Ab initio study of phase stability in doped TiO2

Ab initio density functional theory calculations of the relative stability of the anatase and rutile polymorphs of TiO₂ were carried out using all-electron atomic orbitals methods with local density approximation. The rutile phase exhibited a moderate margin of stability of ~ 3 meV relative to the anatase phase in pristine material. From computational analysis of the formation energies of Si, Al, Fe and F dopants of various charge states across different Fermi level energies in anatase and in rutile, it was found that the cationic dopants are most stable in Ti substitutional lattice positions while formation energy is minimised for F^? doping in interstitial positions. All dopants were found to considerably stabilise anatase relative to the rutile phase, suggesting the anatase to rutile phase transformation is inhibited in such systems with the dopants ranked F?>?Si?>?Fe?>?Al in order of anatase stabilisation strength. Al and Fe dopants were found to act as shallow acceptors with charge compensation achieved through the formation of mobile carriers rather than the formation of anion vacancies.     

Ab initio study of structural,elastic and vibrational properties of praseodymium chalcogenides

The plane-wave pseudopotential approach to the density-functional theory within the GGA approximation implemented in Vienna Ab-initio Simulation Package (VASP) code have been used to calculate structural, elastic and lattice dynamical properties of praseodymium chalcogenides: PrS, PrSe and PrTe. The structural properties are performed in five different crystal structures: NaCl (space group Fm3m(225)), CsCl (space group Pm3m(221)), ZB (space group F43m(216)), WC (space group P6m2(187)) and CuAu (P4/mmm(123)). Our results showed that these chalcogenides are stable in NaCl phase which are in agreement with the experimental works. The phonon dispersion curves are showed that the gap between the optic and acoustic branches decreases on going from PrS to PrTe. We have also estimated some thermodynamical properties such as entropy and heat capacity at the temperature range 0–1000 K.     

Ab initio vibrational and thermal properties of AlN nanowires under axial stress

The vibrational and thermal properties of small diameter AlN nanowires are investigated using first principles calculations. Upon applying external pressure, the nanowires suffer a stress induced phase transition from a würtzite (WZ) to a graphite-like (GL) phase. The thermal conductance displays a nearly identical behavior for all systems in the temperature regime governed by acoustical modes, while at higher temperatures the conductance is systematically enhanced for nanowires in the GL phase. The heat capacity points out the different phonon group velocities for the acoustical modes in the two structural configurations. Our DFT-based calculations are consistent with experimental data for bulk AlN, in terms of phonon spectrum and temperature dependent heat capacity in the large nanowire limit.     

Ab initio prediction of molecular crystal structures

It is presently not possible to predict the structure(s)_of molecular crystals from the given structure of a single, general molecule. However, various computational methods are available to generate a set of possible crystal structures and it is likely that this set will contain the experimentally observed structure(s). The usually corrected ranking of the generated structures (with respect to the probability of occurrence) presumably is not mainly due to inaccuracies of the evaluation of energy. It appears that a full solution to the problem of molecular crystal structure prediction requires the (at least partial) inclusion of kinetic and thermodynamic effects.     

Elastic, optoelectronic, and thermal properties of cubic CSi2N4: an ab initio study

The mechanical, optoelectronic, and thermodynamic properties of carbon silicon nitride spinel compound have been investigated using density functional theory. The exchange–correlation potential was treated with the local density approximation (LDA) and the generalized gradient approximation of Perdew–Burke and Ernzerhof (PBE-GGA). In addition, the Engel–Vosko generalized gradient approximation (EV-GGA) and the modified Becke–Johnson potential (TB-mBJ) were also applied to improve the electronic band structure calculations. The ground state properties, including lattice constants and bulk modulus, are in fairly good agreement with the available theoretical data. The elastic constants, Young’s modulus, shear modulus, and Poisson’s ratio have been determined by using the variation of the total energy with strain. From the elastic parameters, it is inferred that this compound is brittle in nature. The results of the electronic band structure show that CSi₂N₄ has a direct energy band gap (Γ–Γ). The TB-mBJ approximation yields larger fundamental band gaps compared to those of LDA, PBE-GGA, and EV-GGA. In addition, we have calculated the optical properties, namely, the real and the imaginary parts of the dielectric function, refractive index, extinction coefficient, reflectivity, and energy loss function for radiation up to 40.0 eV. Using the quasi-harmonic Debye model which considers the phononic effects, the effect of pressure P and temperature T on the lattice parameter, bulk modulus, thermal expansion coefficient, Debye temperature, and the heat capacity for this compound were investigated for the first time.     

Ab initio insights into the visible luminescent properties of ZnO

The luminescence spectrum of ZnO exhibits, besides a UV band-edge recombination line, a broad visible band around 2.2-2.4 eV whose origin has not been satisfactorily established. Recently, analysis of the luminescence of nanostructured materials with high surface-to-volume ratios has led some authors to suggest that the band could be related to surface states. This work presents a novel ab initio study of the most relevant ZnO surfaces and their intrinsic point defects. It reveals the existence of intragap surface states 0.5 eV above the valence band maximum. If additional bulk defect levels are considered, several bulk-to-surface transitions are compatible with the observed visible luminescence.     

Ab initio prediction of new 3D-like phases ThCuSiAs, ThCuGeAs and their structural, mechanical, and electronic properties

New 1111-like quaternary tetragonal Th-based phases: silicide arsenide ThCuSiAs and germanide arsenide ThCuGeAs are proposed in this article. The stabilities of these materials are verified, and the value of their structural, elastic (elastic constants, bulk, shear, and Young’s moduli, compressibility, Pugh’s indicator, Poisson’s ratio, indexes of elastic anisotropy), electronic (densities of electronic states, electronic heat capacity, molar Pauli paramagnetic susceptibility, and the so-called metallicity indexes), and some other properties (Vickers hardness and melting temperatures) are predicted from the first principles calculations. The proposed phases are the first Th-based oxygen-free 1111-like materials, and their syntheses seem very attractive to expand the rather rare group of non-conventional quasi-three-dimensional 1111-like materials.     

Ab initio study of the structural, elastic, electronic and optical properties of the antiperovskite SbNMg3

A theoretical study of structural, elastic, electronic and optical properties of the cubic antiperovskite SbNMg₃ is presented using the pseudo-potential plane wave method (PP-PW) within the generalized gradient approximation (GGA). Results are given for lattice constant, elastic constants and their pressure dependence. Band structure, density of states and pressure coefficients of energy gaps are also given. Furthermore, the optical reflectivity, refractive index, extinction coefficient, dielectric function and electron energy loss are calculated for radiation up to 30 eV. The results are compared with the available theoretical and experimental data.     

Ab initio calculation of the structural and dynamical properties of layered semiconductors

We present a theoretical first-principles investigation of the structure and lattice dynamics of several layered semiconductors. The equilibrium structure as obtained by minimization of the total energy of the bulk materials is in good agreement with experiment. Furthermore, we have investigated the surface of these materials in order to obtain information on the van der Waals epitaxial growth. We found that the relaxed atomic positions at the surface deviate from the ideal ones in the bulk by less than 1%, which is obviously a consequence of the weak interlayer forces. Additionally, bulk phonon-dispersion curves have been calculated along several high symmetry directions within the density-functional perturbation theory (DFPT). The weak interlayer interaction makes the vibrational properties of the bulk very similar to those of the surface. In fact, our ab initio calculations for the bulk reproduce well both the experimental bulk phonon frequencies obtained by inelastic neutron scattering and the experimental surface phonon dispersion measured with inelastic He-atom scattering (HAS).     

二维二轴编织复合材料几何模型及弹性性能预测

提出了二维二轴1×1和2×2编织复合材料的几何模型,模型考虑了纤维束的相互挤压及横截面的变化。基于细观分析和体积平均法,建立了预测二维二轴编织复合材料弹性性能的理论分析方法。数值结果与试验结果吻合,表明该方法行之有效,且具有运算快、精度高、适合工程分析等优点。分析了编织角、纤维体积含量和纤维束横截面形状对材料弹性常数的影响。研究表明,编织角对弹性常数的影响具有互补性,材料弹性模量与纤维体积含量成正比,纤维束截面形状变化对材料弹性常数影响不大。     

Ab initio calculations of the hydroxyl impurities in BaF2

OH^? impurities in BaF₂ crystal have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. Three different configurations of OH^? impurities were investigated and the (1 1 1)-oriented OH^? configuration is the most stable one. Our calculations show that OH^? as an atomic group has a steady geometrical structure instead of electronic properties in different materials. The studies on band structures and density of states (DOS) of the OH^?-impurity systems indicate that there are two defect levels induced by OH^? impurities. The two superposed occupied OH^?-bands located 1.95 eV above the valance bands (VB) at Γ point mainly consist of the O p orbitals, and the H s orbitals do the major contribution to the empty defect level located 0.78 eV below the conduction bands (CB). The optical absorption due to the doped OH^? is centered around 8.61 eV.     

First principles prediction of structural stability,elastic, lattice dynamical and thermal properties of osmium carbides

Abstract

First principles calculations are performed to investigate the structural stability, elastic, lattice dynamical and thermal properties of osmium carbides with various crystal structures. Our calculation indicates that the I₄Te type structure is energetically the most favourable for Os₄C. Based on stress–strain relationships, elastic constants are obtained, and the relevant mechanical properties are also discussed. The phonon dispersion relation and the dynamical stability are also predicted. We have found that the predicted structures are mechanically stable as well as dynamically stable except for cubic-Os₄C. Through the quasi-harmonic Debye model, the temperature and pressure effects on the bulk modulus, thermal expansion coefficient, heat capacity, Grüneisen parameter and Debye temperature are presented.     

Ab initio study of electronic,magnetic and optical properties of CuWO4 tungstate

Ab initio calculations based on the density-functional theory have been employed to study the electronic and magnetic properties of copper tungstate CuWO₄, as well as its optical characteristics in the ultraviolet region, up to 40 eV. The electronic structure around the band gap is dominated by the O p- and the Cu d-states and it is quite different from the recent spin-restricted calculations of the same compound. The most stable antiferromagnetic state and the values of magnetic moments at Cu sites and O(3) oxygen atomic sites (closest to Cu²⁺ ions in zigzag antiferromagnetic chains) are in agreement with experiments. The gap is found to be indirect with the acceptable value only after use of the LDA + U rotationally invariant self-consistent full potential linearized augmented plane wave (FP LAPW) approach. The optical spectra are analyzed, compared, and interpreted in terms of calculated band structures. It is shown that absorption process involves significant energy flow from the O ions to the Cu ions.     

Ab initio Calculations of Magnetic Properties of Fe16N2

Pseudo-potential and plane wave basis-set under the framework of density functional theory have been employed to study the electronic and magnetic properties of Fe162, and to have an insight look on the subject of giant magnetic moments reported in Fe16N2. After geometrical optimization, band structures and densities of states have been evaluated together with the atom resolved band populations and magnetic moments. In this paper, we report a theoretical effort to look into the various aspects of the magnetic properties of Fe16N2, including volume effect and distortion effect.     

TiO_2纳米管阵列热驱动掺银制备TiO_2-Ag复合涂层及其光催化性能

无定形结构TiO2纳米管阵列(titania nanotube arrays,TNA)在浸泡AgNO3溶液后,再经适当的热处理,不仅可使TNA的晶体结构转变为锐钛矿型或金红石型,还可以制得海绵状TiO2-Ag复合涂层。深入研究了AgNO3溶液浓度、热处理温度等对TiO2-Ag复合涂层结构和光催化制氢性能影响的影响。研究结果表明,AgNO3溶液浓度对TiO2-Ag复合涂层形貌结构和晶体结构具有重要影响。当AgNO3溶液浓度为0.1~0.5 mol/L、热处理温度为500℃时,可制得海绵状TiO2-Ag复合涂层,该涂层在紫外-可见光光照条件下,光催化制氢速率为2.14μmol/cm2·h。     

Ab initio investigation of Al/Mo2B interfacial adhesion

First-principles calculations were performed to study the adhesion and the interfacial electronic structure of aluminum/molybdenum semi-boride (Al/Mo₂B) interface. The work of adhesion (W_ad) was calculated for both terminations of the Mo₂B surface and it was found that Mo-terminated has larger W_ad than the B-terminated one. It was shown that interfacial Al and B atoms form polar covalent bonds, while bonding of interfacial Al and Mo atoms mainly presents metallic character.     

Ab initio simulation of magnetic tunnel junctions

In this paper, we present the mathematical and implementation details of an ab initio method for calculating spin-polarized quantum transport properties of atomic scale spintronic devices under external bias potential. The method is based on carrying out density functional theory (DFT) within the Keldysh non-equilibrium Green's function (NEGF) formalism to calculate the self-consistent spin densities. We apply this method to investigate nonlinear and non-equilibrium spin-polarized transport in a Fe/MgO/Fe trilayer structure as a function of external bias voltage.