crystal phase transition in 2D materials or thin membranes. Finally, a better control of the microscopic mechanisms responsible for the observed memristive effect in MoS2 is important for the implementation of future devices.

Atomistic structure learning algorithm with surrogate energy model relaxation

The recently proposed Atomistic Structure Learning Algorithm (ASLA) builds on neural network enabled image recognition and reinforcement learning. It enables fully autonomous structure determination when used in combination with a first-principles total energy calculator, e.g. a density functional theory (DFT) program. To save on the computational requirements, ASLA utilizes the DFT program in a single-point mode, i.e. without allowing for relaxation of the structural candidates according to the force information at the DFT level. In this work, we augment ASLA to establish a surrogate energy model concurrently with its structure search. This enables approximative but computationally cheap relaxation of the structural candidates before the single-point energy evaluation with the computationally expensive DFT program. We demonstrate a significantly increased performance of ASLA for building benzene while utilizing a surrogate energy landscape. Further we apply this model-enhanced ASLA in a thorough investigation of the c(4x8) phase of the Ag(111) surface oxide. ASLA successfully identifies a surface reconstruction which has previously only been guessed on the basis of scanning tunnelling microscopy images.

Gaining insights into the formation of high temperature GeMn nanocolumns: the effect of substrate crystalline orientation on their structural and magnetic properties

Structural and magnetic characterisations along with first-principles total energy calculations based on the density functional theory have been combined to investigate the effects of (001) and (111)-oriented Ge substrates on the formation of GeMn nanocolumns. The samples were grown by means of molecular beam epitaxy (MBE) at the growth temperature (TS) of 130 °C, the Mn concentration of ∼6% and the film thickness of ∼80 nm. We found that due to the surfactant effect of Mn atoms, Ge0,94Mn0,06 films grown on the Ge(001) substrate exhibit the nanocolumn structure along the growth direction and the Curie temperature (TC) is higher than 400 K. On the other hand, for the Ge0,94Mn0,06 films grown on the Ge(111) substrate, Mn adatoms can easily diffuse into deeper layers through the interstitial sites, resulting in the formation of Ge3Mn5 streaks along the preferred direction [110] and surrounded by diluted matrix. The physical origin of the contrasting behaviour of Mn atoms is therefore strongly related to the different surface reconstruction.

In-plane ordering of oxygen vacancies in a high- Tc cuprate superconductor with compressed Cu-O octahedrons: An automated cluster expansion study

A recently discovered high-Tc cuprate superconductor Ba2CuO$_{4-\delta}$ exhibits exceptional Jahn-Teller distortion, wherein the CuO6 octahedrons are compressed along the c axis. As a consequence, the O vacancies prefer to reside in the CuO2 plane, but the exact structure is not known. By combining first-principles total energy calculation with the automated structure inversion method, the effective cluster interactions of O vacancies are mapped out. Around $\delta$=0.8, where the 73K superconductivity was observed experimentally, we predict that the ordered O vacancies slice the CuO2 plane into not only 1D chains and but also two-leg ladders. A Monte Carlo simulation is performed based on the effective cluster interaction model, showing that such an ordering pattern is stable up to ~900 K. Our results put forth a concrete structural basis to discuss the underlying superconducting mechanism.

Effect of doping Zn atom on the structural stability, mechanical and thermodynamic properties of AlLi phase in Mg–Li alloys from first-principles calculations

ABSTRACT This work uses first-principles total energy calculations on the basis of density functional theory to predict the structural stability, mechanical and thermodynamic properties of Zn atom doped AlLi phase in Mg–Li–Al–Zn alloy. The values of the equilibrium lattice parameters and the formation of enthalpy are highly consistent with the experimental and previous calculations results available. Negative enthalpies of formation ΔH are predicted for all AlLi phase doped concentrations which have positive consequences for its structural stability. The elastic modulus is deduced by Voigt–Reuss–Hill arithmetic approximation. The bulk modulus of the Al–Li–Zn compounds increases as the doping concentrations increase, which are larger than the value of the AlLi phase. In particular, the stability and mechanical anisotropy of the Al–Li–Zn compounds are discussed. The charge density cloud map is drawn to reveal the bonding characteristics of four compounds. The changes in thermodynamic properties are derived by the phonon frequencies within the quasi-harmonic approximation.

Investigation of GaAs and AlAs atomic-layer epitaxial growth mechanism based on experimental results and first-principles total energy calculation

In GaAs and AlAs atomic-layer epitaxy (ALE), it was experimentally found that the AlAs layer is deposited in two monolayers (2 ML) per ALE cycle, while the GaAs layer is deposited in 1 ML. In order to elucidate this growth mechanism, we compared the stability of Ga and Al atoms on the GaAs(100) surface by the first-principles total energy calculation based on density functional theory. Comparing adsorption energies, we found that Ga and Al stably adsorbed on the As-terminated GaAs surface in 1 ML. It was also revealed that Al can adsorb on an As-terminated GaAs(100) surface in 2 ML because excess Al atoms can be adsorbed by forming a metallic Al(110) plane-like structure. These results well explain the experimental results for ALE-GaAs and ALE-AlAs.

Simulations of X-ray absorption spectroscopy and energetic conformation of N-heterocyclic carbenes on Au(111).

It has recently been demonstrated that N-heterocyclic carbenes (NHCs) form self-assembled monolayers (SAMs) on metal surfaces. Consequently, it is important to both characterize and understand their binding modes to fully exploit NHCs in functional surface systems. To assist with this effort, we have performed first-principles total energy calculations for NHCs on Au(111) and simulations of X-ray absorption near edge structure (XANES). The NHCs we have considered are N,N-dimethyl-, N,N-diethyl-, N,N-diisopropylbenzimidazolylidene (BNHCX, with X = Me, Et, and iPr, respectively) and the bis-BNHCX-Au complexes derived from these molecules. We present a comprehensive analysis of the energetic stability of both the BNHCX and the complexes on Au(111) and, for the former, examine the role of the wing group in determining the attachment geometry. Further structural characterization is performed by calculating the nitrogen K-edge X-ray absorption spectra. Our simulated XANES results give insight into (i) the relationship between the BNHCX/Au geometry and the N(1s) → π*/σ*, pre-edge/near-edge, absorption intensities, and (ii) the contributions of the molecular deformation and molecule-surface electronic interaction to the XANES spectrum. These simulated spectra work not only as a map to the BNHCX conformation, but also, combined with electronic structure calculations, provide a clear understanding of recent experimental XANES findings on BNHCX/Au.

Structural, electronic, and optical properties of cubic formamidinium lead iodide perovskite: a first-principles investigation.

Hybrid organic–inorganic perovskites have been one of the most active areas of research into photovoltaic materials. Despite the extremely fast progress in this field, the electronic properties of formamidinium lead iodide perovskite (FAPbI3) that are key to its photovoltaic performance are relatively poorly understood when compared to those of methylammonium lead iodide (MAPbI3). In this study, first-principles total energy calculations based on density functional theory were used to investigate the favored orientation of FA. Different theoretical methods, with or without incorporation of spin-orbit coupling (SOC) effects, were used to study the structure, electronic properties, and charge-carrier effective mass. Also the SOC-induced Rashba k-dependent band splitting, density of states and optical properties are presented and discussed. These results are useful for understanding organic–inorganic lead trihalide perovskites and can inform the search for new materials and design rules.

Pressure-Induced Structural Phase Transition and Superconductivity in NaSn5

The structural and electronic properties of the tin-rich compound NaSn5 were investigated under pressures of up to 10 GPa on the basis of the evolutionary algorithm (EA) technique coupled with first-principles total energy calculations. Upon compression, the known metallic tetragonal P4̅21m phase transforms into a metallic hexagonal P6/mmm phase at 1.85 GPa accompanied by an unusual change in the existing form of Sn atoms. The P6/mmm phase can be interpreted as a quasi-layered sandwich structure with two Sn layers and one sodium layer. The presence of softening phonon modes and the existence of Fermi pockets together with the obvious Fermi surface nesting indicate a strong electron-phonon coupling (EPC) and thus potential superconductivity in the P6/mmm phase. The strong EPC in the P6/mmm phase is mainly attributed to the phonons from Sn1 atoms together with electrons from the Sn1 py and Sn1 pz states. The calculated superconducting critical temperature Tc of the P6/mmm phase is 5.91 K at 1.85 GPa. This study provides a new clue for designing intercalated compounds with superconductivity.

Electronic property enhancement of zinc oxide by surface decoration with carbon nanotubes: experimental and theoretical studies

Experimental and theoretical investigations are performed to study the electronic property enhancement of zinc oxide (ZnO) thin films when their surfaces are decorated with carbon nanotubes (CNTs). Sample preparations have been achieved using the spray pyrolysis method to obtain ZnO (0001) surfaces decorated with carbon nanotubes. Atomic force microscopy and scanning electron microscopy studies allow for the elucidation of surface roughness and morphology. In the current–voltage (I–V) curves, conductivity has improved as induced by surface decoration of ZnO with CNTs. When the thin films are irradiated with visible light, they present an increased current from 3.8 to 100 nA, but when irradiated with UV light, a greater increase from 4.5 to 100 nA is observed. To explain these experimental measurements, first-principles total energy calculations have been performed to determine the electronic properties of the ZnO (0001) surface decorated with carbon nanotubes. Results indicate that the density of states of the ZnO (0001) surface is modified when carbon nanotubes are adsorbed. Electron states emerge at proximities of the Fermi level as a consequence of the nanotube presence. This may help to explain the current increase in the sample as decorated by the carbon nanotubes.