Structural properties,phase stability,elastic properties and electronic structures of Cu–Ti intermetallics

The structural properties, phase stabilities, anisotropic elastic properties and electronic structures of Cu–Ti intermetallics have been systematically investigated using first principles based on the density functional theory. The calculated equilibrium structural parameters agree well with available experimental data. The ground-state convex hull of formation enthalpies as a function of Cu content is slightly symmetrical at CuTi with a minimal formation enthalpy (–13.861 kJ/mol of atoms), which indicates that CuTi is the most stable phase. The mechanical properties, including elastic constants, polycrystalline moduli and anisotropic indexes, were evaluated. G/B is more pertinent to hardness than to the shear modulus G due to the high power indexes of 1.137 for G/B. The mechanical anisotropy was also characterized by describing the three-dimensional (3D) surface constructions. The order of elastic anisotropy is Cu₄Ti₃ > Cu₃Ti₂ > α-Cu₄Ti > Cu₂Ti > CuTi > β-Cu₄Ti > CuTi₂. Finally, the electronic structures were discussed and Cu₂Ti is a semiconductor.     

Ab initio study on the ground states,phase stability,and mechanical properties of the Au–Pt system

For the equilibrium immiscible Au–Pt system, ground states are studied based on the results of the cluster expansion method combined with ab initio calculations. The obtained results show that there is no stable phase for the Au–Pt system at 0 K. The further obtained enthalpies of formation for hypothetical crystalline L1₂, D0₁₉, D0₃ structured Au₃Pt and AuPt₃, as well as L1₀ structured AuPt compounds also have positive values. Moreover, elastic constants are predicted from ab initio for the first time for the metastable L1₂ Au₃Pt, AuPt₃ and L1₀ AuPt compound. Finally, there is an imaginary phonon appearing in the obtained phonon spectra, implying an internal instability of the positions of the nuclear coordinates of the L1₂Au₃Pt compound.     

Theoretical study on the influence of different NˆN ligands on the electronic structures and optoelectronic properties of heteroleptic Iridium(III) complexes

DFT/TDDFT calculations were carried out to investigate the electronic structures, absorption and phosphorescence properties of a series of heteroleptic Ir(III) complexes consisting of two N-heterocyclic carbene ligands and a conjugated bicyclic N,N′-heteroaromatic (N?N) ligand. On the basis of the results reported herein, we attempt to explain the experimental observations according to which complex (mpmi)₂Ir(pybi) (1) [Hmpmi = 1-(4-tolyl)-3-methyl-imidazole; Hpybi = 2-(pyridin-2-yl)-1H-benzo[d]imidazole] emits green light with an extremely high-quantum phosphorescence efficiency (Φ _PL ) of 79.3%, while a relatively lower Φ _PL (only 11%) was measured for (fpmi)₂Ir(tfpypz) (2) [fpmi = 1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C, C^2′; tfpypz = 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridinato] emitting blue light by tuning the N?N ligands. Besides, we also designed (fpmi)₂Ir(pyN3) (3) [pyN3H = 2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine] and (fpmi)₂Ir(pyN4) (4) [pyN4H = 2-(1H-tetrazol-5-yl)pyridine] to explore the influence of electron-withdrawing substituents on N?N ligands on the electronic and optical properties of these Ir(III) complexes. The results revealed that electron-withdrawing substituents can stabilise both HOMOs and LUMOs and induce HOMO–LUMO energy gap change. Moreover, the emission properties can be significantly tuned by introducing different N?N ligands. While new insights were gained on structural and electronic properties, the extremely high Φ _PL of 1 was found to be not inherent to spin-orbital coupling effects, but determined by its large transition dipole moment (μ_{S 1}) upon S ₀–S ₁ transition compared with that of 2. On the basis of these results, the designed complexes 3 and 4 are considered to be the promising candidates for blue-emitting phosphorescence materials with higher Φ _PL than the complex 2.     

Elasticity of the B2 phase and the effect of the B1–B2 phase transition on the elasticity of MgO

A study of the high-pressure anisotropy of MgO was conducted using first-principles calculations based on density functional theory within the generalized gradient approximations. The pressure dependence of the elastic stiffness coefficients and the anisotropy parameters, in both B1 and B2 phases, shows that for high-hydrostatic compression the easiest deformation is the shear along (100) plane and the the material's response to deformation and to shearing strains is quite the same. According to the calculations of the velocities of propagation of elastic waves, we deduced that MgO develop an elastic anisotropy, especially, in the B1 phase. We present the B2 phase elastic properties which are not already studied under high pressure.     

Crystal structures and decomposing of B–P compounds under pressure

We have systematically studied the structures, electronic properties, and lattice dynamics of B–P compounds at high pressures. BP and B_6 P are found to be thermodynamically stable below 100 GPa, and other stoichiometries are decomposable under pressure. The predicted structures of F-43 m BP and R-3 m B_6 P are in good agreement with the experimental results by comparing the powder diffraction file(PDF) standard cards with our simulated x-ray diffractions. The bonding properties of BP and B_6 P have also been analyzed by electronic localization functions, charge density difference, and Bader charge analysis. Our results show that BP and B_6 P decompose into B and P under high pressure, which is proven to be dominated by the volumes of them. Furthermore, the infrared and Raman spectra of F-43 m and R-3 m are investigated at selected pressures and will provide useful information for future experimental studies about B–P compounds.     

Study of cohesive,electronic and magnetic properties of the Ni–In intermetallic system

Cohesive, electronic and magnetic properties of the intermetallic system Ni–In, specifically the stable phases Ni₃In-hP8, Ni₂In-hP6, NiIn-hP6 and Ni₂In₃-hP5, have been investigated. At present, these materials are of great interest in connection to the application of the In–Sn alloys as lead-free micro-soldering alloys, and considering Ni as the contact material. In spite of this, scarce literature regarding basic thermodynamic properties of the Ni–In intermetallic phases has been found. Full-Potential Linear Augmented Plane Wave method (FP-LAPW) within the framework of the Density Functional Theory (DFT) with exchange and correlation effects in the Generalized Gradient (GGA) and Local Density (LDA) approximations is used. All the calculations include spin polarization. Structural parameters, formation energies and cohesive properties of the different phases are studied through minimization of internal parameters. The electronic density of states (DOS) is analyzed for each optimized structure. We found that the NiIn-hP6 phase is the most stable one and only the Ni₃In-hP8 phase exhibits magnetic properties.     

Theoretical study of novel B–C–O compounds with non-diamond isoelectronic

Two novel non-isoelectronic with diamond(non-IED)B–C–O phases(tI16-B₈C₆O₂and mP16-B₈C₅O₃)have been unmasked.The research of the phonon scattering spectra and the independent elastic constants under ambient pressure(AP)and high pressure(HP)proves the stability of these non-IED B–C–O phases.Respective to the common compounds,the research of the formation enthalpies and the relationship with pressure of all non-IED B–C–O phases suggests that HP technology performed in the diamond anvil cell(DAC)or large volume press(LVP)is an important technology for synthesis.Both tI16-B₈C₆O₂and tI12-B₆C₄O₂possess electrical conductivity.mP16-B₈C₅O₃is a small bandgap semiconductor with a 0.530 eV gap.For aP13-B₆C₂O₅,mC20-B₂CO₂and tI18-B₄CO₄are all large gap semiconductors with gaps of 5.643 eV,6.113 eV,and 7.105 eV,respectively.The study on the relationship between band gap values and pressure of these six non-IED B–C–O phases states that tI16-B₈C₆O₂and tI12-B₆C₄O₂maintain electrical conductivity,mC20-B₂CO₂and tI18-B₄CO₄have good bandgap stability and are less affected by pressure.The stress-strain simulation reveals that the max strain and stress of 0.4 GPa and 141.9 GPa respectively,can be sustained by tI16-B₈C₆O₂.Studies on their mechanical properties shows that they all possess elasticity moduli and hard character.And pressure has an obvious effect on their mechanical properties,therein toughness of tI12-B₆C₄O₂,aP13-B₆C₂O₅,mC20-B₂CO₂and tI18-B4CO4 all increases,and hardness of mP16-B₈C₅O₃continue to strengthen during the compression.With abundant hardness characteristics and tunable band gaps,extensive attention will be focused on the scientific research of non-IED B–C–O compounds.     

First principles studies on the electronic structures of LiMxFe1-xPO4 （M = Co, Ni and Rh）

The local crystal structures and electronic structures of LiMxFe1-xPO4 （M = Co, Ni, Rh） are studied through first-principles calculations. The lattice constants and unit cell volumes are smaller for the Co and Ni doped materials than for pure LiFePO4, while larger than for the Rh doped material. The local structures around M atoms in the doped materials are studied in details. The total density of states （DOS） and atomic projected DOS （PDOS） are all calculated and analysed in detail. The results give a reasonable prediction to the improvement of electronic conductivity through Fe-site doping in LiFePO4 material.     

Effect of gallium alloying on the structure,the phase composition,and the thermoelastic martensitic transformations in ternary Ni–Mn–Ga alloys

The effect of gallium alloying on the structure, the phase composition, and the properties of quasibinary Ni₅₀Mn_50–zGa_z (0 ? z ? 25 at %) alloys is studied over a wide temperature range. The influence of the alloy composition on the type of crystal structure in high-temperature austenite and martensite and the critical martensitic transformation temperatures is analyzed. A general phase diagram of the magnetic and structural transformations in the alloys is plotted. The temperature–concentration boundaries of the B2 and L2₁ superstructures in the austenite field, the tetragonal L1₀ (2M) martensite, and the 10M and 14M martensite phases with complex multilayer crystal lattices are found. The predominant morphology of martensite is shown to be determined by the hierarchy of the packets of thin coherent lamellae of nano- and submicrocrystalline crystals with planar habit plane boundaries close to {011}_B2. Martensite crystals are twinned along one of the 24 \(24\left\{ {011} \right\}{\left\langle {01\bar 1} \right\rangle _{B2}}\) “soft” twinning shear systems, which provides coherent accommodation of the martensitic transformation–induced elastic stresses.     

Theoretical study of the effect of temperature differential and ionizing radiation on the current–voltage characteristics of HEM transistors

Requirements to the modeling of the effects of temperature differential and ionizing radiation on the current–voltage characteristics of high electron mobility transistors (HEMTs) were formulated. The results of modeling of the effects of temperature differential on the current–voltage characteristics of HEMTs were described. The results of analysis of the effects of ionizing radiation on the current–voltage characteristics of HEMTs were given. The results of modeling of the effects of ionizing radiation on the current–voltage characteristics of HEMTs were presented.     

Influence of gamma radiation on morphology structure,electrochemical corrosion behavior and hardness of Ni–Cr based alloys

This study evaluates the effects of gamma radiation on structure, electrochemical corrosion behavior and Vickers hardness of commercial dental Nikkeli–Kromi–Polttosekoitus [Ni_65.2Cr_22.5Mo_9.5X_2.8 (X=Nb, Si, Fe and Mn)] alloy. The corrosion rate of Ni_65.2Cr_22.5Mo_9.5X_2.8 (X=Nb, Si, Fe and Mn) alloy with 0.5 M HCl is increased with increasing the exposure rate of gamma radiation. The corrosion resistance of Ni_65.2Cr_22.5Mo_9.5X_2.8 (X=Nb, Si, Fe and Mn) is varied and reaches a minimum value at 30 KGy. The corrosion potential value also is varied and reaches its highest value at 30 KGy. The Vickers hardness value of Ni_65.2Cr_22.5Mo_9.5X_2.8 (X=Nb, Si, Fe and Mn) alloy is decreased by increasing the gamma radiation dose. Also it is obvious from our results that the effects of gamma radiation at the surface are much higher as compared with deeper parts and the structure of the alloy is changed due to its exposure to gamma radiation.     

First principle investigation of the structural,electronic and elastic properties of the Laves phase compounds SrX2 (X = Pd and Pt)

Structural, electronic and elastic properties of the cubic Laves phases SrX₂ (X = Pd and Pt) with Fd-3 m (No. 227) space group and crystallize in the MgCu₂ structure are studied using the all-electrons full potential linearized augmented plane wave (FP-LAPW) method as implemented in the Wien2k code. The exchange-correlation potential (E_xc) was treated within the scheme of Perdew, Burke, and Ernzerhof generalized gradient approximation (PBE-GGA). In present calculations, studied compounds show a metallic characteristic. When the palladium element was replaced with the platinum, the Fermi level was observed to be increased, indicating an increase in the additional valence electrons that filled the hybridized bonding states. The elastic constants were calculated for both compounds and show that the two compounds suitably verify the Born's mechanical stability criteria and have the ductile manner behaviors.     

Ab initio study of the composite phase diagram of Ni–Mn–Ga shape memory alloys

The magnetic and structural properties of a series of nonstoichiometric Ni–Mn–Ga Heusler alloys are theoretically investigated in terms of the density functional theory. Nonstoichiometry is formed in the coherent potential approximation. Concentration dependences of the equilibrium lattice parameter, the bulk modulus, and the total magnetic moment are obtained and projected onto the ternary phase diagram of the alloys. The stable crystalline structures and the magnetic configurations of the austenitic phase are determined.     

X-ray photoelectron studies of changes in the electronic structure upon phase transitions in Co–Ni and Co–Fe alloys

The changes in the electronic structure of Co–Ni and Co–Fe systems upon phase transitions are studied. X-ray photoelectron study of the valence-band spectra and the parameters of the multiplet splitting of Co, Ni and Fe 3s spectra is carried out at different temperatures. It is established that the ordering–separation phase transition in Co–Ni alloys takes place in the temperature range of 600–700°C. As opposed to Co–Ni alloys, in the Fe–Co alloy, ordering–separation–ordering phase transitions are observed. High-temperature ordering of the Fe₅₀Co₅₀ alloy is observed above 1200°C. The transition from ordering to separation is shown to lead to changes in the d electron spectra of the valence band and in the parameters of the multiplet splitting of the 3s spectra.     

Density functional study of α-graphyne derivatives: Energetic stability,atomic and electronic structure

The energetic stability, atomic and electronic structures of α-graphyne and its derivatives (α-GYs) with extended carbon chains were investigated by density functional (DF) calculations in this work. The studied α-GYs consist of hexagon carbon rings sharing their edges with carbon atoms N=1–10. The structure and energy analyses show that α-GYs with even-numbered carbon chains have alternating single and triple C–C bonds (polyyne), energetically more stable than those with odd-numbered carbon chains possessing continuous double C–C bonds (polycumulene). The calculated electronic structures indicate that α-GYs can be either metallic (odd N) or semiconductive (even N) depending on the parity of number of atoms on hexagon edges despite the edge length. The semiconducting α-graphyne derivatives are found to possess Dirac cones (DC) with small direct band gaps 2–40 meV and large electron velocities 0.554×10⁶–0.671×10⁶ m/s, 70–80% of that of graphene. Our DF studies suggest that introducing sp carbon atoms into the hexagon edges of graphene opens up an avenue to switch between metallic and DC electronic structures via tuning the parity of the number of hexagon edge atoms.     

Magnetic,electronic and electron spin resonance studies of the (Gd1−xYx)Ni2 compounds

Magnetization, electron spin resonance and heat capacity measurements have been made on the (Gd_1-xYx)Ni₂ compounds. Magnetization measurements reveal a moment deficiency for gadolinium in this series. In GdNi₂ a value of 6.92μ_B is obtained for the gadolinium moment, which decreases further with yttrium substitution. In the concentration range 0.2 ⩽ x ⩽ 0.8 the samples show features seen in inhomogeneous ferromagnets.The ESR measurements reveal a negative g-shift and a very shallow ESR bottleneck in this system.The specific heat of the samples in the concentration range 0.2 ⩽ x ⩽ 0.8 show complex behaviour: downturn at low temperatures in the usual (C/T) against T² graphs, anomalously large γ-values and so on. In the remaining compounds the specific heat shows normal behaviour. At the YNi₂ end of the series a rapid increase in the host density of state takes place when gadolinium is substituted for yttrium.     

Calculation of the structure properties and P–T phase diagram of hafnium

Abstract

The full-potential linear muffin-tin orbitals (FP-LMTO) method is used to calculate the total energy and equilibrium lattice properties for the observed phases of Hf. The temperature and pressure dependences of the Gibbs energy are found for these structures within the Debye model. A quantitative agreement with the experimental points of the P-T phase diagram is obtained.     

Theoretical investigation on structures,stability and properties of [P,X, Y] (X=C,Si; Y = O,S) isomers

The structures, energetics and stability of the [P, X, Y] (X?=?C, Si; Y?=?O, S) radicals are explored by means of the density functional theory and ab initio levels. Seventeen [P, X, Y] isomers and 14 interconversion transition states are obtained at the B3LYP/6-311G(d) level. At the CCSD(T)/6-311?+?G(2df)//QCISD/6-311G(d)?+?ZPVE level, the lowest-lying isomers are the linear PCO 1a (0.0?kcal/mol), PCS 1b (0.0) and the three-membered ring cPSiO 1c (0.0), cPSiS 1d (0.0) on their respective potential energy surfaces. These four isomers exhibit considerably not only thermodynamic but also kinetic stabilities. Additionally, the cyclic cPCS 2b (32.8) and linear PSiS 2d (18.6) possess also high kinetic stability. All of six isomers 1a, 1b, 2b, 1c, 1d and 2d are considerably stabilized by a barrier of at least 20?kcal/mol, and may be detected in the laboratory or interstellar space. Their valence bond structures and possible formation strategies in the laboratory and space are discussed in detail. Finally, the similarities and discrepancies on structures and stabilities between [P, X, Y] (X?=?C, Si; Y?=?O, S) isomers are compared. These predicted results are highly expected to be informative for the future identification of [P, X, Y] (X?=?C, Si; Y?=?O, S) in the laboratory and space.