0.5NdAlO3-0.5CaTiO3电子结构及光学性质的第一性原理计算

采用基于第一性原理密度泛函理论的平面波赝势方法,对0.5NdAlO₃-0.5CaTiO₃晶体进行结构优化,并对其能带结构,态密度和光学性质进行了理论计算.结构优化后晶格参数与实验数据相符合,误差小于1%;能带计算结果表明0.5NdAlO₃-0.5CaTiO₃为间接带隙,带隙值为0.52eV;费米面附近的能带由Nd-4f,O-2p,Nd-4p,Al-3p,Ti-4d层的电子态密度确定.同时也计算了该结构的介电函数,反射率和复折射率等光学性质.     

Electronic structure of AgNbO3 and NaNbO3 studied by X-ray photoelectron spectroscopy

The XPS examinations of the AgNbO₃ and NaNbO₃ single crystals and ceramics allowed estimate their average composition as Ag_1.1Nb_0.9O₃ and Na_1.2Nb_0.9O_2.9. The valence bands of the AgNbO₃ compound, formed mainly of the Nb 4d, Ag 4d and O 2p states, show an energy gap about 3 eV while for the NaNbO₃ compound consist of the O 2p states hybridized with the Nb 3d states and show an energy gap about 4 eV. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds. The broadening of the core level lines of AgNbO₃ suggests a stronger structural disorder in comparison with NaNbO₃ compound.     

First principle study of cubic perovskites: AgTF3 (T=Mg, Zn)

The structural, electronic and optical properties of AgTF₃ (T=Mg, Zn) are calculated for the first time using the full-potential linearized augmented plane wave method within the generalized gradient approximation. Structural parameters of the compounds are found to be in reasonable agreement with the available literature. Both compounds are found to have narrow and indirect band-gaps. The calculated band gap for AgMgF₃ is 0.78 eV and 0.75 eV for AgZnF₃. It is observed that Ag-4d, Zn-3d and Ag-5s states controls the electronic properties of AgMgF₃ and AgZnF₃. The nature of chemical bonding in these compounds is discussed by the electron density plots. The results of complex dielectric constant, refractive index, normal-incidence reflectivity and optical conductivity are also presented in the incident photon energy range of 0-35 eV. The wide absorption energy range makes these materials suitable for different devices applications.     

First-principles study of the structural, elastic, electronic and optical properties of the monoclinic BiScO3

The structural, elastic, electronic and optical properties of the monoclinic BiScO₃ are investigated in the framework of the density functional theory. The calculated structural parameters are in agreement with the experimental values. Moreover, the structural stability of BiScO₃ system has been confirmed by the calculated elastic constants. The band structure, density of states, charge transfers and bond populations are also given. The results indicate that BiScO₃ has a direct band gap of 3.36 eV between the occupied O 2p states and unoccupied Bi 6p states, and its bonding behavior is a combination of covalent and ionic nature. Finally, the absorption spectrum, refractive index, extinction coefficient, reflectivity, energy-loss function and dielectric function of the monoclinic BiScO₃ are calculated. In addition, the variation of the static dielectric constants ε₁(0) as a function of pressure for BiScO₃ is also discussed.     

钴磷族化合物BaT2P2和BaT2As2(T=Co,Rh, Ir) 的电子结构

基于密度泛函理论(DFT)在广义梯度(GGA)下计算了钴磷族化合物BaT₂P₂和BaT₂As₂(T=Co, Rh, Ir)的电子结构.研究发现在BaCo₂P₂和BaCo₂As₂中,由于范霍夫畸点位于费米面附近使得费米能级处的态密度非常高,从而导致由斯通纳机理引起的巡游铁磁不稳定性.在从Co到 关键词： 电子结构 范霍夫畸点 斯通纳不稳定性     

Ab initio study of the structural,electronic and elastic properties of AgSbTe2, AgSbSe2, Pr3AlC,Ce3AlC,Ce3AlN,La3AlC and La3AlN compounds

In this paper, we study the structural, electronic and elastic properties of the ternary AgSbTe₂, AgSbSe₂, Pr₃AlC, Ce₃AlC, Ce₃AlN, La₃AlC and La₃AlN compounds using the full-potential linearized augmented plane wave (FP-LAPW) scheme and the pseudopotential plane wave (PP-PW) scheme in the frame of generalized gradient approximation (GGA). Results are given for the lattice parameters, bulk modulus, and its pressure derivative. The calculated lattice parameters are in good agreement with experimental results. We have determined the full set of first-order elastic constants, shear modulus, Young's modulus and Poisson's ratio of these compounds. Also, we have presented the results of the band structure, densities of states, it is found that this compounds metallic behavior, and a negative gap Г→R for Pr₃AlC. The analysis charge densities show that bonding is of covalent–ionic and ionic nature for AgSbSe₂ and AgSbTe₂ compounds.     

First-principles study of the structural, electronic, and magnetic properties of InCCo3 and InNCo3

Spin-polarized calculations were performed to investigate the structural, elastic, electronic, and magnetic properties of InCCo₃ and InNCo₃. The deviation of our calculated lattice parameters and equilibrium volume from experimental results is less than 0.8% and 2.5%, respectively. The obtained values of elasticity moduli C_ij, bulk modulus B, and shear modulus G are discussed. From the calculated band structure and the total and partial densities of states, we have concluded that these compounds are electrical conductors; moreover, they are bonded by a mixture of covalent, ionic, and metallic bonds. Our calculations show that InCCo₃ has nonmagnetic properties, while InNCo₃ could have a magnetic behaviour, with an average magnetic moment about 0.54 μ_B/atom, which is mostly derived from d electrons of the cobalt atoms in the energy range from −5 eV up to the Fermi level.     

Orbital-decomposed electronic and magnetic properties of the double perovskite Sr2FeReO6

The detailed orbital-decomposed electronic structures and magnetic properties of the double perovskite Sr₂FeReO₆ have been studied using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA). Both occupied and unoccupied s and three p states of Fe³⁺ ion are located far away from the Fermi level, while all up-spin states and most down-spin states are completely filled for the s and three p states of Re⁵⁺ ion. The octahedral crystal field of the oxygen atoms around transition-metal (TM) sites splits the five-fold degenerate d states of the free TM atoms into triply degenerate t_2g states with smaller bonding-antibonding splitting and doubly degenerate e_g states with larger bonding-antibonding splitting. The Fe³⁺ and Re⁵⁺ ions are in the states (3d⁵, S=5/2) and (5d², S=1) with magnetic moments 3.70 and −0.86μ_B, respectively and thus antiferromagnetic coupling via oxygen between them. There are no direct interactions between two nearest Fe-Fe or Re-Re pairs, whereas along each Fe-O-Re-O-Fe or Re-O-Fe-O-Re chains, the hybridizations between Fe 3d and 4s, O 2s and 2p, as well as Re 5p, 5d and 6s orbitals are fairly significant.     

The effect of cation doping on spinel LiMn2O4: a first-principles investigation

The effect of the cation doping on the electronic structure of spinel LiM_yMn_2−yO₄ (M=Cr, Mn, Fe, Co and Ni) has been calculated by first-principles. Our calculation shows that new M-3d bands emerge in the density of states compared with that in LiMn₂O₄. Simultaneously, the new O-2p bands appear accordingly in almost the same energy range around the Fermi energy owing to the M-3d/O-2p interaction. It is found that the appearance of new O-2p bands in the lower energy position results in a higher intercalation voltage. Consequently, the origin of higher intercalation voltage in LiM_yMn_2−yO₄ can be ascribed to the lower O-2p level introduced by the doping cation M.     

Structural, elastic, electronic and optical properties of a new layered-ternary Ta4SiC3 compound

We propose a new layered-ternary Ta₄SiC₃ with two different stacking sequences (α- and β-phases) of the metal atoms along c axis and study their structural stability. The mechanical, electronic and optical properties are then calculated and compared with those of other compounds M₄AX₃ (M=V, Nb, Ta; A=Al, Si and X=C). The predicted compound in the α-phase is found to possess higher bulk modulus than these compounds. The independent elastic constants of the two phases are also evaluated and the results discussed. The electronic band structures for α- and β-Ta₄SiC₃ show metallic conductivity. Ta 5d electrons are mainly contributing to the total density of states (DOS). We see that the hybridization peak of Ta 5d and C 2p lies lower in energy and the Ta 5d-C 2p bond is stronger than Ta 5d-Si 3p bond. Further an analysis of the different optical properties shows the compound to possess improved behavior compared to similar types of compounds.     

Elastic, electronic, and optical properties of hypothetical SnNNi3 and CuNNi3 in comparison with superconducting ZnNNi3

The elastic, electronic, and optical properties of MNNi₃ (M=Zn, Sn, and Cu) have been calculated using the plane-wave ultrasoft pseudopotential technique, which is based on the first-principle density functional theory (DFT) with generalized gradient approximation (GGA). The optimized lattice parameters, independent elastic constants (C₁₁, C₁₂, and C₄₄), bulk modulus B, compressibility K, shear modulus G, and Poisson's ratio υ, as well as the band structures, total and atom projected densities of states and finally the optical properties of MNNi₃ have been evaluated and discussed. The electronic band structures of the two hypothetical compounds show metallic behavior just like the superconducting ZnNNi₃. Using band structures, the origin of features that appear in different optical properties of all the three compounds has been discussed. The large reflectivity of the predicted compounds in the low energy region might be useful in good candidate materials for coating to avoid solar heating.     

BiFeO3中各离子在铁电相变中作用本质的第一性原理研究

采用基于密度泛函理论的广义梯度近似方法和赝势平面波法,对多铁材料BiFeO₃的铁电反铁磁相和可能的高温顺电相的电子结构进行了第一性原理研究.计算验证了BiFeO₃基态为G型反铁磁有序,Fe离子的理论磁矩与实验值相符.铁电相变发生后,Bi-6s态和6p态发生了电荷转移,Bi-6s电子的作用更加明显.Born有效电荷的研究表明铁电畸变主要表现为Bi原子的位移,并且电极化强度计算值很好地符合薄膜实验结果.部分态密度的计算表明Bi-6p态的成键轨道与反键轨道间的能量劈裂 关键词： 第一性原理 铁电性 铁电畸变 反铁磁性     

Syntheses and X-ray excited luminescence properties of Ba3BP3O12 , BaBPO5 and Ba3BPO7

Ba₃BP₃O₁₂, BaBPO₅ and Ba₃BPO₇ have been synthesized by the solid-state reaction method. The X-ray-excited luminescent properties of these compounds were investigated, all of these compounds show broad emission band with the peak center at about 400 nm. The room temperature fluorescent decay of these compound powders was measured. Compared to the other two compounds, Ba₃BPO₇ has the maximum emission intensity while Ba₃BP₃O₁₂ has the fastest decay time with 56.3 ns(99.7%). Combining with the X-ray excited luminescent spectra and fluorescent decay profiles of these compounds, it is supposed that all of these compounds should correspond to two different luminescent mechanisms; the luminescent mechanisms of these compounds were discussed. The potential applications of these compounds are pointed out.     

First-principles study of the structural properties and magnetism of R2CoIn8 (R=Y,Pr, Nd,and Dy) intermetallic compounds

The electronic structure of R₂CoIn₈ (R=Y, Pr, Nd, and Dy) intermetallic compounds is calculated from first principles based on the density functional theory (DFT). The Kohn–Sham single-particle equations of the DFT are solved using two independent computational methods, namely APW+lo and FPLO. First the structural properties of Y₂CoIn₈ are studied. Good agreement of calculated equilibrium volume and c/a ratio with the experimental data is found. Also we minimize the forces at equilibrium volume and calculate the symmetry-free structural parameters of the space group P4/mmm for Y₂CoIn₈. In Y₂CoIn₈, the Co 3d states are almost fully occupied and situated below the Fermi level. We applied the fixed-spin-moment method and a stable paramagnetic ground state for Y₂CoIn₈ was found. Finally, the crystal field (CF) parameters were calculated for R=Pr, Nd and Dy from first principles. The microscopic tetragonal CF Hamiltonian was diagonalized and the obtained eigenvalues and eigenfunctions were used to predict the anisotropy of the magnetic properties of R₂CoIn₈ single crystals.     

First-principles calculations of structural, electronic, optical and elastic properties of magnesite MgCO3 and calcite CaCO3

Detailed ab initio calculations of the structural, electronic, optical and elastic properties of two crystals - magnesite (MgCO₃) and calcite (CaCO₃) - are reported in the present paper. Both compounds are important natural minerals, playing an important role in the carbon dioxide cycling. The optimized crystal structures, band gaps, density of states diagrams, elastic constants, optical absorption spectra and refractive indexes dependence on the wavelength all have been calculated and compared, when available, with literature data. Both crystals are indirect band compounds, with calculated band gaps of 5.08 eV for MgCO₃ and 5.023 eV for CaCO₃. Both values are underestimated by approximately 1.0 eV with respect to the experimental data. Although both crystals have the same structure, substitution of Mg by Ca ions leads to certain differences, which manifest themselves in noticeable change in the electronic bands profiles and widths, shape of the calculated absorption spectra, and values of the elastic constants. Response of both crystals to the applied hydrostatic pressure was analyzed in the pressure range of phase stability, variations of the lattice parameters and characteristic interionic distances were considered. The obtained dependencies of lattice constants and calculated band gap on pressure can be used for prediction of properties of these two hosts at elevated pressures that occur in the Earth's mantle.     

Comparative study of the magnetism of SrTcO3 and Ca(Sr)MnO3

By the first-principles calculations, we studied the electronic structures and the magnetic properties of SrTcO₃ and Ca(Sr)MnO₃. We found the strikingly high Néel temperature of SrTcO₃ is mostly due to the strong Tc(4d)–O(2p) hybridizations, since the Tc-4d states are more extended than the Mn-3d states. Such Tc(4d)–O(2p) hybridizations increase the super-exchange constants, hence increased the Néel temperatures.     

Structural, electronic and optical properties of orthorhombic distorted perovskite TbMnO3

This paper calculates the structural parameters, electronic and optical properties of orthorhombic distorted perovskite-type TbMnO3 by first principles using density functional theory within the generalised gradient approximation. The calculated equilibrium lattice constants are in a reasonable agreement with theoretical and experimental data. The energy band structure, density of states and partial density of states of elements are obtained. Band structures show that TbMnO3 is an indirect band gap between the O 2p states and Mn 3d states, and the band gap is of 0.48 eV agreeing with experimental result. Furthermore, the optical properties, including the dielectric function, absorption coefficient, optical reflectivity, refractive index and energy loss spectrum are calculated and analysed, showing that the TbMnO3 is a promising dielectric material.     

Electronic structure of ZrTiO4 and HfTiO4: Self-consistent cluster calculations and X-ray spectroscopy studies

Total and partial densities of states of the constituent atoms of ZrTiO₄ and HfTiO₄ titanates have been calculated using a self-consistent cluster method as incorporated in the FEFF8 code. The calculations reveal the similarity of the electronic structure of both titanates and indicate that the valence band of the compounds under consideration is dominated by contributions of O 2p states. These states contribute throughout the whole valence-band region; however their maximum contributions occur in the upper portion of the band. Other significant contributors in the valence-band region are Ti 3d and Zr 4d states in ZrTiO₄ and Ti 3d and Hf 5d states in HfTiO₄. All the above d-like states contribute throughout the whole valence-band region of the titanates; however maximum contributions of the Ti 3d states occur in the upper portion, whilst those of the Zr 4d (Hf 5d) states are in the central portions of the valence band. The FEFF8 calculations render that the bottom of the conduction band of ZrTiO₄ and HfTiO₄ is dominated by contributions of Ti 3d^? states, with also smaller contributions of Zr 4d^?/Hf 5d^? and O 2p^? states. To verify the above FEFF8 data, the X-ray emission bands, representing the energy distributions of mainly O 2p, Ti 3d and Zr 4d states, were measured and compared on a common energy scale. These experimental data are found to be in agreement with the theoretical FEFF8 results for the electronic structure of ZrTiO₄ and HfTiO₄ titanates. Additionally, X-ray photoelectron valence-band and core-level spectra were recorded for the constituent atoms of the titanates under study.     

Electronic structure of HgGa2S4

The electronic structure and chemical bonding in HgGa₂S₄ crystals grown by vapor transport method are investigated with X-ray photoemission spectroscopy. The valence band of HgGa₂S₄ is found to be formed by splitted S 3p and Hg 6s states at binding energies BE=3-7 eV and the components at BE=7-11 eV generated by the hybridization of S 3s and Ga 4s states with a strong contribution from the Hg 5d states. At higher binding energies the emission lines related to the Hg 4f, Ga 3p, S 2p, S 2s, Hg 4d, Ga LMM, Ga 3p and S LMM states are analyzed in the photoemission spectrum. The measured core level binding energies are compared with those of HgS, GaS, AgGaS₂ and SrGa₂S₄ compounds. The valence band spectrum proves to be independent on the technological conditions of crystal growth. In contrast to the valence band spectrum, the distribution of electron states in the bandgap of HgGa₂S₄ crystals is found to be strongly dependent upon the technological conditions of crystal growth as demonstrated by the photoluminescence analysis.     

三方和四方相PbZr0.5Ti0.5O3 的结构稳定性和电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理超原胞方法和虚晶近似方法, 在局域密度近似和广义梯度近似下系统研究了三方相和四方相 PbZr_0.5Ti_0.5O₃的能量稳定性、原子结构以及电子结构. 计算结果表明三方相的能量比四方相低, 说明三方相结构更加稳定, 并且发现利用广义梯度近似计算的结构参数与实验值符合得更好. 电子结构表明, 两种相的Ti/Zr的3d电子和O的2p电子间存在明显的轨道杂化, 并且Ti-O之间的作用比Zr-O作用更强;Pb的6s和5d电子与O的2s和2p电子也分别存在轨道杂化. 而三方相中Pb的5d电子与O的2s电子杂化比四方相更强, 进一步说明三方相比四方相结构更加稳定.