Ab initio calculations of Born charges in ferroelectrics with a perovskite structure

A method has been proposed for calculating Born effective charges in compounds with a cubic perovskite structure. The method is based on the first-principles calculation of individual contributions from the short-range interaction and the intercell dipole-dipole interaction to the Born tensor Z _ii (s) for crystalline dielectrics. It has been shown that the contribution from the short-range interaction Z _ii ^sr (s) to the Born tensor components can be derived from ab initio calculations performed for polyatomic clusters. The results of the calculations of the short-range interactions Z _ii ^sr (s) for the cubic phases of the BaTiO₃, SrTiO₃, CaTiO₃, PbTiO₃, BaZrO₃, PbZrO₃, KNbO₃, and KTaO₃ compounds with the use of the electronic structure calculations within the Hartree-Fock approximation and the density functional theory are presented. For the BaTiO₃, SrTiO₃, CaTiO₃, PbTiO₃, KNbO₃, and KTaO₃ compounds, the components of the complete Born tensor have been also calculated. The obtained values of Z _ii (s) are in good agreement with the results of the calculations in terms of the linear response theory and the Berry phase approach.     

Point defects in ferroelectrics with perovskite structure

First-principle calculations of the geometry and electronic structure of lattice defects (substituents, vacancies, and binary complexes) in perovskites are considered. A supercell method and modifications of a cluster method for simulating point lattice defects in insulators are discussed. Experimental data on dielectric relaxation in magnesium- and manganese-doped strontium titanate and possible mechanisms of the formation of reorientable polar defects are considered.     

Ab initio calculations of electronic structure of anatase TiO2

This paper presents the results of the self-consistent calculations on the electronic structure of anatase phase of TiO2. The calculations were performed using the full potential-linearized augmented plane wave method (FP-LAPW)in the framework of the density functional theory (DFT) with the generalized gradient approximation (GGA). The fullyoptimized structure, obtained by minimizing the total energy and atomic forces, is in good agreement with experiment.We also calculated the band structure and the density of states. In particular, the calculated band structure prefers an indirect transition between valence and conduction bands of anatase TiO2, which may be helpful for clarifying theambiguity in other theoretical works.     

Dielectric properties of perovskite ferroelectrics at the nonequilibrium concentrations of point defects

Dielectric properties of ceramic barium titanate modified by the addition of aluminum oxide are studied via impedance spectroscopy in weak electric fields in frequency range of 0.3–10³ kHz. For better separation of the properties of the inner regions of crystallites and the boundary regions by quenching, an excess concentration of point defects is created, leading to the formation of bulk charge. The experimental results are performed using the Maxwell-Wagner capacitor model and the Schottky barrier model.     

Ab initio calculation of intrinsic point defects in CuInSe2

Preliminary ab initio calculation of different point defects energy and electronic density of states have been performed on the prototype chalcopyrite semiconductor CuInSe₂. The simulation method used is based on the density functional theory within the framework of pseudo-potentials and plane waves basis. The isolated neutral defects considered are: V_Cu, V_In, V_Se, Cu_i, In_i, In_Cu, Cu_In and the complex defects are 2Cu_i+Cu_In, In_Cu+Cu_In and 2V_Cu+In_Cu, some of which being computed for the first time by advanced ab initio techniques. In agreement with previous results, we show that some point defects (such as V_Cu) and pair defects (2V_Cu+In_Cu) have very low formation energies. Some energies of formation were found significantly lower than previous estimations. The comparison of the formation energies with the exchange correlation (LDA or GGA) is discussed. The perturbation induced by the presence of some of these ideal defects on the density of states is also presented.     

Ab initio calculations of yttrium nitride: structural and electronic properties

Using first principles total energy calculations within the full-potential linearized augmented plane wave method, we have studied the structural and electronic properties of yttrium nitride (YN) in the three phases, namely wurtzite, caesium chloride and rocksalt structures. The calculations are performed at zero and under hydrostatic pressure. In agreement with previous findings, it is found that the favored phase for YN is the rocksalt-like structure. We predict that at zero pressure YN in the rocksalt structure is a semiconductor with an indirect bandgap of 0.8 eV. A phase transition from a rocksalt to a caesium chloride structure is found to occur at ∼134 GPa. Besides, a transition from an indirect (Γ−X) bandgap semiconductor to a direct (X−X) one is predicted at pressure of ∼84 GPa. For the electron effective mass of rocksalt YN, these are the first results, to our knowledge. The information derived from the present study may be useful for the use of YN as an active layer in electronic devices such as diodes and transistors.     

Ab initio investigation of the electronic structure of AgCl

The structural and electronic behavior of a prototype silver halide, namely AgCl, has been investigated by means of the full-potential linearized augmented plane wave method with the local density approximation. The aim of our study is to focus on the influence of the d metal orbitals through a detailed analysis of the electronic energies. The presence the d Ag orbitals leads to a strong hybridization between them and the p halogen ones, giving rise to specific electronic states organization and charge distribution. As expected from its rock-salt structure, AgCl is as an ionic compound characterized by a mixed p–d top valence band.     

Effect of pressure on the atomic and electronic structure of enstatite MgSiO3: Ab initio calculations

Simulation results for the effect of pressure on the atomic and electronic structure of MgSiO₃ are presented. They are in good agreement with experimental data. It is shown that, when the pressure rises from 0 to 2 GPa, the bandgap increases from 4.67 to 4.74 eV because of the electron charge redistribution between atoms in the enstatite structure.     

Ab initio calculations of electronic and optical properties in O-doped LiF crystal

We present a first-principle study of electronic and optical properties in pure LiF and O-doped LiF crystals. The pure LiF crystal exhibits a wide band gap while the O-doped LiF crystal shows the less band gap due to the contribution of O 2p. Some optical constants, such as dielectric functions, reflectivity and the refractive index, have been performed. The calculated reflectivity and refractive index from the pure LiF crystal agree with the experimental and recently calculated results in the low-energy range. Meanwhile, the optical properties have also been predicted from the O-doped LiF crystal. The absorption band in 200 nm has been observed, which is relatively close to the experimental result.      

Ab initio study of electronic structure of strained

Micro-hardness and scratch adhesion testing are the most commonly used techniques for assessing the mechanical properties of thin films. Both of these testing methods utilize single-point contact and induce plastic deformation in the substrate and film. However, the influence of adhesion on the measured hardness has been seldom reported so far. In our experiments, diamond-like carbon (DLC) and silicon carbide (SiC) films deposited on silicon and nickel-based alloy substrates by pulsed laser ablation were indented and scratched by a Vickers micro-hardness tester and a diamond-cutter, respectively. It was found that the composite hardness decreased more rapidly for poor adhesion when increasing the indentation load. The result was explained by the elastic-plastic deformation mode of indentation and helped us to understand the physical meaning of one parameter commonly introduced in the models used to separate film hardness from the composite hardness. Received 30 June 1998     

B80 fullerene: an Ab initio prediction of geometry, stability, and electronic structure

The geometry, electronic, and structural properties of an unusually stable boron cage made of 80 boron atoms are studied, using ab initio calculations. The shape of this cluster is very similar to that of the well-known C60 fullerene, but in the B80 case, there is an additional atom in the center of each hexagon. The resulting cage preserves the Ih symmetry, has a relatively large highest occupied and lowest unoccupied energy gap ( approximately 1 eV) and, most importantly, is energetically more stable than boron double rings, which were detected in experiments and considered as building blocks of boron nanotubes. To our knowledge, this is the most stable boron cage studied so far.     

Ab initio calculations on oxygen vacancy defects in strained amorphous silica

The effects of uniaxial tensile strain on the structural and electronic properties of positively charged oxygen vacancy defects in amorphous silica(a-SiO2)are systematically investigated using ab-initio calculation based on density functional theory.Four types of positively charged oxygen vacancy defects,namely the dimer,unpuckered,and puckered four-fold(4×),and puckered five-fold(5×)configurations have been investigated.It is shown by the calculations that applying uniaxial tensile strain can lead to irreversible transitions of defect structures,which can be identified from the fluctuations of the curves of relative total energy versus strain.Driven by strain,a positively charged dimer configuration may relax into a puckered 5×configuration,and an unpuckered configuration may relax into either a puckered 4×configuration or a forward-oriented configuration.Accordingly,the Fermi contacts of the defects remarkably increase and the defect levels shift under strain.The Fermi contacts of the puckered configurations also increase under strain to the values close to that of Eα′center in a-SiO2.In addition,it is shown by the calculations that the relaxation channels of the puckered configurations after electron recombination are sensitive to strain,that is,those configurations are more likely to relax into a two-fold coordinated Si structure or to hold a puckered structure under strain,both of which may raise up the thermodynamic charge-state transition levels of the defects into Si band gap.As strain induces more puckered configurations with the transition levels in Si band gap,it may facilitate directly the development of oxide charge accumulation and indirectly that of interface charge accumulation by promoting proton generation under ionization radiation.This work sheds a light on understanding the strain effect on ionization damage at an atomic scale.     

Ab initio many-body treatment of the electronic structure of metals

We propose and apply a combination of an ab initio (band-structure) calculation with a many-body treatment including screening effects. We start from a linearized muffin-tin orbital (LMTO) calculation to determine the Bloch functions for the Hartree one-particle Hamiltonian, from which we calculate the static susceptibility and dielectric function within the standard random phase approximation (RPA). From the Bloch functions we obtain maximally localized Wannier functions, using a method proposed by Marzari and Vanderbilt. Within this Wannier basis all relevant one-particle and unscreened and screened Coulomb matrix elements are calculated. This yields a multi-band Hamiltonian in second quantization with ab initio parameters, for which screening has been taken into account within the simplest standard approximation. Then, established methods of many-body theory are used. We apply this concept to a simple metal, namely lithium (Li). Here the maximally localized Wannier functions turn out to be of the sp³-orbital kind. Furthermore, only the on-site contributions of the screened Coulomb matrix elements are relevant, and a generalized, four-band Hubbard model is justified. The screened on-site Coulomb matrix elements are considerably smaller than the band width because of which it is sufficient to calculate the selfenergy in weak-coupling approximation. We compare results obtained within the screened Hartree-Fock approximation (HFA) and within the second-order perturbation theory (SOPT) in the Coulomb matrix elements for Li and find that many-body effects are small but not negligible even for this simple metal.     

非晶碳结构建模和电子结构的第一性原理研究

基于密度泛函理论，采用了一种更为精确的交换相关泛函OLYP(OPTX+LYP)，对密度范围从2.0到3.2 g/cm³的非晶碳进行结构建模. 模拟得到的5个碳网络结构无论从径向分布函数还是sp³含量都与实验符合得很好. 对非晶碳电子结构的研究表明费米能级附近的电子态密度主要是sp²碳原子的贡献. 随着密度的增加，sp³碳原子增加，费米能级附近的态密度越来越小. 小环结构增加了费米能级附近的电子态密度，缺陷态在费米能级形     

非晶碳结构建模和电子结构的第一性原理研究

基于密度泛函理论,采用了一种更为精确的交换相关泛函OLYP(OPTX+LYP),对密度范围从2.0到3.2 g/cm³的非晶碳进行结构建模. 模拟得到的5个碳网络结构无论从径向分布函数还是sp³含量都与实验符合得很好. 对非晶碳电子结构的研究表明费米能级附近的电子态密度主要是sp²碳原子的贡献. 随着密度的增加,sp³碳原子增加,费米能级附近的态密度越来越小. 小环结构增加了费米能级附近的电子态密度,缺陷态在费米能级形 关键词： 非晶碳 密度泛函理论 电子结构     

Li_2NH晶体结构建模和电子结构的第一性原理研究

基于密度泛函理论的赝势-平面波方法,采用三种模型分别研究Li,N原子对N—H键方位的影响以及N—H键间的相互影响,得到了Li2NH的晶体结构和H原子的占位位置.计算结果表明:Li2NH晶体为层状结构,空间群为P42,晶胞中4个N—H键分为两层,层内N—H键为反平行排列,层间N—H键为垂直排列.态密度和电子局域函数(ELF)分析表明,N—H键呈明显的共价键特性,Li和N—H键呈明显的离子键相互作用.可逆储氢反应Li2NH+H2/LiNH2+LiH在温度0K时的反应焓为69.6kJ/molH2,与实验结果66kJ/molH2符合得较好.     

Ab initio study of oxygen point defects on tungsten trioxide surface

The gas response of tungsten trioxide (WO3) based sensors strongly depends on the surface properties. Reconstructed surfaces and oxygen point defects at the surface of the monoclinic WO3 are studied using a self-consistent scheme based on first-principle. The oxygen vacancy is found to be the predominant defect independently of the oxygen partial pressure. Indeed, under rich oxygen atmosphere the formation enthalpies are found to be 1.45 eV in LDA (1.28 eV in GGA) for the oxygen vacancy instead of 2.70 eV (2.42 eV) for the oxygen adatom. When the oxygen partial pressure is lowered, the oxygen vacancy formation enthalpy decreases and becomes exothermic under very O-poor condition (? 1.65 eV in LDA and ? 1.36 eV in GGA). On the other hand, the formation enthalpy of an oxygen adatom rises. Finally, the oxygen vacancy formation acts as a n-doping by introducing negative charge carriers at the bottom of the conduction band. All these results can be very helpful in order to explain the electrical resistivity measurements.