Nudged Elastic Band Research Articles

Diffusion, permeation and solubility of hydrogen, deuterium and tritium in crystalline tungsten: First principles DFT simulations

Tungsten (W) has shown promise to be the most favored plasma facing material in nuclear fusion with deuterium (D) and tritium (T) as fuel. In order to identify a better material, atomistic understanding of the behavior of D/T with W is highly desirable. Here, we report the mechanistic pathways of diffusion of H, D and T in bcc W employing density functional theory (DFT) using PBE and PBE-D3 functional and harmonic transition state theory. The activation diffusion barrier was determined using nudge elastic band techniques. The zero point energy (ZPE) was incorporated by phonon calculations to include the isotope effects. The surface adsorption of H, D and T is predicted to be exothermic, whereas surface to sub-surface and bulk absorption is endothermic but the vacancy induced absorption in bulk W was predicted to be exothermic. H, D and T atoms were observed to be diffused preferably from tetrahedral site to the nearest tetrahedral site. The calculated diffusion coefficients, rate constants, permeability constants and solubility are found to be higher for H compared to its heavier isotopes D and T. The trends of diffusion, permeation and solubility for H, D and T are well in agreement with the experimental results. The diffusion values predicted for H isotopes in bcc W from PBE-D3 method is quite close to the experimental values compared to PBE method. But, the calculated and experimental ratio of diffusivity, permeability and solubility of any two H isotopes follow the similar trend and no considerable change is noticed between PBE and PBE-D3 methods. Further, the calculated diffusion coefficients are shown to be one order of magnitude lower than Cr and two orders than Fe and thus might be the basis for considering W as plasma facing materials. The present study might be useful in modeling the behavior of tritium in W metal for which data is either scattered or least available.

Modified Single Iteration Synchronous-Transit Approach to Bound Diffusion Barriers for Solid-State Reactions.

Herein, we detail an approach to accelerate the computational screening of materials for properties dictated by the kinetics of solid-state diffusion through reliably and rapidly identifying upper and lower bounds to the transition state (TS) energy through our proposed modified single iteration synchronous-transit (MSIST) approach. While this sacrifices providing detailed information of the explicit TS structure, it requires only 30% of the force evaluations of a full nudged elastic band (NEB) TS search and reduces the computational demand to compute estimated diffusion barriers by ∼70% on average. In all 53 cases in which we explicitly compared our results to those of an NEB calculation, the upper and lower bounds identified using this approach bracketed the TS energy calculated with explicit NEB calculations. We use the applications of diffusion of Na+ in potential sodium-ion battery electrodes and oxygen vacancy diffusion in solid-oxide fuel cell electrodes and redox mediators for solar thermochemical hydrogen production to demonstrate the power of MSIST for analyzing the kinetics of bulk diffusion. For Na+ diffusion through 13 proposed electrode materials in which the average diffusion barrier was 0.28 eV, the average difference between the upper and lower bounds was 0.08 eV. An iterative application of this approach to the three materials with the largest difference between their upper and lower bounds further narrowed the average range of the bounded TS energies to 0.04 eV while still requiring fewer force evaluations than an NEB TS calculation. When applied in a high-throughput manner to study 514 diffusion pathways in 97 different materials, the average difference between the upper and lower bounds was 0.33 eV and the average barrier, as calculated by the average of all upper and lower bounds, was ∼1.7 eV. Because the MSIST approach produces explicit errors, i.e., the difference between the upper and lower bounds energies, even predicted barrier ranges with large errors can be reliably modeled with weighted regression techniques. MSIST enables the analysis of the kinetics of solid-state diffusion across larger sets of materials and can thus efficiently provide data to train statistically learned models of diffusion and to develop physical insights into the diffusion process.

Data sets and trained neural networks for Cu migration barriers.

Kinetic Monte Carlo (KMC) is an efficient method for studying diffusion. A limiting factor to the accuracy of KMC is the number of different migration events allowed in the simulation. Each event requires its own migration energy barrier. The calculation of these barriers may be unfeasibly expensive. In this article we present a data set of migration barriers on for nearest-neighbour jumps on the Cu surfaces, calculated with the nudged elastic band (NEB) method and the tethering force approach. We used the data to train artificial neural networks (ANN) in order to predict the migration barriers for arbitrary nearest-neighbour Cu jumps. The trained ANNs are also included in the article. The data is hosted by the CSC IDA storage service.

Adsorption of S on Si(111) with M4 × 4 superstructure

The atomic and electronic structures are calculated for the M4 × 4 superstructure of S/Si(111), which was recently identified by Schmidt et al. [Surf. Sci.694 (2020) 121561] by using the spot profile analysis low-energy electron diffraction (SPALEED) and confirmed by scanning tunneling microscopy (STM). The electronic band structures are found to be semiconducting in nature for three different S structures: monomer, Dimer-A and Dimer-B, each of which saturates all the Si dangling bonds. Due to the varying numbers of S atoms per unit cell, we have calculated the lowest energy phase in the accessible S chemical potential range. Dimer-A and Dimer-B are the lower energy phase and their accessible chemical potential value is less than the monomer case, in excellent agreement with the results of Schmidt et al. Dimer-B is slightly more stable geometry than Dimer-A. Application of the nudged elastic band (NEB) method suggests that there is a rotational activation barrier of 40 meV along the Dimer-A–Dimer-B reaction path.

Adsorption and diffusion of Au, Pt, and Co adatoms on SrTiO3(001) surfaces: A density functional theory study

We use Density Functional Theory (DFT) and Hubbard model-based DFT + U calculations to determine preferential adsorption sites and energies of single, isolated Au, Pt, and Co atoms on planar SrTiO3(001) surfaces. Based on these results, we employ a nudged elastic band (NEB) approach to calculate relevant diffusion energy barriers for the three transition metal species on SrO and TiO2 terminated surfaces. This provides valuable quantitative input for future experimental and simulation studies and a sound basis to guide research aiming at controlling the microstructure of transition-metal-doped SrTiO3 thin films.

Analysis of a preQ1-I riboswitch in effector-free and bound states reveals a metabolite-programmed nucleobase-stacking spine that controls gene regulation.

Riboswitches are structured RNA motifs that recognize metabolites to alter the conformations of downstream sequences, leading to gene regulation. To investigate this molecular framework, we determined crystal structures of a preQ1-I riboswitch in effector-free and bound states at 2.00 Å and 2.65 Å-resolution. Both pseudoknots exhibited the elusive L2 loop, which displayed distinct conformations. Conversely, the Shine-Dalgarno sequence (SDS) in the S2 helix of each structure remained unbroken. The expectation that the effector-free state should expose the SDS prompted us to conduct solution experiments to delineate environmental changes to specific nucleobases in response to preQ1. We then used nudged elastic band computational methods to derive conformational-change pathways linking the crystallographically-determined effector-free and bound-state structures. Pathways featured: (i) unstacking and unpairing of L2 and S2 nucleobases without preQ1—exposing the SDS for translation and (ii) stacking and pairing L2 and S2 nucleobases with preQ1—sequestering the SDS. Our results reveal how preQ1 binding reorganizes L2 into a nucleobase-stacking spine that sequesters the SDS, linking effector recognition to biological function. The generality of stacking spines as conduits for effector-dependent, interdomain communication is discussed in light of their existence in adenine riboswitches, as well as the turnip yellow mosaic virus ribosome sensor.

Modeling competitive precipitations among iron carbides during low-temperature tempering of martensitic carbon steel

Upon tempering of martensitic carbon steels, transition carbides (TCs), as precipitated precursors that compete for available C atoms among multiple iron carbides, have tremendous influences on the microstructural evolution and the final properties of steels. Although two similar structures of TCs, i.e. the hexagonal ε-Fe2C and the orthorhombic η-Fe2C, were successively identified, they still cannot be clearly distinguished experimentally, so far. It is timely to grasp the corresponding atomic-scale nature to predict the real structure of TCs. Based on the maximal entropy production principle and the atomic-scale computations, a Fokker-Planck type equation (FPE) following a modified multi-scale modeling framework, is solved for competitive precipitations among the potential iron carbides (including ε-Fe2C, η-Fe2C and θ-Fe3C) during the low-temperature tempering of carbon steel. The obtained microstructural evolution path dominated by synergy of thermodynamics and kinetics indicates a precipitation sequence for TCs, i.e. ε → η, which is further verified by the minimum energy path in terms of the solid-state nudged elastic band (SSNEB) method. On this basis, the potential hexagonal-orthorhombic transformation (HOT) of TCs is discussed, by which the probable structure of TCs can be accurately identified in experiment. The current modeling framework is expected to be a worthy paradigm for reference to predict the structural evolution of TCs in engineering materials.

Intercalation of Au Atoms into SiC(0001)/Buffer Interfaces-A First-Principles Density Functional Theory Study.

The process of Au intercalation into a SiC/buffer interface has been theoretically investigated here by using density functional theory (DFT) and the nudged elastic band (NEB) method. Energy barriers were at first calculated (using NEB) for the transfer of an Au atom through a free-standing graphene sheet. The graphene sheet was either of a nondefect character or with a defect in the form of an enlarged hexagonal carbon ring. Defects in the form of single and double vacancies were also considered. Besides giving a qualitative prediction of the relative energy barriers for the corresponding SiC/buffer interfaces, some of the graphene calculations also proved evidence of energy minima close to the graphene sheet. The most stable Au positions within the SiC/buffer interface were, therefore, calculated by performing geometry optimization with Au in the vicinity of the buffer layer. Based on these NEB and DFT calculations, two factors were observed to have a great influence on the Au intercalation process: (i) energy barrier and (ii) preferential bonding of Au to the radical C atoms at the edges of the vacancies. The energy barriers were considerably smaller in the presence of vacancies. However, the Au atoms preferred to bind to the edge atoms of these vacancies when approaching the buffer layer. It can thereby be concluded that the Au intercalation will only occur for a nondefect buffer layer when using high temperature and/or by using high-energy impacts by Au atoms. For this type of Au intercalation, the buffer layer will become completely detached from the SiC surface, forming a single layer of graphene with an intact Dirac point.

Adsorption and diffusion of lithium and sodium on the silicon nanowire with substrate for energy storage application: A first principles study

The adsorption and diffusion properties of Li/Na on the Si nanowire (NW) with substrate are analyzed by the density functional theory (DFT) calculations. The Li/Na prefers to adsorb on the corner of the Si NW with substrate and the adsorption energies on different sites vary from −1.32 eV to −0.74 eV. The diffusion pathways and corresponding energy barriers of lithium/sodium on the Si substrate and sidewall of the Si NW are calculated by the nudged elastic band (NEB) method. Moreover, when Li/Na moves around the Si NW, the Si NW would bend with the maximum displacement of ~0.76 Å. Finally, the energy barriers are 0.61–0.81 eV when Li/Na diffuses across the Si NW, indicating the Si NW may be applied as anodes in both lithium and sodium ion batteries. This work would help to investigate the Li/Na adsorption and diffusion properties on various nanowires with substrates and provide guidelines for the design of Si anodes in sodium ion batteries.

Initial Steps of Oxidative Etching of MoS2 Basal Plane Induced by O2

Fil: Farigliano, Lucas Martin. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Cordoba. Instituto de Investigaciones en Fisico-quimica de Cordoba. Universidad Nacional de Cordoba. Facultad de Ciencias Quimicas. Instituto de Investigaciones en Fisico-quimica de Cordoba; Argentina

Vacancy-mediated diffusion of atoms at Ge/Si interfaces: An atomistic perspective

The diffusive transport of atoms at the interface between germanium and silicon is a phenomenon of key importance in understanding the electronic, thermal, and structural behavior of many hetero-epitaxial semiconductor devices and superlattices. Here we explore this phenomenon from the perspective of vacancy-diffusion at the Ge/Si interfaces. By employing the atomistic nudged elastic band method, we obtain the diffusion enthalpies, transition pathways, and activation barriers for the vacancy-mediated diffusion of interfacial atoms. In particular, the {100}, {111}-glide, and {111}-shuffle pseudomorphic interfaces have been examined, and the tendency of a vacancy to cross the interface has been evaluated. Such calculations facilitate the identification of paths and interfaces with minimum and maximum resistance against the vacancy-mediated migration of interfacial atoms. Our computations reveal that the {111}-glide interface exhibits the least tendency of interface-crossing diffusion of vacancies, whereas the {111}-shuffle interface promotes such diffusion. In addition, the atomistic calculations also reveal a unique stable state of Ge monovacancy at the Ge/Si {111}-shuffle interface.

Theoretical study of Li intercalation in TiO2(B) surfaces

TiO2(B) has potential as an anode material for Li-ion batteries. Although theoretical and experimental studies related to the intercalation of Lithium (Li) in TiO2(B) already exist, to the best of our knowledge there are no studies in the open literature related to Li intercalation in low-index surfaces of this oxide phase. Therefore, we study the intercalation of Li in the (100) and (001) surfaces of TiO2(B) using Density Functional Theory (DFT). The DFT + U Hubbard methodology was included to consider the strong correlation of the “d” electron states of the transition metal. Electronic densities of states of the surfaces were analyzed in the diluted limit concentration of Li. We found a small-induced magnetic moment present. The diffusion of Li atoms in the surfaces was studied using the Nudged Elastic Band (NEB) method. Our results indicated that the Li intercalation in the (100) and the (001) surfaces is a favorable process. Moreover, we found diffusion pathways energetically stables for both surfaces. Calculations of charge density difference with Li intercalated in the energetically stable positions on the surfaces indicate that the charge density on Li is a local phenomenon and a charge transfer occurs from Li to the nearest Ti and O atoms.

Enhancement of Ion Diffusion By Targeted Phonon Excitation

We introduce two formalisms that determine the individual contributions of normal modes of vibration (phonons) to the diffusion of atoms/ions through a solid, based on (i) Nudge Elastic Band (NEB) calculations (ii) molecular dynamics (MD) simulations. The combination of the MD and NEB based modal decomposition methods presented herein provide a detailed picture for the atomic/ionic diffusion. MD samples the dominant mechanisms that are present in a natural simulation of the material, while NEB can be used to study specific mechanisms, regardless of the likelihood that they would occur at a given temperature. The results for an example material of Ge-doped Li3PO4 revealed that >87% of the Li-ion diffusion in the lattice originates from a subset of less than 10% of the vibrational modes in the system, which have frequencies between 8-20 THz. By deliberately exciting a very small targeted subset of these contributing modes we observe an increase in diffusion by several orders of magnitude, consistent with what would be observed if the entire material (i.e., all modes) were excited to the same level. This suggests that an entire material need not be heated to elevated temperatures to increase diffusivity, but instead only the modes that contribute need to be excited to elevated temperatures. This new understanding identifies new avenues for increasing diffusivity by engineering the vibrations in a material, and/or increasing diffusivity by external stimuli/excitation of phonons (e.g., via photons or other interactions).

The Influence of Solid Electrolyte Interface (SEI) on Li Dendrites Formation in Li-Metal Battery

The composition, structure and the formation mechanism of the solid-electrolyte interphase (SEI) in lithium-based (e.g., Li-ion and Li-metal) batteries have been widely explored in the literature. However, very little is known about the ions transport through the SEI, specifically through their grain boundaries (GBs) of the inorganic inner layers, and the corresponding mechanism for Li dendrites nucleation. Understanding the underlying ions diffusion processes across the inorganic components of SEI could lead to significant progress, enabling the performance increase and improving the mitigating strategies of dendritic growth and other safety aspects of these batteries. For this reason, in this work, we present the results of a combined ab initio Density Functional Theory (DFT) calculations to study Li diffusion across the SEI and Li nucleation at the Li metal/SEI junction and an extended phase-field model (PFM) for Li migration and electrodeposition that is a step forward toward better understanding of Li dendrites formation and growth.The present DFT calculations are performed using the VASP (Vienna Ab initio Simulation Package)1 code with the plane wave basis sets and the projector augmented wave (PAW)2 pseudo-potentials in the framework of Perdew-Burke-Ernzerhof sol (PBEsol)3 generalized gradient approximation (GGA)4. The migration barriers and diffusion coefficients are evaluated using the Nudged Elastic Band (NEB) method, as implemented in VASP.The major GB structures of SEI that are of interest are the ones between LiF/LiF, Li2O/Li2O and the mixed GBs between LiF/Li2O slabs. The energy barriers for the diffusion of Li vary significantly depending upon the structure of these channels, with the general trend being that Li diffusion in the GB is generally faster than in the neighboring crystalline regions within the grain interiors. The diffusion through LiF/Li2O slabs has the lowest barriers whereas GBs of LiF/LiF slabs has the highest. To understand the role of these GBs in dendrite nucleation, the SEI/electrode interface is analyzed for its stability. The energetics from the DFT analyses depend heavily on the grain structures, with (LiF/LiF)/Li grain structures being most stable and (LiF/Li2O)/Li being the least stable. The interfacial energies in the Li/SEI interfaces show that the interface between (LiF/Li2O)/Li has the least critical length (~1.6 Å) for stable dendritic growth and most favorable for the initiation of crack in the SEI.To quantify the influence of the SEI on the evolution of Li electrodeposits at the continuum scale, we performed the PFM calculations. The PFM is developed employing MOOSE framework5, and the SEI model includes the typical GB structures within 20 nm thickness of the SEI grains. The data from the DFT as described above is used for Li diffusion through the SEI and for mechanical properties of GB and grains. The results from PFM reveal that the anisotropic nature of Li presence at the metal/SEI interface leads to uneven electrodeposition at the Li metal surface. Also, the results allow for monitoring the stress field evolution and its influence on Li filament structure. The present results also show that significant stress is observed at the root of the Li electrodeposits and the triple junction between the GBs.

Understanding Zn Electrodeposits Morphology in Secondary Batteries Using Multiscale Computational Approach

Zinc (Zn) aqueous rechargeable batteries (ZBs) have shown a tremendous success in various applications due to their environmental friendliness, multi-electron capacity, high abundance, safety and low cost; however, analogous to their Li-ion battery counterparts, they suffer from the dendrite formation leading to decrease in capacity and eventual failure. Despite many studies reporting a good performance and partial dendrites suppression1, there have been no systematic studies of Zn electrodeposits morphology analysis in ZBs.In this contribution, we present the results of a combined density functional theory (DFT) calculations of Zn diffusion through the ZnO surface layer (i.e., solid electrolyte interface – SEI) and the phase-field modeling (PFM) of Zn electrodeposition study. The DFT calculations are performed using commercial code VASP (Vienna Ab initio Simulation Package)2 with the projector augmented wave (PAW)3 Perdew-Burke-Ernzerhof (PBE)4 pseudo-potentials within generalized gradient approximation (GGA)5. The Zn path and migration barriers are evaluated using the Nudged Elastic Band (NEB) method. The PFM6,7 is developed in the open source code MOOSE and is formulated in continuum theory based upon diffusion-conservation laws using partial-differential equations and then discretized using finite element method8 and solved computationally.The DFT calculations are performed to reveal the dominant diffusion pathways, energetics and the corresponding diffusion coefficients associated with Zn diffusion through the SEI. The SEI surface layer on Zn electrode in ZBs primarily consists of ZnO. Correspondingly, two cases are considered, i.e. the Zn diffusion through the ZnO grain boundary (GB) formed between the different ZnO surface orientations and the Zn diffusion through the ZnO bulk grains. It is found that Zn diffusion occurs through the different nano-channels inside grains as well as through the GB. The path and the activation energies of Zn diffusion vary significantly depending upon the structure of grains and GB, as well as the presence of additional Zn atoms corresponding to the current density dependency. However, the general trend is that Zn diffusion in a grain is faster comparing to the ZnO/ZnO GB. Using the DFT calculations the appropriate elastic properties of SEI components are calculated as required by the PFM.The 2D PFM model predicts the Zn morphologies, the Zn ions concentration, the electrostatic potential, the stress and the equivalent plastic strain. Three regimes are investigated, specifically, low, intermediate and high current densities, where different Zn morphologies, such as boulders, mossy and dendritic shape are observed. It is found that the stress has a major influence on the electrodeposition, and vice versa the Zn electrodeposits growth is found to affect the stress distribution significantly. The plastic yielding occurs preferentially at the node of the boulders and through the filaments and mossy structures. In addition, based upon the present PFM results alternative strategies for the inclusion of the stress field in the mean-field electrochemical modeling will be discussed.Furthermore, both the DFT and the PFM results are supported by scanning electron (SEM) and scanning transmission electron microscopy (STEM) to reveal micro and nano-scopic details of Zn electrodeposits as well as the surface ZnO layer.The combined results also provide a basis for development and design of novel strategies against unwanted Zn dendrites formation.

Investigation of Electron-Beam Induced Damage in Lithium-Ion Batteries

Lithium-ion batteries are extensively used to transform chemical energy into the electrical energy and vice versa. These batteries are inevitably the best candidates for portable devices such as mobile phones, laptops and specially in the fast-growing market of electrical automobiles. The performance of these batteries is strongly connected to the microstructure of their electrodes. Thus, improvement of lithium-ion batteries requires an accurate structural investigation of their electrodes which is achievable by using electron microscopes.However, the electron microscopes may damage the specimen during the investigation process. Once the beam electrons penetrate into the material, the columbic force of nucleus and electrons can scatter the beam electrons. When beam electrons undergo high angle scattering events, the law of conservation of momentum required momentum transfer to the atoms in the sample. In cases where the transferred energy is large enough to overcome a barrier called displacement energy (Ed), the atom can be displaced from its original lattice site. This displacement of atoms is called knock-on displacement, a significant source of electron beam damage. As a light element, Lithium has very small displacement barrier which makes the lithium base electrodes vulnerable to the high energy beam of electron microscopes.In this study, we use a Density Functional Theory (DFT) based method called Nudged Elastic Band (NEB) to calculate the displacement energies. These values were then applied to the conservation of momentum to find the threshold displacement energy which is the beam energy that creates the knock-on damage inside the material during the electron microscopic investigation. Finding the safe electron beam energy range opens a new door for more accurate structural characterization of lithium compounds as a step for further improvement of lithium ion batteries.

On the slip and twinning mechanisms on first order pyramidal plane of magnesium: Molecular dynamics simulations and first principal studies

Molecular dynamics simulations and first-principles calculations are carried out on first order pyramidal plane (π1) of magnesium to study both compression twinning (CTW) and dislocation slip. To this end, a generalized stacking fault energy analysis is employed on dense (π1D) and loose spaced (π1L) π1 planes. The crystal shearing resistance is extracted by using a minimum-energy path (MEP) finder called the nudged elastic band (NEB) method. The MEP regarding 1¯012101¯1π1L slip system shows that the unfaulted crystal structure is recovered in the middle of the path with non-straight and pronounced curved slip trajectories. Besides, it will be revealed that metastable configurations on the half of the MEP for 1¯012101¯1π1L slip system are indeed related to a dissociated 〈c + a〉 extended dislocation of loose pyramidal-I 〈c + a〉 slip, 1/31¯1¯23101¯1π1L. Also, after extracting the dissociation mechanism related to this dislocation, it will be shown that loose pyramidal-I 〈c + a〉 slip can involve shuffling. Moreover, the MEP for dense pyramidal-I 〈a〉 slip, 1/31¯21¯0101¯1π1D, shows transmutation of this slip into CTW in the middle of the path. This transmutation process will be further examined on CTW growth, and it will be demonstrated that this CTW mechanism is energetically more favorable compared to other twinning mechanisms.

Density functional theory study of the SiF molecule adsorption and decomposition on p(2×2) reconstructed Si(001) surface

The adsorption of the F and Si atoms as well as the SiF molecule on the Si(001)-p(2×2) surface is studied using first-principle calculations. Our results show that the fluorine atom saturates the dangling bond of the surface while the silicon adatom forms two bonds with the surface atoms. For the adsorption of the SiF molecule, we obtained one dissociative state which is the most stable one and seven non-dissociative states. The minimum energy paths for F diffusion and SiF decomposition on the silicon surface are explored using the nudged elastic band (NEB) method. Our study reveals that it is energetically favorable for the fluorine atom to diffuse from the SiF molecule to a surface site than the diffusion from surface site to another one.

Autonomous Discovery of Materials for Intercalation Electrodes

AbstractThe development of automated computational tools is required to accelerate the discovery of novel battery materials. In this work, we design and implement a workflow, in the framework of Density Functional Theory, which autonomously identifies materials to be used as intercalation electrodes in batteries, based on descriptors like adsorption energies and diffusion barriers. A substantial acceleration for the calculations of the kinetic properties is obtained due to a recent implementation of the Nudged Elastic Bands (NEB) method, which takes into consideration the symmetries of the system to reduce the number of images to calculate. We have applied this workflow to discover new cathode materials for Mg batteries, where two of these materials display a threefold increase in the potential of the Chevrel phase, the state‐of‐the‐art cathode in commercial prototype Mg batteries.

Phase stability and temperature effect in ScX (X=S, Se and Te) compounds

Based on first-principles calculations, the structural stability and temperature effect in ScX (X=S, Se and Te) compounds are studied with three typical structures of B1 (NaCl-type), hcp (NiAs-type) and β-hcp (inverse Li2O2-type). Their dynamic stability has been verified using phonon mode analysis and molecular dynamics simulations. From the total energy calculations, we find that the most stable ground state structures are B1 for ScS, and hcp for ScSe and ScTe, respectively. Moreover, structural stabilities at finite temperature are studied with the combination of phonon dependent dynamics analysis and first-principles calculations, which reveals a phase transition from hcp to B1 in ScSe around 230 K and a phase transition from hcp to β-hcp in ScTe around 460 K, in accordance with experimental findings. The energy barrier and pathway along the phase transformation from hcp to β-hcp ScTe are also calculated and analyzed by the solid-state nudged elastic band method.