Calculated electronic structure of the sandwichd 1 metals LaI2 and CeI2: Application of new LMTO techniques

The electronic structures of the cubic layeredd ¹ metals LaI₂ and CeI₂ were calculated using local density-functional theory and the linear muffin-tin orbital method. Special care was taken in the sphere packing used for the atomic spheres approximation. the band structure and the bonding were analysed in terms of projections of the bands onto orthogonal orbitals. The conduction-band structure could be calculated with a down-folded two-orbital basis which then served for the construction of an analytical 2×2 orthogonal, two-center tight-binding Hamiltonian. The conduction band has almost pure Ln-Ln 5d e _g character. Thex ²–y ²contribution dominates and is two-dimensional and short ranged. Strong hybridization with the 3z ²–1 orbital occurs near the saddle point, which is thereby lowered in energy and bifurcated due to thek _z -dispersion provided by the 3z ²–1 orbital. This strengthens the metal-metal bonds and prevents the nesting instability of the Fermi surface of the half filledx ²–y ²band. Within the limited accuracy of the LDA, the band structure of CeI₂ was found to be identical to that of LaI₂. The conduction-band 4f hybridizationV _{d f} ² (0) was analysed and found to be several times smaller than in fcc -Ce, in qualitative agreement with recent photoemission results [1]. Of importance for this reduction seems to be that the conduction band is formed by essentially only one orbital, ,that the number of Ce nearest-neighbors is small, and that the Ce–Ce distance is relatively large.     

Energy band structure and modulation spectra of A2B4C25 semiconductors

The results of the energy band structure calculations of A²B⁴C₂⁵ compounds are reviewed, and the differences in the energy spectra passing from A³B⁵ semiconductors to their closest ternary analogs are described. The origin of the lowest conduction band minima of A²B⁴C₂⁵ compounds was determined from the slopes of the fundamental absorption edge and the pressure coefficients of the energy gap. The investigations of the valence band structure from electroreflectance (ER), thermoreflectance (TR) and wavelength modulated absorption (WMA) spectra are reviewed. In the higher energy region the ER and TR spectra of A²B⁴C₂⁵ compounds were found to be more complicated in comparison with those of their binary analogs. A model of an assignment of the structures in optical spectra of A²B⁴C₂⁵ compounds is discussed.     

Electronic structure of the Sr2MgSi2O7:Eu persistent luminescence material

The electronic structures of the distrontium magnesium disilicate (Sr₂MgSi₂O₇(:Eu²⁺)) materials were studied by a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structures were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands and the band gap energy of the material as well as the position of the Eu²⁺ 4f ground state were calculated. The calculated band gap energy (6.7 eV) agrees well with the experimental value of 7.1 eV. The valence band consists mainly of the oxygen states and the bottom of the conduction band of the Sr states. The calculated occupied 4f ground state of Eu²⁺ lies in the energy gap of the host though the position depends strongly on the Coulomb repulsion strength. The position of the 4f ground state with respect to the valence and conduction bands is discussed using the theoretical and experimental evidence available.     

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.     

Ba0.5Sr0.5TiO3电子结构和光学性质的第一性原理研究

利用第一性原理研究了Ba_0.5Sr_0.5TiO₃的能带结构和光学性质.结果表明,导带和价带都来源于钛原子3d轨道和氧原子2p轨道的杂化.导带主要由钛原子的3d轨道贡献,价带主要由氧原子的2p轨道贡献.吸收系数为10⁵ cm^-1量级,且吸收主要集中在低能区.折射率为n（0）=2.1,结果与实验符合. 关键词： 第一性原理 能带结构 光学性质     

Electronic properties of anatase TiO2 doped by lanthanides: A DFT+U study

The effects of mono-doping of 4f lanthanides with and without oxygen vacancy defect on the electronic structures of anatase TiO₂ have been studied by first-principles calculations with DFT+U (DFT with Hubbard U correction) to treat the strong correlation of Ti 3d electrons and lanthanides 4f electrons. Our results revealed that dopant Ce is easy to incorporate into the TiO₂ host by substituting Ti due to its lower substitutional energy (∼−2.0 eV), but the band gap of the system almost keeps intact after doping. The Ce 4f states are located at the bottom of conduction band, which mainly originates from Ti 3d states. The magnetic moment of doped Ce disappears due to electron transfer from Ce to the nearest O atoms. For Pr and Gd doping, their substitutional energies are similar and close to zero, indicating that both of them may also incorporate into the TiO₂ host. For Pr doping, some 4f spin-down states are located next to the bottom of the conduction band and narrow the band gap of the doping system. However, for Gd doping, the 4f states are located in deep valence band and there is no intermediate band in the band gap. The magnetic moment of dopant Gd is close to the value of isolated Gd atom (∼7 μB), indicating no overlapping between Gd 4f with other orbitals. For Eu, it is hard to incorporate into the TiO₂ host due to its very higher substitutional energy. The results also indicated that oxygen vacancy defect may enhance the adsorption of the visible light in Ln-doped TiO₂ system.     

Line emission of SrBe2Si2O7:Eu2+ and BaBe2Si2O7:Eu2+

The compounds SrBe₂Si₂O₇ and BaBe₂Si₂O₇ both have the barylite structure. With 254 nm excitation, the Eu²⁺-activated compounds give UV emission peaking at 360 nm (Sr) and at 375 nm (Ba). Maximum quantum efficiencies of 40% (Sr) and 65% (Ba) were measured. The emission consists of a 5d-4f band emission as well as 4f-4f line emission, in contrast to many other Eu²⁺-activated oxides which generally show only 5d-4f band emission. At 77°K, both compounds show only the 4f-4f line emission peaking at 360 nm. At higher temperatures, 5d-4f band emission shows up at the cost of the line emission. A thermal equilibrium is assumed between the lowest excited 5d and 4f levels. The energy difference between these levels, calculated from the variation in the line-band intensity ratio with temperature, was computed to be 0.15 eV (Sr) and 0.09 eV (Ba). The occurrence of the line emission in the barylites is correlated with the weakness of the crystal field at the Eu²⁺ ions and with the high quenching temperature of the 5d-4f band emission.     

First-principles study on electronic structure of Sr2Bi2O5 crystal

The electronic structure of Sr₂Bi₂O₅ is calculated by the GGA approach. Both of the valence band maximum and the conduction band minimum are located at Γ-point. This means that Sr₂Bi₂O₅ is a direct band-gap material. The wide energy-band dispersions near the valence band maximum and the conduction band minimum predict that holes and electrons generated by band gap excitation have a high mobility. The conduction band is composed of Bi 6p, Sr 4d and O 2p energy states. On the other hand, the valence band can be divided into two energy regions ranging from −9.5 to −7.9 eV (lower valence band) and from −4.13 to 0 eV (upper valence band). The former mainly consists of Bi 6s states hybridizing with O 2s and O 2p states, and the latter is mainly constructed from O 2p states strongly interacting with Bi 6s and Bi 6p states.     

Single crystal growth,electronic structure and optical properties of Cs2HgBr4

We report on successful synthesis of high-quality single crystal of cesium mercury tetrabromide, Cs₂HgBr₄, by using the vertical Bridgman–Stockbarger method as well as on studies of its electronic structure. For the Cs₂HgBr₄ crystal, we have recorded X-ray photoelectron spectra for both pristine and Ar⁺ ion-bombarded surfaces. Our data indicate that the Cs₂HgBr₄ single crystal surface is rather sensitive with respect to Ar⁺ ion-bombardment. In particular, such a treatment of the Cs₂HgBr₄ single crystal surface alters its elemental stoichiometry. To explore peculiarities of the energy distribution of total and partial densities of states within the valence band and the conduction band of Cs₂HgBr₄, we have made band-structure calculations based on density functional theory (DFT) employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package. The APW+lo calculations allow for concluding that the Br 4p states make the major contributions in the upper portion of the valence band, while its lower portion is dominated by contributors of the Hg 5d and Cs 5p states. Further, the main contributors to the bottom of the conduction band of Cs₂HgBr₄ are the unoccupied Br p and Hg s states. In addition, main optical characteristics of Cs₂HgBr₄ 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 have been explored from the first-principles band-structure calculations.     

Energy band structure of I–III–VI2 semiconductors

We review recent studies of the energy band structure of I-III-VI₂ semiconductors. The structure of the uppermost valence bands of a I-III-VI₂ compound is profoundly influenced by the proximity of noble metal d levels in the valence band. The direct energy gaps observed in I-III-VI₂ compounds are low relative to the energy gaps in the II–VI analogs by amounts up to 1.6eV, and the spin-orbit splittings observed in the ternaries are low relative to the values observed in the binary analogs, owing to a partial cancellation of the positive spin-orbit parameter for p levels and the negative spin-orbit parameter for d levels. The presence of the noble metal d levels in the valence band has been confirmed directly by the observation of electroreflectance structure due to transitions from the d levels themselves to the lowest conduction band minimum.     

Electronic structure of MgS and MgYb2S4: Electron Energy-Loss Spectroscopy and self-consistent multiple scattering calculations

The electronic structure of MgS and MgYb₂S₄ have been studied using the fine structure of the Mg-K, S-K, Mg-L_2,3, S-L_2,3 and Yb-N₅ edges measured by electron energy-loss spectroscopy (EELS). Our experimental results are compared with real-space full multiple scattering calculations as incorporated in the FEFF9.6 code. All edges are very well reproduced. Total and partial densities of states have been calculated. The calculated densities of states of Mg and S are similar in both compounds. The energy distribution of these states suggests a covalent nature for both materials. For MgYb₂S₄ a band gap smaller than for MgS is predicted. In this compound the top of the valence band and the bottom of the conduction band are dominated by Yb states.     

Luminescent mechanism of Y2SiO5:Ce phosphor powder

A luminescent mechanism was constructed for the broad band emission spectra of the X₁ phase of the Y₂SiO₅:Ce phosphor powder. Four Gaussian peaks fit to the cathodoluminescent (CL) and photoluminescent (PL) spectra were attributed to the two different sites (A1 and A2) of the Ce³⁺ ion in the host matrix and the difference in orientation of the neighbour ions in the complex crystal structure. Each Ce³⁺ site gives rise to transitions from the 5d to the two (therefore two peaks) 4f energy levels (²F_5/2 and ²F_7/2 due to crystal field splitting). Energy transfer from other defect levels in the matrix was also observed.     

Ce(Ⅳ)和Ce(Ⅲ)化合物系列的X射线光电子能谱研究

用XPS研究了标称四价铈盐系列:硫酸铈铵、硫酸铈、二苯基羟乙酸铈、碘酸铈、过氧化铈、二氧化铈以及三价铈盐系列:草酸亚铈、硫酸亚铈、二苯基羟乙酸亚铈、碘酸亚铈和氯化亚铈.结果表明,上述标称四价铈盐都属于混合价化合物,其Ce3d电子能谱呈现Ce⁴⁺和Ce³⁺的二组谱线结构的混合.在真空中加热后CeO₂的Ols电子能谱有两个氧峰,这说明 Ce(4f⁰)O(2p^π)←→Ce(4f¹)O(2p^n-1) 类型价态波动的可能性较大.三价铈盐的3d谱线的低结合能端约3.5—4eV处则存在摔落伴峰.     

Thermoelectric power and ac conductivity of A-type Nd2O3

Measurement of thermoelectric power Θ of pressed pellets of A-type Nd₂O₃ from 550 to 1180K and electrical conductivity (σ) at dc, 50 Hz, 1.542 kHz and 3 kHz at different temperatures is reported. It is concluded that electrical conduction at high temperature (T>600K) in this solid is due to positive large polarons in O²⁻ : 2p (valence) band and negative intermediate polarons in Nd³⁺ : 5d (conduction band). The energy band gap of the solid has been found to be 2.44 eV. At low temperatures, conduction by hopping of charge carriers from one impurity centre to another has been predicted.     

Band structure of the quasi two-dimensional purple molybdenum bronze

The molybdenum purple bronze KMo₆O₁₇ is quasi two-dimensional (2D) metallic oxide that shows a Peierls transition towards a metallic charge density wave state. Since this specific transition is directly related to the electron properties of the normal state, we have investigated the electronic structure of this bronze at room temperature. The shape of the Mo_K1s absorption edge reveals the presence of distorted MoO₆ octahedra in the crystallographic structure. Photoemission experiments evidence a large conduction band, with a bandwidth of 800 meV and confirm the metallic character of this bronze. A wide depleted zone separates the conduction band from the valence band that exhibits a fourfold structure, directly connected to the octahedral symmetry of the Mo sites. The band structure is determined by ARUPS in two main directions of the (0 0 1) Brillouin zone. It exhibits some unpredicted features but corroborates the earlier theoretical band structure and Fermi surface. It confirms the hidden one-dimensionality of KMo₆O₁₇ that has been proposed to explain the origin of the Peierls transition in this 2D compound.     

InP-SiO2 metal-insulator-semiconductor structure parameters investigated with DLTS and other capacitance techniques

In this paper, we report the study of a n-type InP-SiO₂ Metal-Insulator-Semiconductor structure by means of Deep Level Transient Spectroscopy and two complementary techniques : Capacitance versus Voltage and Conductance versus Frequency measurements. We have observed two bulk traps probably related to impurities in the InP crystal. Majority carrier interface states have been studied : the three methods bring similar density profiles showing a minimum value about 10¹² eV^-1 cm^-2 in the energy range 0.4–0.7eV below the conduction band edge. Moreover, we have detected a “missing Phosphorus” interface level at 0.3 eV below the conduction band with a density about 4 x 10¹⁰ cm^-2.     

Electronic structure of CeAl2 thin films studied by X-ray absorption spectroscopy

We report X-ray absorption near edge structures (XANES) study of CeAl₂ thin films of various thicknesses, 40-120 nm, at Al K- and Ce L₃-edges. The threshold of the absorption features at the Al K-edge shifts to the higher photon energy side as film thickness decreases, implying a decreased in Al p-orbital charges. On the other hand, from Ce L₃-edge spectra, we observed a decrease in the 5d4f occupancy as the surface-to-bulk ratio increases. The valence of Ce in these thin films, as revealed by the Ce L₃-edge spectral results, is mainly trivalent. From a more detailed analysis we found a small amount of Ce⁴⁺ contribution, which increases with decreasing film thickness. Our results indicate that the surface-to-bulk ratio is the key factor which affects the electronic structure of CeAl₂ thin films. The above observations also suggest that charge transfer from Al to Ce is associated with the decrease of the film thickness.     

A first-principles study of the electronic structure of the sulvanite compounds

We have investigated by means of first-principles total energy calculations the electronic structure of the sulvanite compounds: Cu₃VS₄, Cu₃NbS₄ and Cu₃TaS₄; the later is a possible candidate as a p-type transparent conductor with potential applications in solar cells and electrochromic devices. The calculated electronic structure shows that these compounds are indirect band gap semiconductors, with the valence band maximum located at the R-point and the conduction band minimum located at the X-point. The character of the valence band maximum is dominated by Cu d-states and the character of the conduction band minimum is due to the d-states of the group five elements. From the calculated charge density and electron localisation function we can conclude that the sulvanite compounds are polar covalent semiconductors.