Aid Of First-principles Calculations Research Articles

Thermodynamic assessment of the Cr–Si binary system

A thermodynamic evaluation of the Cr–Si system was performed concerning the latest experimental phase diagram and with the aid of first-principles calculations. The thermodynamic parameters for the Gibbs energy of pure Cr were modified based on the new experimental value of 1861 °C for the melting point of pure Cr, which was previously reported as 1907 °C in the SGTE database. The Gibbs energy descriptions of the Cr3Si and Cr5Si3 phases were revised using Cr3(Cr,Si)- and (Cr,Si)5Si3-type two-sublattice models to reproduce the latest experimental results of their solubility composition ranges, that is, Cr3Si extending toward the Cr-rich side and Cr5Si3 extending toward the Si-rich side from the stoichiometry. The CrSi and CrSi2 phases were considered line compounds with no solubility range, and the solubility of Cr in the Si phase was ignored. A set of self-consistent thermodynamic parameters for the Cr–Si system was obtained using the CALPHAD technique. The phase diagrams calculated using the optimized parameters showed reasonable agreement with the latest phase equilibrium data and thermodynamic property data in the literature, including the enthalpy of mixing, formation enthalpy, and chemical potential diagrams of Cr and Si in the equilibrium phases.

Topological semimetal phases in a family of monolayer X3YZ6 (X=Nb,Ta, Y=Si,Ge,Sn, Z=S,Se,Te) with abundant nodal lines and nodes

The electronic and topological properties of single-layer X3YZ6 (X=Nb,Ta, Y=Si,Ge,Sn, Z=S,Se,Te) materials have been studied with the aid of first principles calculations. This kind of materials belong to topological semimetals (TMs) with abundant nodal lines and nodes. Considering their similar properties, we focus on the analysis of Ta3SnTe6 and Ta3SiSe6. The presence of spin–orbit coupling (SOC) leads to the transition from type-I nodal lines to Dirac points as well as the disappear of type-II Dirac points. The three-dimensional (3D) plot of the band structure reproduces vividly the characteristics of nodes and nodal lines. The appearance of the flat bands in (110) edge states further confirm their nontrivial topological properties. We also explore the relationship among different nodal lines (nodes), crystal symmetry and SOC. The type-I nodal lines are protected by Mz and Mỹ symmetry in the absence of SOC. Symmetry breaking leads to band splitting even in the presence of SOC. The single-layer X3YZ6 can be used as candidates for two-dimensional (2D) Dirac materials and provide a platform for further study of interesting physical phenomena.

Lattice dynamics and in-plane antiferromagnetism in MnxZn1−xPS3 across the entire composition range

Antiferromagnetic MnPS${}_{3}$ and diamagnetic ZnPS${}_{3}$ belong to a family of two-dimensional compounds that exhibit a rich variety of magnetic ordering. Low-temperature Raman spectroscopy and SQUID magnetometer measurements are performed on both compounds and their alloy in order to evaluate a possible Jahn-Teller distortion as well as to reveal magnetic and alloy signatures. With the aid of first-principles calculations, a description of the lattice dynamics of the system is presented, together with the magnetic phase diagram across the entire compositional range.

Raman spectroscopy of a few layers of bismuth telluride nanoplatelets.

We can shape the electronic and phonon properties of Bi2Te3 crystals via the variation of the number of layers. Here, we report a Raman study with the aid of first-principles calculations on few-layered Bi2Te3 systems ranging from 5 to 24 nm layer thickness using 1.92, 2.41 and 2.54 eV excitation energies. We examine how the frequency position, intensity and lineshape of the main Raman modes (A11g, E2g, and A21g) behave by the variation of the layer thickness and excitation energy. We observed a frequency dispersion on the number of layers of the main modes, indicating changes in the inter- and intra-layers interaction. A resonant Raman condition is reached for all modes for samples with 11 and 18 nm thickness because of van Hove singularities at the electronic density of states. Also, the Breit-Wigner-Fano line shape of the A21g mode shows an increase of electron-phonon coupling for thick layers. These results suggest a relevant influence of numbers of layers on the Raman scattering mechanics in Bi2Te3 systems.

Unravelling the regulating role of graphene coating on improving the electrochemical performance of pyrophosphate cathode material: A first-principles study

Pyrophosphate (Li2FeP2O7, LFPO) is proposed as a promising cathode material for Li-ion batteries on account of the high voltage and the two-dimensional lithium ions diffusion channel. Graphene coating could effectively improve the inherent poor ionic and electronic conductivities of Li2FeP2O7 material, however, the interfacial interaction and regulating role of the graphene coating on the graphene/Li2FeP2O7 (G/LFPO) composite structures are lack of research at the atomic scale. With the aid of first-principles calculations and ab initio molecular dynamics (AIMD) simulations, the interaction mechanism of graphene on the LFPO cathode material has been systematically investigated in terms of the geometrical structure, thermodynamical stability, interfacial charge distribution and electronic property. The analysis of electron localization function (ELF), charge density differences, and density of states strongly suggest the G/LFPO(001) and G/LFPO(010) heterostructures exhibit high electrochemical performance. The AIMD simulations show that the parallel and perpendicular G/LFPO(001) and G/LFPO(010) heterostructures have high thermodynamical stabilities and electronic conductivities. The diffusion coefficients of Li+ ions in the G/LFPO heterostructures are noticeably enhanced. Our present work would not only provide a fundamental understanding of interfacial interaction and electrochemical properties of G/LFPO composite, but also shed light on the regulating mechanism for the potential phosphate-based cathode material with high electrochemical performance.

Evidence for topological features in the electronic and phononic bands of ZGeSb (Z = Hf, Zr, Ti) class of compounds

Nontrivial topological properties in materials have been found in either the electronic or the phononic bands, but they have seldom been shown in both for a compound. With the aid of first-principle calculations, our paper attempts to find topological features in the electron and phonon band structures of ZGeSb (Z = Hf, Zr, Ti) class of compounds. The electron band structure exhibits two nodal rings in each of these compounds. Furthermore, drumhead surface states (DSS) have also been shown. The phonon band structure depicts one nodal ring in each of these compounds. DSS is also seen in the phonon surface states. Layering possibility has also been explored in HfGeSb, which admits a nodal ring each in its electronic and phononic band structure. Finally, these compounds (bulk and mono-layer) possess Dirac points robust to spin–orbit coupling effects, with at least one such Dirac point with its linear dispersion extending to the Fermi energy. Therefore, these compounds fall under the topological nodal line metals class, which is rarely seen in materials. These compounds’ theoretical nontrivial topological nature in their electronic and phononic band structure provides a profound grasp of electronic and phononic nodal-line physics and is a good candidate for experimental verification. The existence of Dirac points close to the Fermi level could also motivate one to look for extreme magnetoresistance in these compounds. Moreover, given their largely metallic nature, these compounds become an excellent arena for novel device applications.

Study on Stability and Elastic Properties of β-TiX (X=Nb, Ta) Alloys From First-Principles Calculations

In this article, the phase stability, elastic properties, and electronic structure of the β-TiX (X = Nb, Ta) alloy body-centered cubic (bcc) structure were systematically studied with the aid of first-principles calculations. The results show that the phase stability and elastic properties of the β-TiX alloys are closely related to the contents of alloying element X. For β-TiX alloys, the contents of Nb and Ta that satisfy their mechanical stability are 10% and 13%, respectively; at room temperature, both β-TiNb and β-TiTa alloys can reach a thermodynamically stable state when the content of Nb or Ta is 25%. In terms of elastic properties, the content of alloying element X is positively correlated with the elastic constant, Young’s modulus, and shear modulus of the β-TiX alloys. The elastic modulus reaches its minimum when the X content is 25%, and the smallest direction of Young’s modulus appears in the <111> direction. The calculation results of the electronic structure show that the bonding strength between the Ti atom and X atom increases with the content of alloying element X, which leads to improvement of phase stability and elastic modulus.

Computational screening of single transition metal atom embedded in nitrogen doped graphene for CH4 detection

Sensitive and selective detection of methane (CH4) is crucial for human activities, but is severely challenging due its volatility and nonpolar nature. Herein, with the aid of first-principles calculations, we explore the potential of single transition metal atoms (Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt and Au) embedded in graphene with four pyridine nitrogen atoms doped (TMN4 @G) as efficient sensor materials towards CH4. It is found that AgN4 @G enables outstanding sensitivity, selectivity and recovery ability for CH4 detection in ambient condition. Further calculations in the presence of O2 or H2O revealed that the sensitivity of the system is remarkably improved, indicating its applicability and robustness for CH4 sensing in ambient or humid conditions.

Conversion of CO2 into Formic Acid on Transition Metal-Porphyrin-like Graphene: First Principles Calculations.

Recently, transition metal (TM)-porphyrin-like graphene has been predicted to be a promising material for CO2 capturing under favorable conditions. Such materials can capture CO2 at 300 K and release it at 450 K. However, the captured CO2 gas is mostly stored in oceans. With the aid of first principles calculations, we herein propose a method in which the captured CO2 is converted into an environmentally friendly product, formic acid. Addition of H2 to CO2 molecules adsorbed on Sc- and Ti-porphyrin-like graphene was found to catalyze this conversion. We also performed nudged elastic band calculations and thermodynamic analysis using the first-order Polanyi–Wigner equation and equilibrium statistical mechanics to investigate the chemical reactions involved in this conversion. In addition, we performed Bader charge analysis to obtain insights into the mechanism of charge transfer and adsorption throughout the conversion. Our study presents a novel method in which the captured CO2 is treated by converting it into an environmentally friendly product. Since this method does not require CO2 storage, it is expected to be an effective strategy to manage the rising CO2 level in the environment.

Experimental and theoretical investigation of Na4MnAl(PO4)3 cathode material for sodium-ion batteries

Finding appropriate structures of cathode materials is crucial to the development of sodium-ion batteries (SIBs). A novel NASICON-related structure (R-32 space group) of Na4MnAl(PO4)3 is synthesized, which exhibits a high specific capacity (116.8 mAh g−1) with two voltage plateaus of ~ 3.56 V and ~ 4.10 V for Mn3+/Mn2+ and Mn4+/Mn3+ redox couples, respectively. The sodium ion storage mechanism and diffusion mechanism are investigated by ex-situ XRD and ex-situ XPS, with the aid of first principle calculations. Both two-phase and solid solution reactions take place in the charge/discharge process, and the diffusion path along Na(1) ↔ Na(2) ↔ Na(3) is revealed for the first time in NASICON-structured materials. In addition, a voltage plateau of ~ 4.48 V of the Mn5+/Mn4+ redox couple is suggested by the DFT calculations in Na4MnAl(PO4)3, which may be more easily accessed than that of other compounds such as Na3MnTi(PO4)3 and Na3MnZr(PO4)3 reported in literature, leaving great potential in elevating the voltage and specific capacity for Na4MnAl(PO4)3 cathode. Because of high theoretical specific capacity, high voltage plateaus and low cost, Na4MnAl(PO4)3 is a strong contender in cathode material for SIBs.

Topological effects of phonons in GaN and AlGaN: A potential perspective for tuning phonon transport

Tuning thermal transport in semiconductor nanostructures is of great significance for thermal management in information and power electronics. With excellent transport properties, such as ballistic transport, immunity to point defects and disorders, and forbidden backscattering, topological phonon surface states show remarkable potential in addressing this issue. Herein, topological phonon analyses are performed on hexagonal wurtzite gallium nitride (GaN) to check the topological characteristics of phonons. And other nitrides of the same family, i.e., AlN and AlGaN crystals, are also calculated from a topological phonon phase transition perspective. With the aid of first-principle calculations and topological phonon theory, Weyl phonon states, which host surfaces states without backscattering, are investigated for all these materials. The results show that there is no nontrivial topological phonon state in GaN. However, by introducing Al atoms, i.e., in wurtzite type AlN and AlGaN, more than one Weyl phonon point is found, confirmed by obvious topological characteristics, including non-zero integer topological charges, source/sink in Berry curvature distributions, surface local density of states, and surface arcs. As AlN and AlGaN are typical materials in AlGaN/GaN heterostructure based electronics, the existence of topological phonon states in them will benefit thermal management by facilitating the design of one-way interfacial phonon transport without backscattering.

Electronic structure of graphene/TiO2 interface: Design and functional perspectives

We propose the design of low strained and energetically favourable mono and bilayer graphene overlayer on anatase TiO2 (001) surface and examined the electronic structure of the interface with the aid of first principle calculations. In the absence of hybridization between surface TiO2 and graphene states, dipolar fluctuations govern the minor charge transfer across the interface. As a result, both the substrate and the overlayer retain their pristine electronic structure. The interface with the monolayer graphene retains its gapless linear band dispersion irrespective of the induced epitaxial strain. The potential gradient opens up a few meV bandgap in the case of Bernal stacking and strengthens the interpenetration of the Dirac cones in the case of hexagonal stacking of the bilayer graphene. The difference between the macroscopic average potential of the TiO2 and graphene layer(s) in the heterostructure lies in the range 3–3.13 eV, which is very close to the TiO2 bandgap (~3.2 eV). Therefore, the proposed heterostructure will exhibit enhanced photo-induced charge transfer and the graphene component will serve as a visible light sensitizer.

Structurally Regulating Cation Migration Suppresses Voltage Fade and Redox Asymmetry in Lithium-Rich Layered Oxides

Although lithium-rich layered oxides hold the greatest promise to transcend energy-density limitations in rechargeable lithium-ion batteries because of their high reversible capacity (exceeding 250 mAh g−1) and high-voltage anionic redox chemistry, the inevitable voltage decay, or gradual decrease in the average discharge voltage during cycling remains one some of the most pernicious problems jeopardizing their real-world application. Of paramount importance in understanding the fundamentals of the voltage decay is that its essential determinant is not the transition metal (TM) migration itself but the resulting confinement of TM ions in the Li layer. Therefore, a substantive key lies in improving the intra-cycle reversibility of TM migration. In this work, reversible intra-cycle TM migration is first demonstrated by modifying the oxygen lattice of lithium-rich layered oxides. O2-type stackings inherently allow reversible intra-cycle TM migration, thus delivering outstanding voltage retention over extended cycling and far outperforming their O3-phase counterparts and other lithium-rich layered 3d metal oxides. Suppressed voltage decay arose from the retention of the pristine layered structure with highly reversible TM migration over extended cycling was revealed by various structural characterizations with the aid of first-principles calculations. Our findings indicate that tailoring the migration path of TM ions provides a viable strategy to address the critical issues of voltage decay, which may help rejuvenate the research field of lithium-rich layered oxides.

Linear scaling relations for N2H4 decomposition over transition metal catalysts

Catalytic decomposition of hydrazine monohydrate (N2H4·H2O) represents a promising hydrogen storage/production technology. Nowadays, the development of N2H4 decomposition catalysts is sluggish due primarily to a lack of universal screening rules. Herein, with the aid of first-principles calculations, we identify a reliable descriptor, N2H4 adsorption energy, to properly address N2H4 decomposition kinetics on TM catalysts. Particularly, we extract linear scaling relations between the adsorption energetics of key intermediates, H (and N2H3) vs. NH2 that exert a fundamental limitation on the H2 selectivity of N2H4 decomposition over elementary transition metal catalysts. We propose that catalysts with optimum H2 selectivity may be achieved by independently stabilizing and destabilizing adsorption of H (or N2H3, or both) and NH2, respectively. The disclosed dissociation behaviors of N2H4 and their physical origins are expected to advance the rational design of advanced catalysts for selectively promoting H2 production from N2H4.

Surface Adsorption and Vacancy in Tuning the Properties of Tellurene.

The emerging two-dimensional tellurene has been demonstrated to be a promising candidate for photoelectronic devices. However, there is a lack of comprehensive insight into the effects of vacancies and common adsorbates (i.e., O2 and H2O) in ambient conditions, which play a crucial role in semiconducting devices. In this work, with the aid of first-principles calculations, we demonstrate that H2O and O2 molecules behave qualitatively differently on tellurene, while water adsorption can be remarkably promoted by adjacent preadsorbed O2. Upon the formation of Te vacancies, the adsorption of both O2 and H2O molecules is enhanced. More importantly, the existence of H2O and Te vacancies can dramatically facilitate the dissociation of O2, suggesting that tellurene may be readily oxidized in humid conditions. In addition, it is found that the electronic properties of tellurene are well preserved upon either H2O or O2 adsorption on the surface. In sharp contrast, vacancies enable significant modification on the band structure. Specifically, an indirect-to-direct band gap transition is found at a vacancy concentration of 5.3%.

Thermodynamic modelling of Y–H and Y–Zr–H system aided by first-principles and its application in bulk hydride moderator fabrication

A self-consistent thermodynamic description of Y–H binary system has been derived using the CALPHAD method with the aid of first-principles calculations and applied in the fabrication of crack-free bulk Zr–Y hydride neutron moderator. The enthalpies of formation of the end-members calculated by first-principles were adopted for the thermodynamic assessment. Only four model parameters were needed and optimized with the available experimental data from literature. The calculated phase equilibria and thermodynamic data agree well with experimental data. Y–Zr–H ternary system was constructed by extrapolation of three binary sub-systems. According to the thermodynamic calculation results, the hydrogenation of Zr–Y alloys was divided into three stages: isothermal hydriding, isobaric hydriding and terminal hydriding. By carefully controlling the temperature and hydrogen pressure, the transformation from metal to hydride of yttrium and zirconium can be separated and carried out in different stages, so that the cracks formation during hydrogenation was successfully inhibited. With addition of yttrium, the stress induced by the martensitic transformation from δ to ε zirconium hydride would be relaxed. As a result, hydride embrittlement can be mitigated, and the bulk Zr–Y hydrides without micro-cracks can be fabricated.

High-pressure studies on l,l-dileucine crystals by Raman spectroscopy and synchrotron X-ray diffraction combined with DFT calculations.

The vibrational properties of the dipeptide l-leucyl-l-leucine hydrate were investigated through Raman and infrared spectroscopy. With the aid of first principle calculations using the density functional theory, the assignment of the vibrational modes from the material was furnished. In addition, the behavior of the crystal under high pressure was investigated using Raman spectroscopy (~8GPa) and synchrotron X-ray diffraction (~26GPa). The results show significant changes in both the X-ray diffractogram and the Raman spectra, suggesting that l-leucyl-l-leucine hydrate undergoes a phase transition between 2.3 and 2.9GPa. Finally, for pressures above 16GPa the broadening of X-ray peaks suggests a disorder in the crystal lattice induced by high-pressure effects.

First-principles determination of electronic charge transport properties in polymer dielectrics using a crystalline-based model system

There are various models for describing the charge transport in polymers but it is unclear how to choose the appropriate one for a given system. In this study, we determine the relevant charge transfer model for several insulating polymers by evaluating the Marcus parameters for electron and hole transfer with the aid of first-principles calculations. As expected, except for electron transfer in polyethylene and isotactic polypropylene, non-adiabatic (polaron) hopping takes place in most polymers, owing to the small inter-chain electronic interactions (10–100 meV) and large polaron formation energy (100–1000 meV). Therefore, the first-principles based parameter-free, multi-scale modeling approach, which we developed for evaluating the hole transfer property in polyethylene, can be used to study the electronic carrier transport properties in wide variety of polymers. The computed Marcus parameters imply that the electron and hole mobilities in syndiotactic polystyrene and polytetrafluoroethylene are larger than the hole mobility in polyethylene and isotactic polypropylene, which agrees with experimental data. The strong chain-length dependence of the computed Marcus parameters indicates that slight change in the polymer structure can result in significant variations in the charge mobility.

First-principles modeling of GaN(0001)/water interface: Effect of surface charging.

The accumulation properties of photogenerated carriers at the semiconductor surface determine the performance of photoelectrodes. However, to the best of our knowledge, there are no computational studies that methodically examine the effect of "surface charging" on photocatalytic activities. In this work, the effect of excess carriers at the semiconductor surface on the geometric and electronic structures of the semiconductor/electrolyte interface is studied systematically with the aid of first-principles calculations. We found that the number of water molecules that can be dissociated follows the "extended" electron counting rule; the dissociation limit is smaller than that predicted by the standard electron counting rule (0.375 ML) by the number of excess holes at the interface. When the geometric structure of the GaN/water interface obeys the extended electron counting rule, the Ga-originated surface states are removed from the bandgap due to the excess holes and adsorbates, and correspondingly, the Fermi level becomes free from pinning. Clearly, the excess charge has a great impact on the interface structure and most likely on the chemical reactions. This study serves as a basis for further studies on the semiconductor/electrolyte interface under working conditions.

Catalytic CO oxidation by Fe doped penta-graphene: A density functional study

Single atom catalysts (SAC) have been investigated extensively for CO oxidation, and penta-graphene (PG) as a new allotrope of carbon has been studied due to its excellent physical and chemical properties. In order to explore the mechanism of CO oxidation and develop the application of PG in the field of SAC, series of metals doped PG were examined to catalyze CO oxidation with the aid of first-principles calculations. Fe doped at 3-coordinated carbon site of PG is found favorable with large binding energy of -12.73 eV, more importantly, exhibits high affinity to the CO and O2. Furthermore, CO oxidation via Eley-Rideal (ER) mechanism is prior compare with Langmuir-Hinshelwool (LH) mechanism, the first CO molecule oxidation is the rate-determining step (RDS) with the energy barrier of 0.51 eV, which is comparable with other catalysts. In the case of Fe doped at 4-coordinated carbon site, ER and LH mechanisms are almost the same because of the similar energy barrier (0.60 eV and 0.59 eV).On the basis of reasonable first-principles calculations, the proposed Fe doped PG is believed to be a promising single atom catalyst for CO oxidation, and our results provide deep insights to design new heterogeneous catalysts based on PG.