Pressure induced structural phase transition and electronic properties of actinide monophospides: Ab-initio calculations

We have investigated the structural and electronic properties of monophospides of thorium, uranium and neptunium. The total energy as a function of volume is obtained by means of the self-consistent tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). From the present study with the help of total energy calculations it is found that ThP, UP and NpP are stable in NaCl-type structure at ambient pressure. The structural stability of ThP, UP and NpP changes under the application of pressure. We predict a structural phase transition from NaCl-type (B₁-phase) structure to CsCl-type (B₂-phase) structure for these phospides in the pressure range of 37.0-24.0 GPa (ThP-NpP). We also calculate lattice parameter (a₀), bulk modulus (B₀), band structure and density of states. From energy band diagram it is observed that ThP, UP and NpP exhibit metallic behavior. The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.     

Theoretical study of the structural, elastic, electronic and thermal properties of the MAX phase Nb2SiC

Structural, elastic, electronic and thermal properties of the MAX phase Nb₂SiC are studied by means of a pseudo-potential plane-wave method based on the density functional theory. The optimized zero pressure geometrical parameters are in good agreement with the available theoretical data. The effect of high pressure, up to 40 GPa, on the lattice constants shows that the contractions along the c-axis were higher than those along the a-axis. The elastic constants C_ij and elastic wave velocities are calculated for monocrystal Nb₂SiC. Numerical estimations of the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, average sound velocity and Debye temperature for ideal polycrystalline Nb₂SiC aggregates are performed in the framework of the Voigt-Reuss-Hill approximation. The band structure shows that Nb₂SiC is an electrical conductor. The analysis of the atomic site projected densities and the charge density distribution shows that the bonding is of covalent-ionic nature with the presence of metallic character. The density of states at Fermi level is dictated by the niobium d states; Si element has a little effect. Thermal effects on some macroscopic properties of Nb₂SiC are predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the primitive cell volume, volume expansion coefficient, bulk modulus, heat capacity and Debye temperature with pressure and temperature in the ranges of 0-40 GPa and 0-2000 K are obtained successfully.     

First principles study of structural,electronic, mechanical and thermal properties of A15 intermetallic compounds Ti3X (X=Au,Pt, Ir)

The structural, electronic, elastic, mechanical and thermal properties of Ti₃Au, Ti₃Pt and Ti₃Ir intermetallic compounds crystallizing in A15 structure have been studied using density functional theory within generalized gradient approximation (GGA) for the exchange correlation potential. Elastic properties such as Young's modulus (E), rigidity modulus (G), bulk modulus (B), Poisson's ratio (σ) and elastic anisotropic factor (A) have been calculated. From the present study it is noted that Ti₃Ir is the hardest compound among the three materials studied due to its larger bulk modulus. Also, it is more ductile in nature.     

Vacancy-defect effect on the electronic and optical properties of Pmm2 BC2N: A first-principles study

Orthorhombic-Pmm2-BC₂N as a superhard photocatalyst simulates great interests in the researches of materials-design and application. To promote the studies of Pmm2 BC₂N as a multifunctional material with both great hardness and good optical properties, we investigated the electronic and optical properties of Pmm2 BC₂N with various vacancy-defects by the systematic first-principles density functional theory (DFT) calculations in this work. The absorption, refractivity, reflectivity, and photoconductivity of considered structures were calculated and explored. The various characteristics of the optical properties were analyzed based on relative computed density of states (DOS).     

Structural,electronic, optical and bonding properties of strontianite,SrCO3: First-principles calculations

The first-principles calculations are performed within the density functional theory to investigate the crystal structure, energy band structure, density of states, optical properties, and bonding properties of strontianite. The optimized structure parameters and bonding results with the generalized gradient approximation (GGA) functional and the localized density approximation (LDA) functional are in good agreement with the earlier experimental data. The band structure, density of states and chemical bonding of strontianite have been calculated and analyzed. The indirect band gap of strontianite is estimated to be ~4.45 eV (GGA) or ~4.24 eV (LDA). The absorption, reflectivity, refractive index and extinction coefficient have been calculated using the imaginary part of the dielectric function. The calculated results of the optical properties show that strontianite has an optical anisotropy along [100] (or [010]) and [010] polarization directions of incoming light. Furthermore, the calculated results of the density of states and Mulliken population indicate that the interactions among atoms are both ionic and covalent bonding in strontianite.     

Modeling electronic,mechanical, optical and thermal properties of graphene-like BC6N materials: Role of prominent BN-bonds

We model monolayer graphene-like materials with BC₆N stoichiometry where the bonding between the B and the N atoms plays an important role for their physical and chemical properties. Two types of BC₆N are found based on the BN bonds: In the presence of BN bonds, an even number of π-bonds emerges indicating an aromatic structure and a large direct bandgap appears, while in the absence of BN bonds, an anti-aromatic structure with an odd-number of π-bonds is found resulting a direct small bandgap. The stress-strain curves shows high elastic moduli and tensile strength of the structures with BN-bonds, compared to structures without BN-bonds. Self-consistent field calculations demonstrate that BC₆N with BN-bonds is energetically more stable than structures without BN-bonds due to a strong binding energy between the B and the N atoms, while their phonon dispersion displays that BC₆N without BN-bonds has more dynamical stability. Furthermore, all the BC₆N structures considered show a large absorption of electromagnetic radiation with polarization parallel to the monolayers in the visible range. Finer detail of the absorption depend on the actual structures of the layers. A higher electronic thermal conductivity and specific heat are seen in BC₆N systems caused by hot carrier–assisted charge transport. This opens up a possible optimization for bolometric applications of graphene based material devices.     

First-principles study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallics

First-principles calculations were performed to study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallic compounds (AlCu₃, AlCu₂Zr, and AlZr₃). The calculated results show that the lattice parameters obtained after full relaxation of crystalline cells are consistent with experimental data. The calculation of cohesive energies indicated that the structure stability of these Al-based intermetallics will become higher with increasing Zr element in crystal. The calculations of formation energies showed that AlCu₂Zr has the strongest alloying ability, followed by AlZr₃ and finally the AlCu₃. The further analysis find out that single-crystal elastic constants at zero-pressure satisfy the requirement of mechanical stability for cubic crystals. The calculations on the ratio of bulk modulus to shear modulus reveal that AlCu₂Zr can exhibit a good ductility, followed by AlCu₃, whereas AlZr₃ can have a poor ductility; however, for stiffness, these intermetallics show a converse order. The calculations on Poisson's ratio show that AlCu₃ is much more anisotropic than the other two intermetallics. In addition, calculations on densities of states indicate that the valence bonds of these intermetallics are attributed to the valence electrons of Cu 3d states for AlCu₃, Cu 3d, and Zr 4d states for AlCu₂Zr, and Al 3s, Zr 5s and 4d states for AlZr₃, respectively; in particular, the electronic structure of the AlZr₃ shows the strongest hybridization, leading to the worst ductility.     

Theoretical and experimental investigation on structural and electronic properties of Al/O/Al,O-doped WS2

Effects of the doping atom (O, Al, and (Al, O)) on structural and electronic properties of the monolayer WS₂ have been studied by using first-principles calculations. Results show that the covalent character of W–S bonding has been enhanced after doping. Meanwhile, W–O, Al–S and W–S bonds of (Al, O) co-doped WS₂ monolayer have higher covalent character compared with O-doped and Al-doped WS₂ monolayer of this work. After doping with Al (or Al, O) atoms, Fermi level moves close to the valence band and the dopant atoms produce the defect energy levels, indicating that Al doped and (Al, O) co-doped WS₂ monolayer both have p-type conductivity. O-doped and (Al, O) co-doped WS₂ ultrathin films was prepared on Si substrates. Results of Raman spectra show the formation of the O-doped and (Al, O) co-doped WS₂ films. Moreover, compared with the pure WS₂, the approximate reduction of 0.43 eV and 0.46 eV for W 4f and S 2p in binding energy after (Al, O) co-doped shows that p-type doping of (Al, O) co-doped WS₂ has been verified.     

Density functional investigation of structural, electronic and optical properties of Ge-doped ZnO

Recent experiments reported fascinating phenomenon of photoluminescence (PL) blueshift in Ge-doped ZnO. To understand it, we examined the structural, electronic and optical properties of Ge-doped ZnO (ZnO:Ge) systematically by means of density functional theory calculations. Our results show that Ge atoms tend to cluster in heavily doped ZnO. Ge clusters can limit the conductivity of doped ZnO but reinforce the near-band-edge emission. The substitutional Ge for Zn leads to Fermi level pinning in the conduction band, which indicates Ge-doped ZnO is of n-type conductivity character. It is found that the delocalized Ge 4s states hybridize with conduction band bottom, and is dominant in the region near the Fermi level, suggesting that Ge-4s states provides major free carriers in ZnO:Ge crystal. The observed blueshift of PL in Ge-doped ZnO originates from the electron transition energy from the valence band to the empty levels above Fermi level larger than the gap of undoped ZnO. The electron transition between the gap states induced by oxygen vacancy and conduction band minimum may be the origin of the new PL peak at 590 nm.     

Structure twinning,electronic and photoluminescence properties of yavapaiite-type orthophosphate BaTi(PO4)2

A ternary orthophosphate BaTi(PO₄)₂ has been prepared using a high temperature molten salt method and structurally determined by single crystal X-Ray diffraction analysis. It crystallizes in yavapaiite-type structure with monoclinic space group C2/m. The structure was refined by a non-merohedral twinning model with the twin law (−0.435 1.4350 −0.564 −0.435 0 0.097 −0.099 1). Band structure calculation using the density functional theory (DFT) method indicates that BaTi(PO₄)₂ has a direct bond gap of about 3.00 eV, which is well fitted with the experimental value of 2.95 eV. The photoluminescence excitation and emission spectra, decay curve, and the color coordinates for BaTi(PO₄)₂ were investigated. It can be efficiently excited by UV light (270 nm) and presents blue–green emission (centered at 506 nm), which may be attributed to the lattice defect emission.     

A first-principles calculation on the electronic properties of Si/N-codoped TiO2

To deeply understand the effects of Si/N-codoping on the electronic structures of TiO₂ and confirm their photocatalytic performance, a comparison theoretical study of their energetic and electronic properties was carried out involving single N-doping, single Si-doping and three models of Si/N-codoping based on first-principles. As for N-doped TiO₂, an isolated N 2p state locates above the top of valence band and mixes with O 2p states, resulting in band gap narrowing. However, the unoccupied N 2p state acts as electrons traps to promote the electron-hole recombination. Using Si-doping, the band gap has a decrease of 0.24 eV and the valence band broadens about 0.30 eV. These two factors cause a better performance of photocatalyst. The special Si/N-codoped TiO₂ model with one O atom replaced by a N atom and its adjacent Ti atom replaced by a Si atom, has the smallest defect formation energy in three codoping models, suggesting the model is the most energetic favorable. The calculated energy results also indicate that the Si incorporation increases the N concentration in Si/N-codoped TiO₂. This model obtains the most narrowed band gap of 1.63 eV in comparison with the other two models. The dopant states hybridize with O 2p states, leading to the valence band broadening and then improving the mobility of photo-generated hole; the N 2p states are occupied simultaneously. The significantly narrowed band gap and the absence of recombination center can give a reasonable explanation for the high photocatalytic activity under visible light.     

Atomic structures and electronic properties of interfaces between aluminum and carbides/nitrides: A first-principles study

We perform a first-principles investigation of the atomic structures and electronic properties of interfaces between aluminum and four kinds of ceramics, TiC, TiN, VC and VN, under three orientations (001), (110) and (111). We find that the stable interfaces are those with bonding between Al atom and metalloid C (or N) atom, which is attributed to the overlap of p states of Al and d states of metalloid atoms at Femi level forming covalent components. Among the interfaces with the three orientations, the (111) interfaces are found to possess the largest adhesion energy in that the stacking of atoms follows intrinsic atomic distribution and this interfacial bonding is relatively strong. It is also found that the interfaces between Al and metal carbides (TiC and VC) are more stable than those between Al and metal nitrides (TiN and VN).     

Elastic,electronic and thermal properties of topological insulator SmB6: first-principles calculations

Abstract

The pressure dependence of the structural, elastic, electronic and thermal properties of Kondo insulator SmB₆ have been systematically studied by density functional theory combined with the quasi-harmonic Debye model. The calculated structure at zero pressure is in good agreement with the available experimental results at low temperature. The obtained elastic constants, bulk modulus and shear modulus indicate that SmB₆ is mechanically stable and behaves in a brittle manner under the applied pressure 0–20 GPa, consistent with available experimental data. In addition, the elastic-relevant properties, Young’s modulus and the Poisson ratio manifest that increasing pressure results in an enhancement in the stiffness of the compound. It is found that unlike temperature, pressure has little effect on the heat capacity of SmB_6. What more important is that we observed an insulator to metal phase transition at about 5.5 GPa through the disappearance of the band gap, well consistent with the experimental data. This transition has little effect on the physical properties of SmB₆.     

Electronic and thermal properties of B2-type AlRE intermetallic compounds: A first principles study

The ground state electronic structure and thermal properties of B₂-type intermetallic compounds AlRE (RE: Pm, Sm, Eu, Tb, Gd and Dy) have been studied using a self-consistent tight-binding linear muffin-tin orbital (TB-LMTO) method at ambient as well as at high pressure. These compounds show metallic behavior under ambient condition. The band structure, total energy, density of states and ground state properties like lattice parameter, bulk modulus are calculated in the present work. The Debye-Grüneisen model is used to calculate the Debye temperature and the Grüneisen constant. The calculated results are in good agreement with the reported experimental and other theoretical results. The variation in the Debye temperature with pressure has also been reported. We present a detailed analysis of the role of f electrons of RE in the AlRE system.     

Theoretical investigations of phases of AlAs by first-principles

Five potential novel phases of AlAs (Pmn2₁-AlAs, Pbam-AlAs, Pbca-AlAs, bct-AlAs and wz-AlAs) are proposed and the stabilities of them are verified by the enthalpy, independent elastic constants and phonon dispersion spectra. The mechanical, electronic and thermodynamic properties of ten AlAs phases are studied and the electronic properties are calculated by PBE0 hybrid functional. Pmn2₁-AlAs and Pbam-AlAs own direct band gaps, so they have electronic advantages over other phases of AlAs at 0?GPa. Our calculated band gaps are 3.02, 3.07, 3.21, 2.76, 3.12, 1.15, 1.88, 2.53, 2.75 and 2.53?eV for Pmn2₁-AlAs, Pbam-AlAs, Pbca-AlAs, bct-AlAs, wz-AlAs, oC12-AlAs, hP6-AlAs, cI24-AlAs, zb-AlAs and cmcm-AlAs, respectively. Additionally, these phases: Pmn2₁-AlAs, Pbam-AlAs, Pbca-AlAs, bct-AlAs, cI24-AlAs, and cmcm-AlAs behave in a ductile manner, and wz-AlAs, oC12-AlAs, hP6-AlAs and zb-AlAs behave in a brittle manner. Otherwise, cmcm-AlAs has the greatest anisotropy, and hP6-AlAs has the least anisotropy.     

Ab-initio calculations of electronic structure and optical properties of TiAl alloy

The electronic structures and optical properties of TiAl intermetallic alloy system are studied by the first-principle orthogonalized linear combination of atomic orbitals method. Results on the band structure, total and partial density of states, localization index, effective atomic charges, and optical conductivity are presented and discussed in detail. Total density of states spectra reveal that (near the Fermi level) the majority of the contribution is from Ti-3d states. The effective charge calculations show an average charge transfer of 0.52 electrons from Ti to Al in primitive cell calculations of TiAl alloy. On the other hand, calculations using supercell approach reveal an average charge transfer of 0.48 electrons from Ti to Al. The localization index calculations, of primitive cell as well as of supercell, show the presence of relatively localized states even above the Fermi level for this alloy. The calculated optical conductivity spectra of TiAl alloy are rich in structures, showing the highest peak at 5.73 eV for supercell calculations. Calculations of the imaginary part of the linear dielectric function show a prominent peak at 5.71 eV and a plateau in the range 1.1-3.5 eV.     

The structural stability, elastic constants and electronic structure of Al-Sr intermetallics by first-principles calculations

The lattice constants, enthalpies of formation, elastic constants and electronic structures of Al-Sr intermetallics have been calculated by first-principles method within generalized gradient approximation. The calculated lattice constants and enthalpies of formation are in good agreement with experimental and other theoretical results. The polycrystalline bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are also estimated from the calculated single crystalline elastic constants. The total and partial electronic densities of state for the intermetallics were obtained, and the results indicated that Al₂Sr-oI is more stable than Al₂Sr-cF. Finally, longitudinal, transverse and average sound velocities and Debye temperature are estimated.     

Cu-Fe共掺杂ZnO电子结构与光学性质的第一性原理计算研究

基于密度泛函理论的第一性原理平面波超软赝势方法,计算了本征ZnO,Cu、Fe单掺杂和Cu-Fe共掺杂ZnO的电子结构和光学性质.计算结果显示:Cu掺杂属于p型掺杂,Fe掺杂属于n型掺杂,单掺杂时Cu-3d态电子和Fe-3d态电子均在禁带形成杂质能级,从而提高ZnO的载流子浓度,改善ZnO的导电性能,而Cu-Fe共掺杂时ZnO半导体进入简并态,呈现金属特性.掺杂后的ZnO介电函数虚部变化主要集中在低能量区域,光谱吸收系数及反射率曲线发生红移,其中本征ZnO对太阳光谱有较好的透射性,Fe单掺杂和Cu-Fe共掺杂ZnO对可见光谱有相似的吸收效果,而Fe单掺杂ZnO对近紫外区域的光谱透射率更小,适用于制备防紫外线薄膜.     

Structural, electronic properties and stability of tungsten mono- and semi-carbides: A first principles investigation

First principles calculations have been performed with the purpose to understand the comparative peculiarities of the structural, electronic properties and stability for all phases formed in the tungsten-carbon system: hexagonal and cubic mono-carbides WC and four polymorphs (α, β, γ and ε) of semi-carbide W₂C. All calculations were performed by means of the full-potential linearized augmented plane wave method (FLAPW). The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) formalism was used for the exchange and correlation energy functional. The geometries of all WC and W₂C phases were optimized and their structural parameters and theoretical density were established. Besides, we have evaluated the formation energies (E_form) of all the tungsten carbides. Based on our estimations we can arrange all investigated W-C phases depending on their stability in the following sequence: h-WC>ε-W₂C>β-W₂C>γ-W₂C>α-W₂C>c-WC. Here three carbides (h-WC, ε-W₂C and β-W₂C) are stable (E_form<0), γ-W₂C belongs to metastable systems (E_form∼0), whereas α-W₂C and c-WC appear to be unstable (E_form>0). Moreover, band structures, total and partial densities of states were obtained and analyzed systematically for all W-C phases in comparison with other available theoretical and experimental data.