Thermodynamic correlations and band gap calculations in metal oxides

Thermodynamic parameters and energy-band gap of a given metal oxide, are suitable measures of its stability. For this reason, direct correlations between energy-band gap or chemical hardness, and thermodynamic parameters (e.g. standard enthalpy of formation) have been constructed for different metal oxides. Furthermore, a simple relationship to calculate values of energy-band gaps, for metal oxides, from values of their enthalpies of formation, is presented here. When tested, an appreciable number of metal oxides from s- , p- and d-blocks well obeyed the relationship, while a number of metal oxides deviated. A qualitative theoretical account for such different behaviors is presented here.     

Spin-dependent band structures of nanotubes

A quantum-mechanical ab initio method, with inclusion of spin–orbit coupling, has been suggested for the calculation of the electronic structure of carbon chains (carbynes) and nanotubes. Consideration of spin–orbit coupling leads to the formation of spin–orbit gaps with a width of 2–3 meV in carbynes and up to 1 meV in nanotubes, as well as to spin polarization in chiral nanotubes.     

羰基氧化物RR^1COO的电子结构

本文采用6-31G基组的abinitio方法对羰基氧化物RR^1COO(R,R^1=H,F,CH~3)进行几何构型优化计算,研究其基态的电子结构。结果表明,RR^1COO的稳定结构为双自由基型,其单重态和双重态的相对稳定性受取代基的影响。H~2COO、H(CH~3)COO和(CH~3)~2COO的基态为单重态(^1A),HFCOO和F~2COO的基态为三重态(^3A),HFCOO和H(CH~3)COO的顺式结构比反式稳定。     

一氯氧化物及其负离子的分子结构和热力学性质

利用两种杂化DFT方法(BHLYP和B3LYP，两种纯DFT方法(BP86和BLYP)，以DZP 为基函数对ClOO，ClOOO和ClO3及其负离子的平衡构型进行了量子化学计算，研究了它们的几何构型、相对能量、三种电子亲和势(绝热电子亲和势Ead＝E(optimized neutral)-E(optimized anion),垂直电子亲和势Evert=E(optimized neutral)-E(anion at neutral equilibrium geometry)和垂直电子解离能Evd=E(neutral at anion equilibrium)-E(optiminzed anion))和红外振动频率。     

Correlation corrected band structures of homopolypeptides v. B3LYP band structures of 19 homopolypeptides

Ab initio B3LYP crystal orbital (CO) calculations have been performed on the 19 homopolypeptides (PolyGly, PolyAla, PolySer, PolyThre, PolyLeu, PolyiLeu, PolyVal, PolyAspAc, PolyAsp, PolyGlutAc, PolyGlut, PolyHist, PolyProl, PolyCyst, PolyMeth, PolyTyr, PolyPhenAla, PolyArg, and PolyLys) in their β pleated sheet conformation. Keeping the main chain conformation fixed as in PolyGly, the side chain geometries were optimized. For the calculation 2n+1 different k points were used with n = 8 for the case of simpler and n = 10 for more complicated side chains. The basis set applied was the double ζ one of Clementi. According to the results obtained, the conduction bands are shifted upward and the valance band downward, compared with the results of previous BLYP 1 and LDA 7 CO calculations. The bandwidths are similar to the previous cases. The band edges are in many cases not at the endpoints of the first Brillouin zones, causing nonmonotonous dispersion of both the conduction bands (CB) and the valance bands (VB), respectively. The fundamental gap values due to the upward shifts of the CB and downward shifts of the VB are substantially larger than in the case of our previous DFT CO calculations (values 6.0–7.0 eV). They are very close to the gap values, which can be estimated on the basis of experimental ultraviolet (UV) spectra of some homopolypeptides and on the basis of intermediate exciton theoretical calculations (6.5–7.5 eV). These surprisingly good results for the gaps are due to the compensation of errors (LDA or BLYP gives too small and simple HF provides too large gap values) in the B3LYP method. The admixture of the exact HF exchange with a weight of 0.19 obviously compensates the self interaction error occurring in the LDA or BLYP methods. This article discusses whether/how this result could be established by other B3LYP CO calculations on simple polymer chains and on stacked systems (e.g., nucleotide base stacks). Furthermore, a comparative analysis of the ground state DFT methods, the HF method and of the optimized effective potential method could throw more light on our successful theoretical results for the gaps of the homopolypeptides. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Using diatomics-in-molecules in calculating semiempirical band structures

The diatomics-in-molecules approach usually limited to small-sized molecules is applied to infinitely extended systems, thereby relating the empirical potentials used as DIM input data to the well-known Slater-Koster parameters of tight-binding theory. Based on this parameters the band structure and the density of states of the rare-gas (RG) valence bands are calculated. Our results are in very good agreement with experimental data like the width of the bands and the vacuum-energy. Moreover, the characteristic features of the calculated density of states reflect the spectroscopically determined peak positions satisfactorily. We discuss further applications of this work.     

On the crystal structures of the fluorite-related intermediate rare-earth oxides

The binary oxides of the rare-earth elements are, except for the A- and B-type sesquioxides, members of a fluorite-related homologous series R_nO_2n?2 (n = 4, 6, 7, 9, 10, 11, 12, ∞ are well established and a related phase with $\text{n = 10}\text{1}\text{3}$ has been reported). In this paper an electron optical study of members of the series is discussed which reveals the unit cell dimensions and possible space groups of the intermediate phases as well as the transformation matrices in terms of the fluorite substructure. This information reveals the structural relationships among the members of the series as well as the highest common structural feature involved. Structures are proposed for members of this series consistent with the new results obtained and with the data already in existence.     

SEELFS对Mg-Al复合氧化物结构的研究

应用EXAFS类似原理,对MgO和Mg-Al复合氧化物[Mg(Al)O]中Mgk-边的SEELFS(SurfaceExtendedenergyLossFineStructure)一阶微分谱进行数据处理,得到了Mg近邻原子的配位距离。研究发现,同MgO比较,由于Al^3^+的引入,使[Mg(Al)O]中的Mg－mg配位距离增加,配位数降低,而Mg－O的配位环境基本不变。     

Manganese oxides: parallels between abiotic and biotic structures

A large number of microorganisms are responsible for the oxidation of Mn(2+)((aq)) to insoluble Mn(3+/4+) oxides (MnO(x)()) in natural aquatic systems. This paper reports the structure of the biogenic MnO(x)(), including a quantitative analysis of cation vacancies, formed by the freshwater bacterium Leptothrix discophora SP6 (SP6-MnO(x)()). The structure and the morphology of SP6-MnO(x)() were characterized by transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), including full multiple-scattering analysis, and powder X-ray diffraction (XRD). The biogenic precipitate consists of nanoparticles that are approximately 10 nm by 100 nm in dimension with a fibrillar morphology that resembles twisted sheets. The results dem-onstrate that this biogenic MnO(x)() is composed of sheets of edge-sharing of Mn(4+)O(6) octahedra that form layers. The detailed analysis of the EXAFS spectra indicate that 12 +/- 4% of the Mn(4+) layer cation sites in SP6-MnO(x)() are vacant, whereas the analysis of the XANES suggests that the average oxidation state of Mn is 3.8 +/- 0.3. Therefore, the average chemical formula of SP6-MnO(x)() is M(n)()(+)(y)()Mn(3+)(0.12)[ square(0.12)Mn(4+)(0.88)]O(2).zH(2)O, where M(n)()(+)(y)() represents hydrated interlayer cations, square(0.12) represents Mn(4+) cation vacancies within the layer, and Mn(3+)(0.12) represents hydrated cations that occupy sites above/below these cation vacancies.     

Superfine and hyperfine structures in the v3 band of 32SF6

We present a first detailed account of our theoretical approach to reproduce observed superfine and hyperfine structures in the ν₃ band of SF₆ and we display various observed and calculated patterns of superfine clusters exhibiting hyperfine effects. The main operators of the hamiltonian are derived and the associated constants are related to molecular parameters. We show that, owing to the off-diagonal terms in the hyperfine hamiltonian, a mixing occurs between vibration—rotation states with different point-group symmetry species. As a consequence, superfine and hyperfine structures have to be considered simultaneously and hyperfine hamiltonian matrices connecting several vibration—rotation states need to be diagonalized to reproduce the spectra. We analyse in greater detail a few typical examples from which several molecular constants have been determined (e.g. t₀₄₄, c_d). For the first time, the sign c_d is obtained. Also an effective change, Δc_d, is found between upper and lower levels which can be readily interpreted as a manifestation of the tensor spin—vibration interaction.     

High-resolution electron microscopy of crystallographic shear structures in tungsten oxides

Crystallographic shear (CS) structure in reduced crystals of W0₃ has been imaged at a resolution of 3–4A?by a high-resolution electron microscope.A?large distortion of the W0₆ octahedra sharing their edges at the CS planes has been directly recognized in electron micrographs.Crystallographic shear occurs preferentially in a particular crystallographic orientation. The preference may be explained by a different degree of distortion along the principal axes in the W0₆ octahedra of the pseudocubic structure of W03 crystal.A model for growth and ordering of the CS planes is discussed.     

On the origin of a band gap in compounds of diamond-like structures

Electronic structure calculations were performed to examine the origin of a band gap present in most 18-electron half-Heusler compounds and its absence in NaTl. In these compounds of diamond-like structures, the presence or absence of a band gap is controlled by the sigma antibonding between the valence s orbitals, and the bonding characteristics of the late-main-group elements depend on the extent of their ns/np hybridization. Implications of these observations on the formal oxidation state and the covalent bonding of the transition-metal atoms in 18-electron half-Heusler and related compounds were discussed.     

The charge-transfer absorption band of Eu3+ in oxides

The position of the charge-transfer band of Eu³⁺ in the absorption spectra of a number of oxides is discussed. It is shown that this position is more or less fixed in octahedral VI coordination and that it varies in VIII and XII coordinations as a function of the effective ionic radius of the relevant host lattice ion.     

Theory for the band structures in liquid crystal polymers under shear

A theory for the shear flow instability in a liquid crystal aligned by the initial flow is presented. We have investigated a periodic distortion of the director and the velocity field in the plane perpendicular to the velocity gradient. We present solutions for the director and velocity field and make a connection with the optical image observed under a polarizing microscope. We include the convective terms in the basic equations neglected previously, and show that they alter the values of the critical parameters, but do not change the instability mechanism. Comparison with experimental data is made and further experiments are suggested.     

Ca2B2O5的电子结构和能带结构

合成了标题化合物,在已测晶体结构的基础上,利用含组态作用的INDO/S方法计算了其他电子能带结构;利用态求和方法计算了微结构分子的线性极化率及其晶体的平均折射率。分析结果表明,价带主要由B^3^+和O^2^-离子的价轨道的贡献,导带底部主要由Ca^2^+离子轨道的贡献,从O^2^-离子到Ca^2^+离子的电荷转移对线性极化率起主要贡献。     

A study of conduction band edge states in complex oxides by X-ray absorption spectroscopy

Conduction band edge d-states are compared for complex oxides: (i) mixed tetravalent–trivalent ZrO₂–Y₂O₃ alloys, (ii) tetravalent Zr(Hf)O₂–TiO₂ alloys, and (iii) trivalent La scandate and aluminate. Low Y₂O₃ content cubic ZrO₂–Y₂O₃ alloys display two crystal-field split 4d-features in O K₁ spectra. Alloys with higher Y₂O₃ content, as well as Zr(Hf)O₂–TiO₂ alloys display increased d-state multiplicity. O K₁ spectra of perovskite-structured LaScO₃ and LaAlO₃ indicate Jahn–Teller d-state term-splittings with contributions from both trivalent atomic species.     

Electron and hole band structures and mobility in γ-polymorph platinum phthalocyanine

Excess electron and excess hole energy bands for γ-PtPc have been calculated using the tight-binding approximation, extended Hückel technique LCAO MO's as basis functions for constructing the Bloch functions and a modified molecular potential. The energy bands obtained are highly anisotropic. It is argued that the holes play a dominant role in the charge carrier transport in PtPc. In the absence of experimental data on PtPc, the calculated results are discussed in the light of the available results for H₂Pc and CuPc.     

Probing the non-analytic properties of exciton band structures by reflection spectroscopy

A method of probing the exciton band structure about the centre of the Brillouin zone is developed. Oblique angle reflectance spectra at angles of incidence up to 80° for a (001) face of an anthracene crystal have been obtained for both an air-crystal and a silica-crystal interface. A method for transforming the reflectance data to optical constants is developed and used. From this method the direction of propagation of radiation k? at any frequency inside the crystal for any incident angle can be determined. For a model of an isotropic crystal consisting of anisotropic molecules, dipole-dipole theory for an arbitrary k? is developed, exciton band structures from this theory are calculated and theoretical spectra based on these calculated band structures are presented. The theoretical spectra are in very good agreement with the experimental data. Information about the exciton density of states is deduced from the results.