Origin of Photocatalytic Activity of BiPO4： the First-principles Calculations

Geometric and electronic structures of three polymorphs of BiPO4（m MBIP, n MBIP and HBIP） have been investigated by the first-principles calculations. The results show that PO4 tetrahedron in n MBIP is distorted most, and m MBIP possesses minimum effective mass of carriers in three polymorphs of BiPO4. Further, the leading role of inductive effect of dipole moment or effective mass of carries in the separation of electron-hole pairs is analyzed. Based on the fact that n MBIP has higher photocatalytic activity than m MBIP, it can be inferred that the inductive effect of dipole moment deriving from distorted PO4 tetrahedron is the dominant factor affecting the separation efficiency of carries. The calculated results represent that n MBIP has more appropriate redox potential and narrower band gap than others. These findings may provide meaningful guidance for further understanding on the relationship between unique crystal structure and photocatalytic activity of BiPO4.

High-Pressure Phase Transitions of PbTe Using the First-Principles Calculations

High-pressure structural phase transitions in PbTe are investigated by means of the first principles total energy calculations within the generalized gradient approximation （GOAl and local density approximation CLDA） by using the density functional theory. First principle calculation shows that PbTe is stable with the NaCl-type （B1） structure under amSient conditions and transforms to the CsCl-type （B2） structure under high pressure via an intermediate phase. Two candidate structures of the intermediate phase, namely Prima and Cmcm, are chosen for total energy calculations and discussed. It indicates that the intermediate phase adopts the Pnma structure rather than the Cmcm structure, and lattice parameters of the Pnma phase calculated by using OGA and LDA are in consistent with experimental results.

Magnetic properties of several potential rocksalt half-metallic ferromagnets based on the first-principles calculations

Several rocksalt Sr4X3N （X = O, S, Se, and Te） are predicted to be potential half-metallic ferromagnets free of transition-metal and rare-earth elements by performing the first-principles calculations. Then their magnetic properties, such as the half metallicity and the crystal-cell magnetic moments are investigated. The Sr4X3N possibly have higher Curie temperatures and have more stable half metallicity than the Sr4X3C. Their crystal-cell magnetic moments are all 1.00 μB. The crystal-cell magnetic moments and the half metallicity arise mainly from the N ions. The main mechanism is the strong covalent interaction leading to the sp2 hybridized orbitals in the Sr4X3N. Then two Sr-5s and three N-2p electrons enter into three sp2 hybridized orbitals. Among these five electrons, four electrons are paired and one is unpaired, so there are three spin-up electrons and two spin-down electrons in these sp2 hybridized orbitals.

The First-Principle Calculation of La-doping Effect on Piezoelectricity in Tetragonal KNN Crystal

The La-dopping effect on the piezoelectricity in the K0.5Na0.5NbO3 （KNN） crystal with a tetragonal phase is investigated for the first time using the first-principle calculation based on density functional theory. The full potentiallinearized augumented plane wave plus local orbitals （APW-LO） method and the supercell method are used in the calculation for the KNN crystal with and without the La doping. The results show that the piezoelectricity originates from the strong hybridization between the Nb atom and the O atom, and the substitution of the K or Na atom by the La impurity atom introduces the anisotropic relaxation and enhances the piezoelectricity at first and then restrains the hybridization of the Nb-O atoms when the La doping content further increases.

Organic Cation Effect on the Physical Properties of CH3NH3PbI3 Perovskite from the First-principles Study

The effect of the distribution of organic cations CH3NH3^＋（MA^＋） on the stability,electronic structures and optical properties of CH3NH3 Pb I3 perovskite have been investigated using the plane-wave ultrasoft pseuudopotentials. Generalized gradient approximation and local density approximation are used to optimize the geometries of six models, which are different in the orientation of organic cations. The results show that model C is more stable than others, and the main contribution to the top of valence band is from I 5p states. In the bottom of conduction bands, the main components are Pb 6s states with an overlapping of I 5p states. When the orientation of organic group is transforming, the Pb I6 octahedra will distort and the band structure will alter with it, which affect the generation and migration of photon-generated carriers and optical properties.

Electronic Structure and Optical-absorption Properties of C-, N-, and S-doped BiOCl: the First-principles Calculations

Impurity formation energy, electronic structure, and photocatalytic properties of C-, N-, or S-doped BiOCl are investigated by density-functional theory plus U calculations(DFT + U). Results show that the doping effect of S is better than that of C or N on the tunable photocatalytic activities of BiOCl. At low concentration, S-doped BiOCl systems are the most stable under Bi-rich growth conditions because of their lower impurity-formation energy. Compared with the electronic structures of S-doped BiOCl, C-or N-doped BiOCl have relatively deeper impurity energy levels appearing in their band gap(except Bi_(36)O_(35)NCl_(36)), which may act as photogenerated carrier-recombination centers and reduce photocatalytic activity. At high concentration, S is substituted on the O lattice site system, whereas some S 3p states mix with the valence band; this mixture leads to an obvious band-gap decrease and continuum-state formation above the valence-band edge of BiOCl. Such activity is advantageous to photochemical catalysis response. Compared with pure Bi OCl and a low-concentration S-doped system, a high-concentration S-doped system shows an obvious redshift on the absorption edge and has better photocatalytic O_2 evolution performance.

Clarifying the atomic origin of electron killers in β-Ga_(2)O_(3) from the first-principles study of electron capture rates

The emerging wide bandgap semiconductorβ-Ga_(2)O_(3) has attracted great interest due to its promising applications for high-power electronic devices and solar-blind ultraviolet photodetectors.Deep-level defects inβ-Ga_(2)O_(3) have been intensively studied towards improving device performance.Deep-level signatures E_(1),E_(2),and E_(3) with energy positions of 0.55–0.63,0.74–0.81,and 1.01–1.10 eV below the conduction band minimum have frequently been observed and extensively investigated,but their atomic origins are still under debate.In this work,we attempt to clarify these deep-level signatures from the comparison of theoretically predicted electron capture cross-sections of suggested candidates,Ti and Fe substituting Ga on a tetrahedral site(Ti_(GaI) and Fe_(GaI))and an octahedral site(Ti_(GaII) and Fe_(GaII)),to experimentally measured results.The first-principles approach predicted electron capture cross-sections of Ti_(GaI) and Ti_(GaII) defects are 8.56×10^(–14) and 2.97×10^(–13) cm^(2),in good agreement with the experimental values of E_(1) and E_(3) centers,respectively.We,therefore,confirmed that E_(1) and E_(3) centers are indeed associated with Ti_(GaI) and Ti_(GaII) defects,respectively.Whereas the predicted electron capture cross-sections of Fe_(Ga) defect are two orders of magnitude larger than the experimental value of the E_(2),indicating E_(2) may have other origins like C_(Ga) and Ga_(i),rather than common believed Fe_(Ga).

Accurate calculations of the high-pressure elastic constants based on the first-principles

The energy term corresponding to the first order of the strain in Taylor series expansion of the energy with respect to strain is always ignored when high-pressure elastic constants are calculated. Whether the modus operandi would affect the results of the high-pressure elastic constants is still unsolved. To clarify this query, we calculate the high-pressure elastic constants of tantalum and rhenium when the energy term mentioned above is considered and neglected, respectively.Results show that the neglect of the energy term corresponding to the first order of the strain indeed would influence the veracity of the high-pressure elastic constants, and this influence becomes larger with pressure increasing. Therefore, the energy term corresponding to the first-order of the strain should be considered when the high-pressure elastic constants are calculated.

The growth ofβphase in Mg-Gd-Y-Ni alloy by experimental and first-principles study

The paper reports on the atomic investigation aboutβphase in Mg_(96)Gd_(2)Y_(1)Ni_(1) alloy by using the first-principles study and the high-angle annular dark-field scanning transmission electron microscope(HAADF-STEM)corrected by atomic Cs.By using HAADF-STEM,the rectangularβphases were observed in the underage and peak aging stages in Mg_(96)Gd_(2)Y_(1)Ni_(1) alloy.Theβphase could be precipitated from the previously precipitatedβphase,and theβphase grew in steps when it was precipitated.A special transition structure of three atomic layer thicknesses was first observed at the edge of theβphase and the structure of this interface is probably as theβ/Mg_(1) interface for the analysis of thermodynamic characterization and electronic characterization.Theβ'phase and theβ_(H) structure were precipitated only at the edge of the length directions of theβphase.Theβ'phase continues to grow into aβphase directly without the formation ofβ_(1) phase,resulting in an increase in the length of theβphase,which is discovered for the first time.

Mechanical Properties and Electronic Structures of M(M=Ti,V,Cr,Mn and Fe)Dopedβ-Si_(3)N_(4) from First-Principle

The structures,mechanical properties and electronic structures of M metals(M=Ti,V,Cr,Mn and Fe)dopedβ-Si_(3)N_(4) were investigated by First-principles calculations within CASTEP.The calculated lattice parameters ofβ-Si_(3)N_(4) were consistent with previous date.The cohesive energy and formation enthalpy show that initialβ-Si_(3)N_(4) has the highest structural stability.The calculated elastic constant and the Voigt-Reuss-Hill approximation indicate that elastic moduli ofβ-Si_(3)N_(4) are slightly reduced by M doping.Based on Poisson’s and Pugh’s ratio,β-Si_(3)N_(4) is a ductile material and the toughness ofβ-Si_(3)N_(4) increases with M doping,and Fe doping exhibited the best toughness.The results of density of states,charge distributions and overlapping populations indicate thatβ-Si_(3)N_(4) has the strong covalent and ionic bond strength between N and Si.

First-Principles Investigation of Charge Transfer Mechanism of B-Doped 3C-SiC Semiconductor Material

This study delves into the charge transfer mechanism of boron (B)-doped 3C-SiC through first-principles investigations. We explore the effects of B doping on the electronic properties of 3C-SiC, focusing on a 12.5% impurity concentration. Our comprehensive analysis encompasses structural properties, electronic band structures, and charge density distributions. The optimized lattice constant and band gap energy of 3C-SiC were found to be 4.373 Å and 1.36 eV respectively, which is in agreement with previous research (Bui, 2012;Muchiri et al., 2018). Our results show that B doping narrows the band gap, enhances electrical conductivity, and influences charge transfer interactions. The charge density analysis reveals substantial interactions between B dopants and surrounding carbon atoms. This work not only enhances our understanding of the material’s electronic properties, but also highlights the importance of charge density analysis for characterizing charge transfer mechanisms and their implications in the 3C-SiC semiconductors.

Highly active Fe_(36)Co_(44) bimetallic nanoclusters catalysts for hydrolysis of ammonia borane: The first-principles study

In this paper,Fe_(36)Co_(44)nanocluster structure is used to catalyze the hydrolysis reaction of ammonia borane to produce H_(2).Firstly,we complete the construction of Fe_(36)Co_(44)cluster structure and calculate the electronic properties of the cluster.By comparing the adsorption process of Ammonia Borane (AB) in active sites of the cluster,which have different Effective Coordination Number (ECN),the qualitative relationship between ECN and the catalytic activation of AB is clarified,and the optimal catalytic active site is obtained.Then,from the perspective of different reaction paths,we study the hydrolysis reaction of AB in multiple paths,and obtain 5 different reaction paths and energy profiles.The calculation results show that in the case of N–H bond priority break (path 5),the reaction has the minimum rate-determining step (RDS) barrier (about 1.02 e V) and the entire reaction is exothermic (about 0.40 e V).So,path 5 is an optimal catalytic reaction path.This study will have an important guiding significance for the study of the AB hydrolysis reaction mechanism.

First-principles calculations of Ni–(Co)–Mn–Cu–Ti all-d-metal Heusler alloy on martensitic transformation,mechanical and magnetic properties

The martensitic transformation,mechanical,and magnetic properties of the Ni_(2)Mn_(1.5-x)Cu_(x)Ti_(0.5) (x=0.125,0.25,0.375,0.5) and Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5)[(x=0.125,y=0.125,0.25,0.375,0.5) and (x=0.125,0.25,0.375,y=0.625)]alloys were systematically studied by the first-principles calculations.For the formation energy,the martensite is smaller than the austenite,the Ni–(Co)–Mn–Cu–Ti alloys studied in this work can undergo martensitic transformation.The austenite and non-modulated (NM) martensite always present antiferromagnetic state in the Ni_(2)Mn_(1.5-x)Cu_(x)Ti_(0.5) and Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5) (y<0.625) alloys.When y=0.625 in the Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5) series,the austenite presents ferromagnetic state while the NM martensite shows antiferromagnetic state.Cu doping can decrease the thermal hysteresis and anisotropy of the Ni–(Co)–Mn–Ti alloy.Increasing Mn and decreasing Ti content can improve the shear resistance and normal stress resistance,but reduce the toughness in the Ni–Mn–Cu–Ti alloy.And the ductility of the Co–Cu co-doping alloy is inferior to that of the Ni–Mn–Cu–Ti and Ni–Co–Mn–Ti alloys.The electronic density of states was studied to reveal the essence of the mechanical and magnetic properties.

A first-principles study on remote van der Waals epitaxy through a graphene monolayer on semiconductor substrates

To investigate the mechanism of remote epitaxy, where the overlayer can follow the same crystalline structure as the underlying semiconductor substrate through a thin two-dimensional interlayer, we systematically study the potential fluctuations of graphene covered Si, Ga As, and Ga N substrates from first-principles. We find that the uneven semiconductor surface, the distorted graphene, and the non-uniform interface charge transfer make significant contributions to the potential fluctuation. The semiconductor substrate with different surface reconstructions and orientations will generate different potential fluctuations through the graphene interlayer. We also calculate and compare the adsorption of adatoms on graphene covered substrates. The adsorption energies of adatoms not only depend on their distances to the underlying semiconductor surface, but are also sensitive to the direction of the charge transfer at the graphene/substrate interface. Changing the semiconductor reconstruction or orientation could even reverse the order of the adsorption energies of cation and anion adatoms by reversing the interface charge transfer direction, leading to a change in the growth orientation of the overlayer.Our study improves the understanding of the mechanism of remote epitaxy, and reveals that it is possible to control the initial nucleation and orientation of overlayers by changing the semiconductor reconstructions and/or orientations in remote epitaxy.

First-principles Study of Electronic Structural and Mechanical Properties of Mg_(x)La(x=1,2,3)Compounds under Pressure

The effects of pressure on structural,elastic and electronic properties of Mg_(x)La(x=1,2,3)compounds are investigated by using CASTEP program based on the density functional theory.The calculated equilibrium lattice parameters at zero pressure agree well with available experimental and theoretical values.The calculated DOS show that the structure of these compounds remains mechanically stable and structural phase transformation is not induced with increasing pressure from 0 to 30 GPa,and their structural stability increases with pressure.The ductility of MgLa can be improved by increasing pressure,which is the same as Mg_(2)La in 0-20 GPa,while brittle behavior turns into ductile behavior in 0-5 GPa for Mg_(3)La.The resistance to volume deformation of Mg_(x)La(x=1,2,3)compounds can be improved as the pressure increases.The shear deformation resistance and elastic stiffness of Mg_(3)La can be enhanced by rising pressure,but MgLa and Mg_(2)La increase first and then decrease when pressure is up to 25 GPa.In addition,the three compounds exhibit the elastic anisotropy with pressure.

Pressure-dependent electronic,optical,and mechanical properties of antiperovskite X_(3)NP(X=Ca,Mg):A first-principles study

Hydrostatic pressure provides an efficient way to tune and optimize the properties of solid materials without chang-ing their composition.In this work,we investigate the electronic,optical,and mechanical properties of antiperovskite X_(3)NP(X^(2+)=Ca,Mg)upon compression by first-principles calculations.Our results reveal that the system is anisotropic,and the lat-tice constant a of X_(3)NP exhibits the fastest rate of decrease upon compression among the three directions,which is different from the typical Pnma phase of halide and chalcogenide perovskites.Meanwhile,Ca_(3)NP has higher compressibility than Mg_(3)NP due to its small bulk modulus.The electronic and optical properties of Mg_(3)NP show small fluctuations upon compression,but those of Ca_(3)NP are more sensitive to pressure due to its higher compressibility and lower unoccupied 3d orbital energy.For example,the band gap,lattice dielectric constant,and exciton binding energy of Ca_(3)NP decrease rapidly as the pressure increases.In addition,the increase in pressure significantly improves the optical absorption and theoretical conversion effi-ciency of Ca_(3)NP.Finally,the mechanical properties of X_(3)NP are also increased upon compression due to the reduction in bond length,while inducing a brittle-to-ductile transition.Our research provides theoretical guidance and insights for future experi-mental tuning of the physical properties of antiperovskite semiconductors by pressure.

Rational design of Fe/Co-based diatomic catalysts for Li–S batteries by first-principles calculations

Fe/Co-based diatomic catalysts decorated on an N-doped graphene substrate are investigated by first-principles calculations to improve the electrochemical properties of Li–S batteries.Our results demonstrate that Fe CoN8@Gra not only possesses moderate adsorption energies towards Li2Snspecies,but also exhibits superior catalytic activity for both reduction and oxidation reactions of the sulfur cathode.Moreover,the metallic property of the diatomic catalysts can be well maintained after Li2Snadsorption,which could help the sulfur cathode to maintain high conductivity during the whole charge–discharge process.Given these exceptional properties,it is expected that Fe CoN8@Gra could be a promising diatomic catalyst for Li–S batteries and afford insights for further development of advanced Li–S batteries.

Thermal transport properties of two-dimensional boron dichalcogenides from a first-principles and machine learning approach

The investigation of thermal transport is crucial to the thermal management of modern electronic devices.To obtain the thermal conductivity through solution of the Boltzmann transport equation,calculation of the anharmonic interatomic force constants has a high computational cost based on the current method of single-point density functional theory force calculation.The recent suggested machine learning interatomic potentials(MLIPs)method can avoid these huge computational demands.In this work,we study the thermal conductivity of two-dimensional MoS_(2)-like hexagonal boron dichalcogenides(H-B_(2)VI_(2);V I=S,Se,Te)with a combination of MLIPs and the phonon Boltzmann transport equation.The room-temperature thermal conductivity of H-B_(2)S_(2)can reach up to 336 W·m^(-1)·K^(-1),obviously larger than that of H-B_(2)Se_(2)and H-B_(2)Te_(2).This is mainly due to the difference in phonon group velocity.By substituting the different chalcogen elements in the second sublayer,H-B_(2)VIV I′have lower thermal conductivity than H-B_(2)VI_(2).The room-temperature thermal conductivity of B2STe is only 11%of that of H-B_(2)S_(2).This can be explained by comparing phonon group velocity and phonon relaxation time.The MLIP method is proved to be an efficient method for studying the thermal conductivity of materials,and H-B_(2)S_(2)-based nanodevices have excellent thermal conduction.

First-principles study of the bandgap renormalization and optical property ofβ-LiGaO_(2)

Theβ-LiGaO_(2)with an orthorhombic wurtzite-derived structure is a candidate ultrawide direct-bandgap semiconductor.In this work,using the non-adiabatic Allen-Heine-Cardona approach,we investigate the bandgap renormalization arising from electron-phonon coupling.We find a sizable zero-point motion correction of-0.362 eV to the gap atΓ,which is dominated by the contributions of long-wavelength longitudinal optical phonons.The bandgap ofβ-LiGaO_(2)decreases monotonically with increasing temperature.We investigate the optical spectra by comparing the model Bethe-Salpether equation method with the independent-particle approximation.The calculated optical spectra including electron-hole interactions exhibit strong excitonic effects,in qualitative agreement with the experiment.The contributing interband transitions and the binding energy for the excitonic states are analyzed.

First-principles prediction of quantum anomalous Hall effect in two-dimensional Co2Te lattice

The quantum anomalous Hall effect(QAHE) has special quantum properties that are ideal for possible future spintronic devices. However, the experimental realization is rather challenging due to its low Curie temperature and small non-trivial bandgap in two-dimensional(2D) materials. In this paper, we demonstrate through first-principles calculations that monolayer Co2Te material is a promising 2D candidate to realize QAHE in practice. Excitingly, through Monte Carlo simulations, it is found that the Curie temperature of single-layer Co2Te can reach 573 K. The band crossing at the Fermi level in monolayer Co2Te is opened when spin–orbit coupling is considered, which leads to QAHE with a sizable bandgap of Eg= 96 me V, characterized by the non-zero Chern number(C = 1) and a chiral edge state. Therefore, our findings not only enrich the study of quantum anomalous Hall effect, but also broaden the horizons of the spintronics and topological nanoelectronics applications.