First principles study of electronic structure and optical properties of CaTiO3

The electronic-energy band structure, site and angular momentum decomposed density of states (DOS) and charge-density contours of perovskite CaTiO ₃ are calculated by the first principles tight-binding linear muffin-tin orbitals method with atomic sphere approximation using density functional theory in its local density approximation. The calculated band structure shows an indirect (R-Γ) band gap of 1.5 eV. The total DOS as well as the partial density of states (PDOS) are compared with the experimental photoemission spectra. The calculated DOS are in reasonable agreement with the experimental energy spectra and the features in the spectra are interpreted by a comparison of the spectra with the PDOS. The origin of the various experimentally observed bands have been explained. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Ca and TiO ₃ is mainly ionic and that the TiO ₃ entities bond covalently. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of CaTiO ₃ . The real and imaginary parts of the dielectric function and hence the optical constants such as refractive index and extinction coefficient are calculated. The calculated spectra are compared with the experimental results for CaTiO ₃ and are found to be in good agreement with the experimental results. The effective number of electrons per unit cell participating in the interband transitions are calculated. The role of band structure calculation as regards the optical properties of CaTiO ₃ is discussed. Received 1 February 2000 and Received in final form 21 July 2000     

Ab initio calculations study of the electronic, optical and thermodynamic properties of NaMgH3, for hydrogen storage

The structural stability, electronic structure, optical and thermodynamic properties of NaMgH₃ have been investigated using the density functional theory. Good agreement is obtained for the bulk crystal structure using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy. It is found from the electronic density of states (DOS) that the valence band is dominated by the hydrogen atoms while the conduction band is dominated by Na and Mg empty states. Also, the DOS reveals that NaMgH₃ is a large gap insulator with direct band gap 3.4 eV. We have investigated the optical response of NaMgH₃ in partial band to band contributions and the theoretical optical spectrum is presented and discussed in this study. Optical response calculation suggests that the imaginary part of dielectric function spectra is assigned to be the interband transition. The formation energy for NaMgH₃ is investigated along different reaction pathways. We compare and discuss our result with the measured and calculated enthalpies of formation found in the literature.     

Electronic structure of wurtzite and zinc-blende AlN

The electronic structure of AlN in wurtzite and zinc-blende phases is studied experimentally and theoretically. By using X-ray emission spectroscopy, the Al 3p, Al 3s and N 2p spectral densities are obtained. The corresponding local and partial theoretical densities of states (DOS), as well as the total DOS and the band structure, are calculated by using the full potential linearized augmented plane wave method, within the framework of the density functional theory. There is a relatively good agreement between the experimental spectra and the theoretical DOS, showing a large hybridization of the valence states all along the valence band. The discrepancies between the experimental and theoretical DOS, appearing towards the high binding energies, are ascribed to an underestimation of the valence band width in the calculations, or to extra states in the optical and ionic gaps due to the presence of point defects or impurities. Differences between the wurtzite and zinc-blende phases are small and reflect the slight variations between the atomic arrangements of both phases.Received: 25 October 2004, Published online: 23 December 2004PACS: 78.70.En X-ray emission spectra and fluorescence - 71.20.Nr Semiconductor compounds - 71.15.Mb Density functional theory, local density approximation, gradient and other corrections     

First principles study of the electronic and optical properties of SbTaO4

The electronic density of states (DOS), band structure and optical properties of orthorhombic SbTaO₄ are studied by first principles full potential-linearized augmented plane wave (FP-LAPW) method. The calculation is done in the framework of density functional theory with the exchange and correlation effects treated using generalized gradient approximation (GGA). We find an indirect band gap of 1.9 eV at the R→Γ symmetry direction of the Brillouin zone in SbTaO₄. It is observed that there is a strong hybridization between Ta-5d and O-2p electronic states which is responsible for the electronic properties of the system. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of SbTaO₄. The real and imaginary parts of the dielectric function of SbTaO₄ are calculated, which correspond to electronic transitions from the valence band to the conduction band. The band gap obtained is in close agreement with the experimental data.     

Valence band densities of states of CdIn2S4 and In2S3 from x-ray photoelectron spectroscopy

X-ray photoelectron spectra of the valence band of CdIn₂S₄ and In₂S₃ single crystals have been measured. The spectrum of CdIn₂S₄ has a strong resemblance to its synthesized spectrum from the In₂S₃ and CdS spectra, which is in good agreement with the theoretical density of states (DOS). The contribution of constituent atoms to the valence band DOS in CdIn₂S₄ is corresponding to those in In₂S₃ and CdS.     

Cd掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法,对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构,态密度以及分态密度的计算和比较,发现Cd_xZn_1-xO价带顶端(VBM)始终由O-2p占据;而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加,决定带隙宽度的CBM的位置下降,同时VBM的位置上升,从而导致了带隙的变窄,出现了红移现象. 此外,Cd掺杂会使晶胞发生膨胀,这种张应变也是导致Cd 关键词： 密度泛函理论 电子结构 Cd掺杂ZnO     

Cd掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法，对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构，态密度以及分态密度的计算和比较，发现Cd_xZn_1-xO价带顶端(VBM)始终由O-2p占据；而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加，决定带隙宽度的CBM的位置下降，同时VBM的位置上升，从而导致了带隙的变窄，出现了红移现象. 此外，Cd掺杂会使晶胞发生膨胀，这种张应变也是导致Cd     

Band structure and cluster model calculations of LaNiO3 compared to photoemission,O 1s X-ray absorption,and optical absorption spectra

We studied the electronic structure of LaNiO₃ using band structure and cluster model calculations. This compound is a paramagnetic metal with a R3?c rhombohedral structure. The band structure was calculated using the generalized gradient approximation (GGA). The cluster model was solved using the configuration interactions (CI) many-body method. We present results for the density of states (DOS), the spectral weight, and the dielectric function ${\epsilon }_2$. The calculations are compared to previous photoemission (PES), O 1s X-ray absorption (XAS), as well as optical absorption spectra. Both band structure and cluster model results are in good agreement with the experimental data. We point out that this concordance is very rare and far from trivial; we argue that this may be due to the unusual characteristics of the ground state of LaNiO₃.     

Electronic structure of CdTe probed by Cd and Te M4,5 X-ray emission spectra

We have investigated the bulk electronic structure of CdTe focusing on the Cd 5p and Te 5p valence states by X-ray emission spectroscopy (XES). Despite the very low fluorescence yields the Cd and Te M_4,5 (5p → 3d_3/2,5/2) spectra have been recorded successfully. A good correspondence has been found between the valence band XES and X-ray photoelectron spectra (XPS) by comparison on a common binding energy scale. We also performed a density functional theory calculation of the CdTe valence band, obtaining the Cd 4d, 5s, 5p and Te 5s, 5p local partial densities of states. The experimental Cd 5p and Te 5p derived from the X-ray emission spectra are in good agreement with the calculation. The intensity ratio of the Cd M_4,5 to the Te M_4,5 spectrum is obtained to be 0.25, in agreement with the ratio of the calculated Cd 5p to the Te 5p density of states in the CdTe upper valence band (0.22).     

高压下闪锌矿InN光电性质的密度泛函研究

 采用密度泛函理论,计算了闪锌矿型InN在压力下的结构、力学性质和光学性质,结果显示,随着压强的增大晶格常数减小。给出了零压下C₁₁、C₁₂、B、C_s、C₄₄的值及至70 GPa压力下弹性常数随压强的变化关系。结果表明,C₁₁、C₁₂、B随压强增大而增大,C_s、C₄₄随压强增大而减小,计算结果与现有实验和理论结果符合较好。在价带区,InN的分态密度（PDOS）有两个带,且在费米面附近密度很小,显示其倾向于形成稳定结构并且导电性较差。对闪锌矿型InN在高压下的光学性质研究发现,导带电子向高能方向偏移,而价带电子向低能方向偏移,结果导致能带间隙增大,光吸收谱在压力的作用发生了“蓝移”。研究结果对认识高压下闪锌矿型InN的结构、电学及光学性质具有重要意义。     

Photoemission valence band spectra and electronic density of states in copper oxides and copper based ceramic super-conductors

Photoemission valence band spectra with three different photon energies (21.2 eV, 40.8 eV, 1253.6 eV) for Cu₂O and CuO and a number of copper oxide based superconducting ceramics are investigated, namely La₂CuO₄ (LCO), Y₁Ba₂Cu₃O₇ (YBCO), Bi₂Sr₂CaCu₂O₈ (BSCCO) and Tl₂Ba₂Ca₂Cu₃O₁₀ (TBCCO). From the Cu₂O data it can be infered, that the He-II (40.8 eV) spectra give a fair representation of the density of states (DOS). In addition the agreement between the calculated density of states (DOS) and the He-II photoemission spectrum is almost perfect for Cu₂O. This agreement is worse for CuO and the other materials which all have a Cu groundstate close to a 3d ⁹ configuration indicating a large contribution of thed-d correlation energy to the excitation spectra which is absent in Cu₂O because of the filled 3d-shell. In all cases the experimental DOS atE _F is very small and only the spectra of BSCCO show a well defined Fermi edge. The relevance of these findings with respect to the theoretical local functional density DOS calculations is discussed.     

Properties and identification of oxygen sites at the V2O5(010) surface: theoretical cluster studies and photoemission experiments

Density functional theory cluster studies and angular resolved photoemission (ARUPS) measurements were performed to examine properties of differently coordinated surface oxygens at the V₂O₅(010) surface. Calculations on embedded clusters as large as V₁₆O₄₉H₁₈ confirm the ionic character of the oxide. The computed width of the O 2sp dominated valence band region of V₂O₅ and the work function value of V₂O₅ (010) are in good agreement with the present photoemission data for freshly cleaved V₂O₅(010) samples. Cluster derived total and partial densities of states (DOS, PDOS) can be used to identify differently coordinated surface oxygens. The PDOS referring to terminal (vanadyl) oxygens is localized near the center of the valence band whereas the PDOS’s of the different bridging oxygens yield a broad distribution covering the full energy range of the valence bands. The shape of the experimental ARUPS curves for V₂O₅(010) is well reproduced by the cluster DOS. Thus, the most prominent central peak in the experimental spectrum can be assigned to emission from terminal oxygen while the peripheral peaks at the top and bottom of the valence energy region are characterized as mixtures of vanadium with bridging oxygen induced contributions. This interpretation forms a basis to get insight into microscopic features at the real V₂O₅(010) surface such as imperfections and adsorbate binding. The present study suggests that the different O 2sp derived peaks observed in the photoemission experiment may be taken as monitors of the differently coordinated oxygens at the oxide surface and can be used to study details of catalytic surface reactions in which these oxygens participate.     

First-principles studies of the electronic structure and optical properties of AgBO<Subscript>3</Subscript> (B=Nb,Ta) in the paraelectric phase

The electronic energy-band structure, density of states (DOS), and optical properties of AgBO₃ in the paraelectric cubic phase have been studied by using density functional theory within the local density approximation for exchange-correlation for the first time. The band structure shows a band gap of 1.533 eV (AgNbO₃)and 1.537 eV (AgTaO₃)at (M-⌈)point in the Brillouin zone. The optical spectra of AgBO₃ in the photon energy range up to 30 eV are investigated under the scissor approximation. The real and imaginary parts of the dielectric function and — thus the optical constants such as reflectivity, absorption coefficient, electron energy-loss function, refractive index, and extinction coefficient — are calculated. We have also made some comparisons with related experimental and theoretical data that is available.      

Magnetic properties and electronic structure of GdNi4Si compound

Electronic structure of the ternary GdNi₄Si compound, crystallizing in hexagonal CaCu₅ structure (P6/mmm space group) was studied by magnetic measurements, X-ray photoelectron spectroscopy (XPS) and ab initio calculations. Core levels and valence band were investigated. The valence band of the XPS spectra is determined mainly by the Ni(3d) and Gd(4f) bands. The peaks’ positions are in good agreement with binding energies of a metallic gadolinium and nickel. The experimental valence band spectrum as well as the calculated density of states exhibit the domination of the Ni(3d) states in region from −4 eV to the Fermi level.     

Electronic structure and optical properties of RbPb2Br5

We report on density functional theory (DFT) calculations of the total and partial densities of states of rubidium dilead pentabromide, RbPb₂Br₅, employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package. The calculations indicate that the Pb 6s and Br 4p states are the dominant contributors to the valence band: their main contributions are found to occur at the bottom and at the top of the band, respectively. Our calculations reveal that the bottom of the conduction band is formed predominantly from contributions of the unoccupied Pb 6p states. Data of total DOS derived in the present DFT calculations are found to be in agreement with the experimental X-ray photoelectron valence-band spectrum of this compound. The predominant contributions of the Br 4p states at the top of the valence band of rubidium dilead pentabromide are confirmed by comparison on a common energy scale of the X-ray emission band representing the energy distribution of the valence Br p states and the X-ray photoelectron valence-band spectrum of the RbPb₂Br₅ single crystal. Main optical characteristics of RbPb₂Br₅, such as dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity are explored for RbPb₂Br₅ by the DFT calculations.     

Electronic structure and optical properties of In-doped SrTiO3 by density function theory

The effect of In doping on the electronic structure and optical properties of SrTiO3 is investigated by the first-principles calculation of plane wave ultra-soft pseudo-potential based on the density function theory （DFT）. The calculated results reveal that due to the hole doping, the Fermi level shifts into valence bands （VBs） for SrTi1-x InxO3 with x = 0.125 and the system exhibits p-type degenerate semiconductor features. It is suggested according to the density of states （DOS） of SrTi0.875In0.125O3 that the band structure of p-type SrTIO3 can be described by a rigid band model. At the same time, the DOS shifts towards high energies and the optical band gap is broadened. The wide band gap, small transition probability and weak absorption due to the low partial density of states （PDOS） of impurity in the Fermi level result in the optical transparency of the film. The optical transmittance of In doped SrTiO3 is higher than 85% in a visible region, and the transmittance improves greatly. And the cut-off wavelength shifts into a blue-light region with the increase of In doping concentration.     

Excitation energy dependence of SL2,3 X-ray fluorescent emission of BaNiS2 near the S 2p threshold

The results of measurements of SL_2,3 X-ray fluorescent spectra of BaNiS₂ near the S 2p threshold using tunable synchrotron radiation are presented. They are computed with FLAPW band structure calculations of BaNiS₂. The excitation energy dependence of the SL_2,3 spectra is found in the range of 163.5–173.5 eV which is attributed to excitation of inequivalent sulphur atoms (S(1) apical and S(2) in-plane sites). It allowed us to map separately the distribution of the S(2) and S(1)+S(2) 3s3d-partial density of states (DOS) in the valence band. We conclude that S 3d states participate in chemical bonding and hybridize with Ni 3d states.     

Structural,electronic and optical properties of cubic SrTiO3 and KTaO3: Ab initio and GW calculations

We present first-principles VASP calculations of the structural, electronic, vibrational, and optical properties of paraelectric SrTiO₃ and KTaO₃. The ab initio calculations are performed in the framework of density functional theory with different exchange-correlation potentials. Our calculated lattice parameters, elastic constants, and vibrational frequencies are found to be in good agreement with the available experimental values. Then, the bandstructures are calculated with the GW approximation, and the corresponding band gap is used to obtain the optical properties of SrTiO₃ and KTaO₃.     

A high-resolution X-ray photoemission study of the total valence-band densities of states of GaSe and BiI3

The total valence band denstiy of states spectra for the semiconducting layered compounds GaSe and BiI₃ were obtained by X-ray photoemission spectroscopy. The results for GaSe were used to test recent band structure calculations and are compared with earlier photoemission results. The BiI₃ data are the first experimental determination of the total valence band density of states for this compound. Since no calculations exist for BiI₃, tentative assignments were made.     

First-principles investigation of impurity concentration influence on bonding behavior,electronic structure and visible light absorption for Mn-doped BiOCl photocatalyst

We performed first-principles calculation to investigate the bonding behavior, electronic structure and visible light absorption of Mn_xBi_1−xOCl (x=0, 0.0625, 0.09375 and 0.125) using density functional theory (DFT) within a plane-wave ultrasoft pseudopotential scheme. The relaxed structural parameters are consistent with the experimental results. The bonding behavior, bond orders, Mulliken charges and bond populations as well as formation energies are obtained. The calculated band structures and density of states show that Mn incorporation results in some impurity energy levels of Mn 3d states in forbidden band as well as valence band and conduction band, and that Mn 3d states, for the modest Mn doping concentration, not only can act as the capture center of excited electrons under longer wavelength light irradiation, but also may trap the photo-excited holes, improving the transfer of photo-excited carriers to the reactive sites. Our calculated optical absorption spectra exhibit that the spectral absorption edge is obviously red-shifted and extends to the visible, red and infrared light region due to the incorporation of Mn. Our calculated absorption spectra are in excellent agreement with the experimental results of Mn-doped BiOCl photocatalyst.