Abstract

First-principles calculations were carried out within the framework of density functional theory to investigate the influence of phase changes on the electronic and optical properties of CsSnI₃. The lattice parameter and band gap of four different phases, i.e., α-, β-, γ-, and δ-phases, of CsSnI₃ are estimated by employing different exchange–correlation functionals in order to explore their ability to reproduce geometric and electronic structures adequately. Comparison of the calculated total energies shows the non-perovskite orthorhombic (δ-phase) modification of CsSnI₃ is the most stable, followed by the orthorhombic (γ-CsSnI₃) perovskite phase. Thermal stability calculations in the form of temperature dependence of entropy as well as the absence of imaginary frequencies in the phonon dispersion diagrams also confirmed the dynamical stability of the δ-CsSnI₃. The influence of the structural phase changes on the band gap and Fermi level shifts of CsSnI₃ were assessed. Contribution of the electronic states on the formation of the valence and conduction band of four phases of CsSnI₃ were determined, which were calculated using various exchange–correlation functionals, including the high-precision hybrid functional HSE06, and compared with available experimental ones. The calculated energy band distribution diagrams showed that all three perovskite modifications of CsSnI₃ have direct transitions, while δ-CsSnI₃ has an indirect transition. It was found that during the transition from δ- to α-phase, the Fermi level descends to the low energy region (towards the valence band), and the band gap decreases from 2.99 eV to 1.33 eV. During the transition from α- to β-phase, the band gap width again decreases to 1.23 eV and the Fermi level mixes by 1.65 eV towards the conduction band (CB). On the contrary, the band gap increases from β- to γ-phase and the Fermi level shifts by 0.41 eV towards the conduction band. The values of the complex dielectric constant and the refractive index of four phases of CsSnI₃ were also calculated.

Abstract

To understand excellent emission and sensitivity for hydrostatic pressure luminescent ions host material, the first principles calculations carried out within density functional theory (DFT) framework are performed to clarify the electronic structure of neat and doped with Ni²⁺ ions KMgF₃ single crystals. The results of band structure calculations show that F2p states are the principal contributors to the KMgF₃ valence band, mainly in its upper and central parts, while in the energy band gap of the KMgF₃:Ni²⁺ phosphor, new electronic states associated with the Ni²⁺ 3d‐orbitals are formed. Furthermore, the zero phonon line (ZPL) spin‐forbidden transition emission energies, (³A₂⇄¹E) _ZPL, (³A₂⇄³T₂) _ZPL, strength of the octahedral crystal field, 10Dq (³A₂→³T₂)_ZPL, are calculated for the KMgF₃:Ni²⁺ phosphor. Any changes of the E_m(³A₂⇄¹E)_ZPL transition energy of the KMgF₃:Ni²⁺ phosphor with pressure increasing from 0 to 20 GPa are not detected, while the crystal‐field strength increases linearly with increasing pressure. Present results bring a foresight tool for predicting physicochemical properties of undoped and doped wide‐gap fluorides; KMgF₃:Ni²⁺, without any toxic/harmful or expensive rare‐earth can be effectively used as an optical manometer in 0–20 GPa, which covers the almost whole pressure range available at present in Diamond anvil cell experiments.

First-principles DFT calculations have been performed to explore the geometric optimized structure, electronic properties and optical transition energy of MgTa2O6:Cr3+ phosphor. In the calculations we use different approaches for the exchange-correlation potential and treat three supercell models for MgTa2O6:Cr3+, which consider replacement of Cr3+ ions on the octahedral positions of Mg2+ ions, Ta5+ ions, and the both of these ions. Based on these models, we calculate values of the zero phonon line (ZPL) transition energies (E (2E → 4A2)ZPL and E (4T2 → 4A2)ZPL). The calculations of the total and partial DOSs of MgTa2O6 indicate that the main contributors to the valence band are O 2p states (mainly in its upper and central parts), while at the bottom of the conduction band unoccupied Ta 5d states dominate. In the energy band gap of MgTa2O6:Cr3+, new electronic states related to the 3d-orbitals of Cr3+ ion are detected that are hybridized with O 2p electronic states indicating the formation of the chemical Cr3+–O2- bonds in the phosphor under consideration. Three types of the O–Cr–O interatomic angles in MgTa2O6:Cr3+ (Cr3+ ↔ Mg2+) and five types of the angles in MgTa2O6:Cr3+ (Cr3+ ↔ Ta5+) are detected. The present DFT calculations indicate that, both octahedral positions (Mg2+ and Ta5+) are very suitable for substitutions by Cr3+ ions when forming the MgTa2O6:Cr3+ phosphor.

Carbonate apatite (CAp) coating has been developed as a bioabsorbable corrosion-control coating for biodegradable Mg alloys. Carbonate content in the CAp coatings can be utilized to control the bioabsorbability and corrosion rate of CAp-coated Mg alloys. In this study, the carbonate content in the CAp coatings of pure Mg was varied by NaHCO3 concentration of the coating solution from 0.25 to 1.0 mol/L, and hydroxyapatite (HAp) was formed without NaHCO3. The CAp and HAp coatings were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The carbonate content in the apatite coatings formed with 0, 0.25, 0.5 and 1.0 mol/L NaHCO3 was determined to be 2, 12, 15 and 18 wt%, respectively, from the apatite 002 plane XRD peak position using the existing conversion constant as well as from the coating composition obtained by the XPS analysis. The values of carbonate content obtained by each method were in good accordance, indicating that the carbonate content can be determined simply by XRD measurements. Two types of carbonate group were present in the CAp coatings, CO32− and HCO3-, and the relative content of HCO3- to CO32− increased with an increase in the carbonate content.

Designing new materials with desired properties is a complex and time-consuming process. One of the most challenging factors of the design process is the huge search space of possible materials. Machine learning methods such as [Formula: see text]-nearest neighbors, support vector machine (SVM) and artificial neural network (ANN) can contribute to this process by predicting materials properties accurately. Properties of multi-principal element alloys (MPEAs) highly depend on alloys’ phase. Thus, accurate prediction of the alloy’s phase is important to narrow down the search space. In this paper, we propose a solution of employing SVM method with hyperparameters tuning and the use of weighted values for prediction of the alloy’s phase. Using the dataset consisting of the experimental results of 118 MPEAs, our solution achieves a cross-validation accuracy of 90.2%. We confirm the superiority of this score over the performance of ANN statistically. On the other dataset containing 401 MPEAs, our SVM model is comparable to ANN and exhibits 70.6% cross-validation accuracy. We also found that additional variables, including average melting temperature and standard deviation of melting temperature, increase prediction accuracy by 3.34% in the best case.

Isostatic pressure effects on the elastic and electronic properties of non-doped and Mn4+-doped K2SiF6 (KSF) have been investigated by first-principles calculations within density functional theory (DFT). Bulk modulus was obtained by the Murnaghan’s equation of states (EOS) using the relationship between volume and pressures at pressures between 0 and 40 GPa, and elastic constants were calculated by the stress–strain relationship giving small distortions at each pressure point. The other elastic parameters such as shear modulus, sound velocity and Debye temperature, which can be obtained from the elastic constants, were also estimated. The influence of external isostatic pressure on the electronic properties, such as crystal field strength 10Dq and emission energy of 2E → 4A2 transition (Eem), of KSF:Mn4+ was also studied. The results suggest that 10Dq and Eem linearly increase and decrease, respectively, with increasing pressure.

Organo-halide perovskite solar cells (PSCs) are lightweight and low cost, and they offer high power conversion efficiencies. PSCs have proven to be useful in terrestrial applications. In addition, they are particularly attractive for space applications because they can offer a higher radiation tolerance than GaAs and Si solar cells. This paper evaluates the damage coefficient for minority-carrier diffusion length K-L of perovskite crystals after 1 MeV electron irradiation by time-resolved photoluminescence measurements to investigate the reason for their high radiation tolerance. Results show that perovskite crystals have a lower damage coefficient K-L than that of InP crystals with a high radiation tolerance. On the other hand, first-principles calculations indicate that the displacement energy of perovskite crystals is as low as that of Si, which does not have a high radiation tolerance. The present results suggest that the annealing effect occurs for PSCs at room temperature.

<title>Abstract</title>The experimental evidence for the contraction of volume of gold implanted with hydrogen at low doses is presented. The contraction of lattice upon the addition of other elements is very rare and extraordinary in the solid-state, not only for gold but also for many other solids. To explain the underlying physics, the pure kinetic theory of absorption is not adequate and the detailed interaction of hydrogen in the lattice needs to be clarified. Our analysis points to the importance of the formation of hydride bonds in a dynamic manner and explains why these bonds become weak at higher doses, leading to the inverse process of volume expansion frequently seen in metallic hydrogen containers.

A current research trend for nanostructured functional materials and their applications are summarized in this paper, for which a recently published special issue in Materials Transactions, Vol. 59, No. 7, is mainly surveyed here. Among the progresses of the functional materials, several important topics were covered in this special issue, which are advanced atomic or nano-meter scale analysis, advanced ab initio simulations, highly correlated electron system, advanced spectroscopy and advanced processing. In addition, contents of some related special issues are introduced.

Numerous attempts have been made to improve the oxidation resistance and electrical conductivity of the interconnectors in solid oxide fuel cells. A Co–W alloy coating on ferritic stainless steel has attracted attention because the Co–W oxide layer formed by the oxidation treatment of the Co–W alloy coating has proven effective in reducing the outward diffusion of Cr and improving oxidation resistance. This study was designed to elucidate the diffusion behavior of elements and the barrier mechanism of the CoWO4 layer. After oxidation in air at 750°C, a dense, multilayered oxide formed, comprising (from the stainless steel substrate to the outer layer) Cr2O3, (Cr,Fe,Co)3O4, CoWO4, (Co,Fe)3O4, and Co3O4 layers. The CoWO4 layer and neighboring oxide layers were carefully analyzed by scanning electron microscopy and transmission electron microscopy with electron energy-loss spectroscopy, confirming the absence of trivalent cations (Co3+, Fe3+, and Cr3+) and the presence of Fe2+ ions in the CoWO4 layer; thus, CoWO4 functions as a selective diffusion barrier to trivalent cations, as hypothesized. The Cr-containing oxide layer grows based on the reaction between the metallic cations from the substrate and the inward-diffusing oxide ions, whereas Fe and Co can diffuse outward through CoWO4 as Fe2+ and Co2+ ions.

Lead halide perovskite single layers with three grain sizes are subjected to proton-beam irradiation in order to assess the durability and radiation tolerance of perovskite solar cells (PSCs) against space radiation. Proton-beam irradiation is chosen because proton beams significantly affect solar cell performance in the space environment. We evaluate the effects of proton beams by focusing on the grain structure, crystal structure, and carrier lifetime of a perovskite single layer by using scanning electron microscopy, X-ray diffraction, photoluminescence (PL) spectra, and time-resolved PL (TRPL). The results show that proton irradiation does not significantly affect the grain structure and crystal structure of perovskite layer; the TRPL results show that the carrier lifetime inside the grain is constant up to a fluence of 1 x 10(14) p(+)/cm(2) and decreases significantly at a fluence of 1 x 10(15) p(+)/cm(2). Proton-beam radiation tolerance of the grain inside the perovskite layer is dominant in the radiation tolerance of PSCs.

The breakdown electric field of several SiC polymorphs has been investigated theoretically using a concept of “recovery rate,” which is obtained by first principles calculations. A good relationship between the experimental breakdown electric fields and the calculated recovery rate of 4H-, 6H-, and 3C-SiC was obtained. In order to examine the stability of SiC polymorphs, the total electronic energies of various types of SiC crystal structures were calculated. Here, two candidates of polymorphs—GeS-type- and 2H-SiC—with energies comparable to those of experimentally well-established structures, have been obtained. The breakdown electric fields of these two polymorphs were estimated using a relationship obtained from the results of 4H-, 6H-, and 3C-SiC. This indicates that one of these polymorphs, GeS-type-SiC, has higher breakdown electric field than any other SiC polymorphs. In addition to the investigation with the recovery rate, relationship between experimental breakdown electric field and calculated band gap with recently developed accurate electron-correlation potential has been also discussed.

Designing new materials with desired properties is a complex and time-consuming process. One of the challenging factors of the design process is the huge search space of possible materials. Machine learning methods such as k-nearest neighbours, support vector machine (SVM) and artificial neural network (ANN) can contribute to this process by predicting materials properties accurately. Properties of multi-principal element alloys (MPEAs) highly depend on alloys’ phase. Thus, accurate prediction of the alloy’s phase is important to narrow down the search space. In this paper, we propose a solution of employing support vector machine method with hyperparameters tuning and the use of weight values for prediction of the alloy’s phase. Using the dataset consisting of the experimental results of 118 MPEAs, our solution achieves the cross-validation accuracy of 90.2%. We confirm the superiority of this score over the performance of ANN statistically. On the other dataset containing 401 MPEAs, our SVM model is comparable to ANN and exhibits 70.6% cross-validation accuracy.

The effects of soft X-ray exposure on structures of CH3NH3PbI3 perovskite were investigated using an X-ray photoelectron spectroscopy (XPS) time-dependent measurement method. A crystalline sample was fabricated with the inverse-temperature crystallization method. The time evolutions of the core-level and valence-band spectra were recorded to determine the compositional ratios and valence band electronic structure of the sample, respectively. In addition, first-principles calculations were conducted to evaluate the valence band XPS spectra. The in situ XPS analysis combined with theoretical calculations demonstrated a degradation of the surface of CH3NH3PbI3 perovskite into PbI2 owing to the evaporation of methylammonium iodide.

X-ray absorption near edge structure (XANES) analysis is an element-specific method for proving electronic state mostly in the field of applied physics, such as battery and catalysis reactions, where the valence change plays an important role. In particular, many results have been reported for the analysis of positive electrode materials of Li-ion batteries, where multiple transition materials contribute to the reactions. However, XANES analysis has been limited to identifying the valence state simply in comparison with reference materials. When the shape of XANES spectra shows complicated changes, we were not able to identify the valence states or estimate the valence quantitatively, resulting in insufficient reaction analysis. To overcome such issues, we propose a valence state evaluation method using K-and L-edge XANES analysis with first-principles simulations. By using this method, we demonstrated that the complicated reaction mechanism of Li(Ni1/3Co1/3Mn1/3)O-2 can be successfully analyzed for distinguishing each contribution of Ni, Co, Mn, and O to the redox reactions during charge operation. In addition to the XANES analysis, we applied resonant photoelectron spectroscopy (RPES) and diffraction anomalous fine structure spectroscopy (DAFS) with first-principles calculations to the reaction analysis of Co and Mn, which shows no or very little contribution to the redox. The combination of RPES and first-principles calculations successfully enables us to confirm the contribution of Co at high potential regions by electively observing Co 3d orbitals. Through the DAFS analysis, we deeply analyzed the spectral features of Mn K-edges and concluded that the observed spectral shape change for Mn does not originate from the valence change but from the change in distribution of wave functions around Mn upon Li extraction. Published by AIP Publishing.

© 2016 Author(s). Polycrystalline Fe-doped BaMgSiO4 is synthesized by the conventional solid state reaction method, which shows strong photochromism. Photochromic property of the synthesized specimens is investigated by measuring the diffuse reflectance spectrum. Local environment of doped Fe ions in BaMgSiO4 has been studied by the analysis of the X-ray absorption near-edge structure (XANES) spectrum with the aid of the first-principles calculations.

We discuss here a self-similar structure of an aggregate of crystals or noncrystalline grains. In particular, the sufficient condition for a polycrystal to be filled with an arbitrary finite number of self-similar crystals is investigated using a topological concept.

Soft X-ray exposure effects on CH3NH3PbI3 perovskite in a patterned device sample with a similar structure to solar cells, a promising candidate for X-ray detectors, have been investigated with an X-ray Photoelectron Spectroscopy (XPS) time-dependent measurement method. Our experimental analyses demonstrate compositional change from CH3NH3PbI3 to PbI2 due to evaporation of methylammonium iodide.

In situ measurements of work function of indium tin oxide after UV/ozone treatment were carried out using a photoemission yield spectrometer with an open counter. Although the work function increased just after UV/ozone treatment, it decreased as time passed and finally returned to the initial value. The continuous change in work function with exposure to air was observed under dry atmosphere and at various temperatures. The returning process at higher temperature proceeded faster than at lower temperature. By contrast, humidity has no influence on the work function recovery. The exponential decay of work function was consistent with the first-order reaction rate equation. The rate constant obeyed Arrhenius' equation, and the activation energy was estimated to be 22 kJ/mol.

The molecular and electronic structure of the 1:1 charge-transfer complex between ferrocene (Fc) and zinc porphyrin (ZnP) are investigated with the aid of dispersion-corrected density functional theory (DFT) calculations. Four stable configurations were obtained, two with the Fe molecule laying on the ZnP plane and the other two where Fe interacts with the porphyrin's perimeter. The dipole moment vectors of these Fc:ZnP complexes indicate that they are stabilized by the transfer of electronic charge density from Fe to ZnP or vice versa.

Na-incorporated beta-tricalcium phosphate (beta-TCP) was synthesized via the solid-state reaction method and a substitution mechanism for Na ions in the synthesized materials has been investigated by X-ray absorption near-edge structure (XANES) analysis. In addition, total electronic energy calculations within density functional theory were also carried out to obtain the most energetically favorable substitution site for Na ions in beta-TCP. Both the spectroscopic and computational analysis indicate that substituted Na ions are likely to occupy Ca(4) site in beta-TCP.

The analytical method of local environment of dilutely doped magnetic elements in functional materials in an atomic scale by the X-ray absorption near-edge structure (XANES) measurements and the first-principles calculations is demonstrated. Results for the lead-free piezoelectric material, Mn and Fe doped Bi4Ti3O12, is shown as an application of the above method.

Single-phased polycrystalline Mn- and Fe-doped Bi4Ti3O12 were fabricated using a solid-state reaction technique, doping with various concentrations of Mn and Fe ions. Substitution mechanism of Mn and Fe ions in Bi4Ti3O12 were investigated with X-ray absorption near-edge structure (XANES) measurements and first-principles calculations. The valence states of the Mn and Fe ions are 4+ and 3+, respectively, inferred from the L-2,L-3-edge XANES profiles. From the K-edge XANES analysis, it is determined that Mn and Fe ions are substituted at one of the Ti sites, i.e., Ti(2a) or Ti(4e) sites. Our first-principles total electronic energy calculations suggest that Mn ions are likely to substitute at Ti(2a) sites rather than at Ti(4e) sites, whereas the opposite is true for Fe substitution. Taken together, these results give a clear description of the locations and charge states of the Mn and Fe dopants in Bi4Ti3O12.

We investigated the desorption of water from a TiO2 surface under a dry atmosphere by collecting the photoemission yield spectra with an open counter. For this purpose, a new attachment for the photoemission yield measurement was prepared. This apparatus is capable of detecting, in the open air, low-energy electrons excited by photons under dried atmospheres; the dew point is below -35 degrees C. A significant change in the photoemission yield spectra due to exposure to a dry atmosphere was observed. To gain a better understanding of these results, observations of the change in the photoemission yield spectra caused by the thermal desorption of adsorbed water were also carried out. The results are consistent with those obtained by exposure to a dry atmosphere. Based on the relationship between the photoemission yield and the thickness of the water layer, the time dependence of the change in the thickness was explained by the second-order reaction rate equation.

Si-doped cubic boron nitride (c-BN) is synthesized at high pressure and high temperature, and the local environment of Si is investigated using X-ray absorption near edge structure (XANES) and first-principles calculations. Si-K XANES indicates that Si in c-BN is surrounded by four nitrogen atoms. According to first-principles calculations, the model for substitutional Si at the B site well reproduces experimental Si-K XANES, and it is energetically more favorable than substitutional Si at the N site. Both the present experimental and theoretical results indicate that Si in c-BN prefers the B site to the N site. (C) 2013 AIP Publishing LLC.

The valence band electronic structures of Mn- and/or Fe-doped In2O3, i.e., In2O3:Mn, In2O3:Fe, and In2O3:(Mn, Fe), are investigated by photoemission yield measurements. Significant changes are observed in the threshold energy of photoemission, depending on the doped magnetic ions, which indicates that an additional occupied band appears above the top of the valence band of In2O3 owing to doping with Mn and/or Fe ions. It is confirmed that the order of the threshold energies of photoemission, E-PET is E-PET(In2O3:Mn)&lt;E-PET(In2O3:(Mn, Fe))&lt;E-PET(In2O3:Fe)&lt;E-PET(In2O3). To gain a better understanding of these results, first-principles molecular orbital calculations are also carried out, which successfully explain the observed changes in the photoemission threshold energies. (C) 2012 Elsevier B.V. All rights reserved.

The valence state of Co ions in Pr1-xCaxCoO3-delta and Pr1-xSrxCoO3-delta has been investigated by an analysis of the Co-L-3 X-ray absorption near-edge structure (XANES) profile. The observed intensity distributions of Co-L-3 XANES change continuously with increasing concentration of alkaline-earth ions. To investigate the origin of this change in the XANES profile, charge transfer multiplet calculations were carried out, which could successfully explain the change in the spectral profile; they also suggest that the valence state of Co ions in Pr1-xCaxCoO3-delta and Pr1-xSrxCoO3-delta is between 3+ and 4+ and increases gradually with the concentration of alkaline-earth ions. (c) 2012 Elsevier B.V. All rights reserved.

The valence states of the Mn ions in Pr(1-x)A(x)MnO(3-delta) (A = Ca, Sr) are investigated by Mn-L-3 X-ray absorption near-edge structure (XANES) analysis. The spectral fine structures in the Mn-L-3 XANES analysis show a significant difference between Pr0.5Ca0.5MnO3-delta and Pr0.5Sr0.5MnO3-delta, a paramagnetic insulator and a ferromagnetic metal, respectively, at room temperature, whereas the spectral structures of Pr0.7Ca0.3MnO3-delta and Pr0.7Sr0.3MnO3-delta, paramagnetic insulators, are almost identical. These results indicate that the valence states of the Mn ions in these materials are highly correlated with their magnetic and electrical properties. A significant difference was also found between the Mn-L-3 XANES profiles of Pr1-xCaxMnO3-delta and the profiles formed by the linear combination of the Mn-L-3 XANES spectra of PrMnO3 (Mn3+) and CaMnO3 (Mn4+). This difference indicates that the Mn ions in these materials do not have a mixed-valence state of 3+ and 4+, but have an intermediate valence state o 3+ and 4+. (c) 2012 Elsevier B.V. All rights reserved.

The charge compensation mechanisms in polycrystalline Pr1-xCaxCoO3-delta and Pr1-xCaxCoO3-delta here synthesized by the solid-state reaction method have been investigated by analyzing the Pr-L-3 and Co-K X-ray absorption near-edge structure (XANES) with the aid of first-principles calculations. The valence states of Pr ions in these materials were determined to be trivalent (Pr3+) and independent of the concentration of alkaline-earth ions observed in the Pr-L-3 XANES profile. The Co-K XANES spectra shifted to the higher-energy side with an increase in alkaline-earth concentration and were examined in detail by first-principles calculations. Using calculated XANES results, we successfully determined that the valence state of Co ions is an intermediate value between 3+ and 4+ and increases with an increase in the concentration of alkaline-earth ions in Pr1-xCaxCoO3-delta (A = Ca, Sr) accompanying oxygen vacancy. (C) 2012 The Japan Society of Applied Physics

The detailed valence states of Mn and Fe ions in room-temperature ferromagnetic In2O3 doped with Mn and Fe ions were investigated by the near-edge X-ray absorption fine structure (NEXAFS) measurements. It was confirmed that Mn2+ and Mn3+ coexist in In2O3. In addition to these two magnetic ions, Sn4+ was incorporated to control the charge states of doped Mn and/or Fe ions, which makes the Mn2+/Mn3+ ratio larger with an increase in the doped-Sn4+ concentration. It was found that the magnetic susceptibility decreased with an increase in Mn2+ because of the Sn4+-doping, which indicated that the coexistence of Mn2+ and Mn3+ is mandatory for room-temperature ferromagnetism in Mn- and Fe-codoped In2O3. (C) 2011 Elsevier Ltd. All rights reserved.

Zn-doped beta-tricalcium phosphate (beta-TCP) is synthesized by the solid-state reaction method. The substitution mechanism of Zn ions in beta-TCP synthesized here is investigated by carrying out a combination of near-edge X-ray absorption fine structure (NEXAFS) measurements and first-principles calculations. From the results of the present study, the substitution site for Zn ions in beta-TCP is successfully determined. (C) 2010 Elsevier B.V. All rights reserved.

Sb-doped BaSnO3 is synthesized by the conventional solid-state reaction method changing a concentration of Sb. Electric resistivity of synthesized specimens decreases with increment of doped Sb concentration. In order to investigate an influence of Sb-doping on the electronic structure of BaSnO3, its valence band electronic structure is examined by the photoemission yield spectroscopy (PYS), which shows additional occupied band above the top of the valence band of BaSnO3 due to the Sb-doping. First-principles calculations are also carried out to obtain change in electronic structures of BaSnO3 by Sb-doping, which supports the PYS results. (C) 2009 Elsevier B.V. All rights reserved.

The first-principles calculations are carried out to investigate the influence of the presence of divalent ions, Mg2+ and Sr2+, on the elastic properties of hydroxyapatite. The calculated elastic moduli for pure hydroxyapatite are in good agreement with the experimental values. After examination of the geometrical structures of hydroxyapatite doped with the divalent ions, the elastic properties of the models are investigated. (C) 2010 The Ceramic Society of Japan. All rights reserved.

Fe-doped hexagonal-structured BaTiO3 is synthesized by the conventional solid-state reaction method. The local environment of the Fe ions doped in the synthesized hexagonal BaTiO3 is then investigated by X-ray absorption near edge structure (XANES) measurements and first-principles calculations. It is confirmed from the Fe L-2,L-3-edge XANES spectrum that the charge state of the Fe ions in the synthesized specimen is trivalent. By analyzing the Fe K-edge XANES spectrum of the present specimen with the aid of the first-principles calculations, it is found that doped Fe ions are substituted at the Ti site in Ti2O9 groups of face-sharing octahedra in hexagonal BaTiO3. (C) 2010 The Japan Society of Applied Physics

First-principles lattice dynamics calculations are systematically made on four polymorphs of SrHfO3. The hierarchy of the total energy at the ground state is Pm (3) over barm &gt; I4/mcm &gt; Cmcm &gt; Pnma, which agrees with the sequence of iterative transitions with temperature as reported by experiments. Three structures, Pm (3) over barm, I4/mcm and Cmcm, are found to be dynamically unstable at the ground state. A soft mode of phonons appears in I4/mcm and Cmcm at the Gamma-point. The atomic displacements for the soft mode are analogous to those of the R-25 mode of Pm (3) over barm, which is related to the rotation of the HfO6 octahedron. Pnma phase shows lowest energy and it is the only dynamically stable structure among the four phases. The volume expansion coefficient, bulk modulus and heat capacity are computed within quasi-harmonic approximations using the vibrational density of states. They are compared to experimental data. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Phonon states of Ag doped potassium halides, KX:Ag+ (X = Cl, Br and 1), are computed by a first principles lattice dynamic method using 64-atoms supercells. Results are compared to experimental data in literature. Phonon density of states of host KCl and KI crystals satisfactorily agree to the experimental inelastic neutron scattering data. Experimental frequencies of the impurity-induced infra-red (IR) and Raman active modes in the low frequency region are reasonably well reproduced because the vibrations are localized within the first nearest neighbour anions of the Ag+-ion. On the other hand, limitations of present calculations to reproduce the high frequency impurity-induced modes are pointed out. They are less localized to the Ag+-ion. [doi: 10.2320/matertrans.MC200815]

We have studied the stability and ferroelectric properties of hexagonal RGaO3 and RInO3 (R: rare-earth elements) by first-principles calculations. Computed spontaneous polarization in the series shows a systematic increase with the rare-earth elements, with values being larger in RInO3 than in the corresponding RGaO3. The largest polarization found is about 10 mu C/cm(2) for ErInO3, which is about twice as large as those observed in hexagonal RMnO3. The polarization can be further increased by applying in-plane compressive stress. The Born effective charges of constituent ions in the compounds are found to be similar to their formal values, implying that the ferroelectric displacements are merely driven by the ionic size effect. A transition to the high-symmetry phase at around 1500 K was confirmed in GdInO3 and DyInO3 by in situ high-temperature powder x-ray diffractometry. The present systems should belong to the family of geometric ferroelectrics.

Three types of functional ceramic materials, (1) dilute magnetic semiconductor, (2) phosphor and (3) electrolyte of a solid fuel cell, are fabricated by the conventional solid state reaction method. Local environments of dopants in these ceramic materials here synthesized are systematically investigated by using the x-ray absorption near-edge structure (XANES) with the aid of first-principles calculations. Our present analytical method by combined use of XANES and first principles calculations has successfully explained the local environment of dopants in the above ceramic materials.

Mn-doped beta-tricalcium phosphate (beta-TCP) is synthesized by the solid-state reaction method. The local environment of doped Mn ions in beta-TCP is investigated by the near-edge X-ray absorption fine structure (NEXAFS) measurements. The first principles calculations are also carried out to obtain the theoretical NEXAFS spectra within the density functional theory (DFT). Observed NEXAFS spectra from the Mn ions in beta-TCP are quantitatively well reproduced by the present theoretical calculations, which show Mn ions are substituted for Ca2+ site in beta-TCP.

Supersaturated ZnO-Al(2)O(3) (&gt;20 at.% Al) thin films are grown by pulsed laser deposition technique on silica glass substrates at 600 degrees C. They are characterized by combining x-ray diffraction, Al-K edge x-ray absorption near edge structures (XANESs), high resolution transmission electron microscope (TEM) imaging, TEM analysis, and a series of first principles calculations. The films are composed of textured wurtzite grains with c planes parallel to the substrate. The distance between c planes expands significantly when the Al concentration is greater than 10 at. %. The expansion disappears after annealing the films at above 800 &apos;C. High density of dislocationlike defects is found in the as deposited film. Any segregation of Al cannot be detected either at the grain boundaries or inside the grains. The lattice expansion toward c axis and the experimental XANES can be satisfactorily explained by taking a hypothetical homologous model with the composition of (ZnO)(3)(Al(2)O(3)) as the local environment of Al in the supersaturated solid solution. Simplified substitutional models with Al at the Zn site in wurtzite ZnO cannot explain these experimental results. First principles calculations show that the homologous phase is energetically more favorable than the simplified substitutional models, although decomposition into ZnO and ZnAl(2)O(4) is more favorable than the homologous phase. The local atomic structures of the supersaturated solid solution are therefore concluded to be analogous to the metastable homologous phase. (c) 2008 American Institute of Physics.

Theoretical calculations of Zn K and Fe K x-ray absorption near-edge structures (XANES) using a first-principles method have been performed to evaluate the degree of cation disordering in spinel zinc ferrite (ZnFe2O4) thin film prepared by a sputtering method, ZnFe2O4 thin films annealed at elevated temperatures, and ZnFe2O4 bulk specimen prepared by a solid-state reaction. Using the full-potential linearized augmented plane-wave + local orbitals method, a theoretical spectrum is generated for the tetrahedral and octahedral environments for each of the two cations. The experimental XANES spectrum of the thin film annealed at 800 degrees C as well as that of bulk specimen is successfully reproduced by using either the theoretical spectrum for Zn2+ on the tetrahedral site (A site) or that for Fe3+ on the octahedral site (B site), which is indicative of the normal spinel structure. For the as-deposited film, on the other hand, excellent agreement between theoretical and experimental spectra is obtained by considering the presence of either ion in both the A and B sites. The degree of cation disordering, x, defined as [Zn1-x2+Fex3+](A)[Znx2+Fe2-x3+](B)O-4, is estimated to be approximately 0.6 in the as-deposited film, which is consistent with the analysis of the extended x-ray absorption fine structure on the Zn K edge. Curious magnetic properties as we previously observed for the as-deposited thin film-i.e., ferrimagnetic behaviors accompanied by large magnetization at room temperature and cluster spin-glass-like behavior-are discussed in connection with disordering of Zn2+ and Fe3+ ions in the spinel-type structure.

The effect of thermal annealing on magnetic properties has been investigated for disordered zinc ferrite (ZnFe2O4) thin films deposited on silica glass substrates using a sputtering method. The magnetization at room temperature is enhanced by annealing at relatively low temperature such as 300 degrees C and decreased by annealing at higher temperatures. X-ray absorption near edge structures indicate that ZnFe2O4 thin film annealed at 300 degrees C possesses disordered cation distribution similar to that for as-deposited thin film. It is presumed that the enhancement of magnetization by annealing at 300 degrees C is due to precipitation of disordered ZnFe2O4 crystal from an amorphous matrix formed in the as-deposited thin film. (C) 2006 Elsevier B.V. All rights reserved.

Room temperature ferromagnetism of GaMnN thin film is awaked by a mild hydrogenation treatment of a sample synthesized by molecular beam epitaxy. Local environment of Mn atoms is monitored by Mn-L-2,L-3 near edge x-ray absorption fine structure technique. Doped Mn ions are present at substitutional sites of Ga both before and after the hydrogenation. No secondary phase can be detected. Major valency of Mn changes from 3+ to 2+ by the hydrogenation. The present result supports the model that the ferromagnetism occurs when Mn2+ and Mn3+ are coexistent and holes in the midgap Mn band mediate the magnetic coupling. (c) 2007 American Institute of Physics.

As a model of aliovalent impurity in functional ceramic, the local environment of dilute Ga in rutile-structured TiO2 is investigated by Ga K-edge X-ray absorption near-edge structure (XANES) spectroscopy. In conjunction with the experiments, first-principles calculations by two methods are systematically made. The projector augmented wave method is used to optimize the local structure and obtain the solution energy. The augmented plane wave plus local orbitals method is adopted to obtain theoretical XANES spectra. A comparison between experimental and theoretical XANES spectra shows that Ga dopants are located at the Ti4+ sites forming Ga3+. Oxygen vacancies are present to maintain the charge balance of the solid solution. Ga atoms and oxygen vacancies are not present at the nearest neighbor sites but stay apart.

Considerable efforts have been devoted recently to synthesizing diluted magnetic semiconductors having ferromagnetic properties at room temperature because of their technological impacts for spintronic devices. In 2001 successful growth of GaMnN films showing room temperature ferromagnetism and p-type conductivity was reported. The estimated Curie temperature was 940 K at 5.7% of Mn, which is the highest among diluted magnetic semiconductors ever reported. However, the electronic mechanism behind the ferromagnetic behaviour has still been controversial. Here we show experimental evidence using ferromagnetic samples that Mn atoms are substitutionally dissolved into the GaN lattice and they exhibit mixed valences of +2 (majority) and +3. The p-type carrier density decreases significantly at very low temperatures. At the same time, magnetization dramatically decreases. The results imply that the ferromagnetic coupling between Mn atoms is mediated by holes in the mid-gap Mn band.

Migration of Li+ ions via the vacancy mechanism in LiX (X = F, Cl, Br, and 1) with the rocksalt and hypothetical zinc blende structures and Li2X (X = O, S, Se, and Te) with the antifluorite structure has been investigated using first-principles projector augmented wave calculations with the generalized gradient approximation. The migration paths and energies, determined by the nudged-elastic-band method, are discussed on the basis of two idealized models: the rigid-sphere and charged-sphere models. The trajectories and energy profiles of the migration in these lithium compounds vary between these two models, depending on the anion species and crystal structure. The migration energies in LiX with both the rocksalt and hypothetical zinc blende structures show a tendency to decrease with increasing periodic number of the anion species in the periodic table. This is consistent with the widely accepted view that anion species with large ionic radii and high polarizabilities are favorable for good ionic conduction. In contrast, Li2O exhibits the lowest migration energy among Li2X compounds, although O is the smallest among the chalcogens, indicating that electrostatic attractive interactions play the dominant role in the inter-ion interactions in Li2O and, therefore, in the ion migration.

The temperature dependence of cationic disorder in MgAl2O4 spinel is investigated using a combination of first-principles total-energy calculations, a cluster expansion, and canonical Monte Carlo simulations. The formation energies of the possible cation-disordered structures within the spinel unit cell are predicted to be all positive, suggesting that the ground state is the normal spinel in consistency with a widely accepted view. The temperature dependence of cationic disorder is well reproduced by considering many effective cluster interactions up to quadruplets. The order-disorder transition temperature is estimated at about 860 K based on the anomaly of specific heat. The cluster expansion of the volume of MgAl2O4 indicates that it decreases as more cations exchange.

The temperature dependence of cationic disorder in Mg Al2 O4 spinel is investigated using a combination of first-principles total-energy calculations, a cluster expansion, and canonical Monte Carlo simulations. The formation energies of the possible cation-disordered structures within the spinel unit cell are predicted to be all positive, suggesting that the ground state is the normal spinel in consistency with a widely accepted view. The temperature dependence of cationic disorder is well reproduced by considering many effective cluster interactions up to quadruplets. The order-disorder transition temperature is estimated at about 860 K based on the anomaly of specific heat. The cluster expansion of the volume of Mg Al2 O4 indicates that it decreases as more cations exchange. © 2006 The American Physical Society.

Both X-ray absorption near edge structures (XANES) and electron energy loss near edge structure (ELNES) are important toots in ceramic science offering information on local environment of selected elements not only in crystals but also in amorphous materials. Recent technological progress enables measurements of XANES of ppm-level dopants using modern synchrotron facilities. Combined with transmission electron microscopy, ELNES can be used to analyze the local structures with sulmanometer spatial resolution. First principles methods to reproduce and interpret the spectra have been established just recently. When a core-hole is adequately taken into account, most of K-edge spectra can be well reproduced using a modern band-structure method within one-electron approximation. The same is true for L-2,L-3-edge spectra of non-transition metal compounds. However, multi-electron calculations are mandatory to reproduce L-2,L-3-edge spectra of 3d transition-metal elements because of strong electronic correlations. In this paper, some recent results obtained in our group by the combination of XANES/ELNES experiments and theoretical calculations are reviewed.

Soft X-ray emission spectra in the Ar L region of various solid substrates doped with Ar atoms were measured to investigate the chemical states of the Ar atoms in the solid matrices. Ar ions were implanted into the solid matrices of Si(111), SiO2, highly oriented pyrolytic graphite (HOPG), Ti, Cr, Ni, and Cu with an acceleration voltage of 5 kV at room temperature. Soft X-ray emission spectra in the Ar L region were measured using synchrotron radiation. Low-energy tailing was observed at the L-3-M-1 and L-2-M-1 X-ray emission peaks in Ar-doped transition metals. The density of states (DOS) of the Ar 3s orbitals indicated that the low-energy tailed DOS can be formed by hybridization with 3d orbitals in the overpressurized Ar clusters. (c) 2005 Elsevier B.V. All rights reserved.

The first-principles band structure calculations are carried out to interpret the spectral fine structure of the near-edge X-ray absorption fine structure (NEXAFS) spectrum of PdO at the 0 K-edge by the full-potential augmented plane wave plus local orbitals (APW+lo) method. The observed NEXAFS fine structure of PdO is quantitatively reproduced by our calculations. Three types of PdO polymorph are examined, which show a significant difference in theoretical NEXAFS profile. They can be used as theoretical finger prints for future studies.

AlN thin films have been grown on c-cut sapphire substrates by pulsed-laser deposition. The film epitaxially grown at 1073 K under vacuum of 5 x 10(-4) Pa was used to examine the crystallographic orientation dependence of Al K-edge x-ray absorption near-edge structures (XANES), which satisfactorily agrees with theoretical spectra obtained by first-principles calculations. The film grown at 1073 K with N-2 backfill of 7 x 10(-2) Pa shows nanotextured structure with its c plane parallel to the substrate. Although the nanotexture is not evident by x-ray diffraction, XANES can unambiguously indicate the texturing. Cross-sectional high-resolution electron microscopy provides the evidence of the nanostructure. &COPY; 2005 American Institute of Physics.

High-resolution near-edge x-ray absorption fine structure (NEXAFS) at Mn K edge is employed to probe the local environment of Mn dopant in ZnO. First-principles supercell calculations are systematically made to obtain theoretical NEXAFS. Mn is found to substitute for Zn up to 5 at.%Mn in polycrystalline samples sintered at 1623 K in air. Presence of Mn3O4 is apparent for samples with higher Mn content. The NEXAFS does not change in the range of Mn concentration from 0.01 to 5 at. %, indicating the absence of Mn precipitates. The results are confirmed by examining the polarization dependence of the NEXAFS for a 5 at. %-doped ZnO thin film. (C) 2005 American Institute of Physics.

Local environments of solutes in β- and spinel Si 6-zAl zO zN 8-z are investigated by means of Al K x-ray absorption near-edge structure. The experimental spectra are found to be the same throughout the wide solubility range. This suggests that the local environments of Al are independent of the solute concentration. First-principles band-structure calculations are systematically made to interpret the experimental spectra. Effect of a core hole was included into the calculation. Theoretical spectra were obtained using variety of different model structures constructed by a set of plane-wave pseudopotentials calculations in our previous study [K. Tatsumi, I. Tanaka, H. Adachi, and M. Yoshiya, Phys. Rev. B 66, 165210 (2002)]. The numbers of models were 51 and 45 for both β and spinel, respectively. They are classified and averaged according to the local atomic structure of Al solutes. The combination of experimental spectra and theoretical results can unambiguously lead to the conclusion that Al atoms are preferentially coordinated by O atoms in both β and spinel phases. This is consistent with the conclusion obtained by the first-principles total-energy calculations. In the spinel phase, Al atoms are found to be located preferentially at the octahedral cationic site. This agrees with the conclusion in a recent report on the nuclear magnetic resonance experiment. ©2005 The American Physical Society.

Local environments of solutes in beta- and spinel Si6-zAlzOzN8-z are investigated by means of Al K x-ray absorption near-edge structure. The experimental spectra are found to be the same throughout the wide solubility range. This suggests that the local environments of Al are independent of the solute concentration. First-principles band-structure calculations are systematically made to interpret the experimental spectra. Effect of a core hole was included into the calculation. Theoretical spectra were obtained using variety of different model structures constructed by a set of plane-wave pseudopotentials calculations in our previous study [K. Tatsumi, I. Tanaka, H. Adachi, and M. Yoshiya, Phys. Rev. B 66, 165210 (2002)]. The numbers of models were 51 and 45 for both beta and spinel, respectively. They are classified and averaged according to the local atomic structure of Al solutes. The combination of experimental spectra and theoretical results can unambiguously lead to the conclusion that Al atoms are preferentially coordinated by O atoms in both beta and spinel phases. This is consistent with the conclusion obtained by the first-principles total-energy calculations. In the spinel phase, Al atoms are found to be located preferentially at the octahedral cationic site. This agrees with the conclusion in a recent report on the nuclear magnetic resonance experiment.

High-resolution X-ray absorption spectra of Al, AIN and Al2O3 are measured at the Al K-edge, which have revealed a chemical shift in the threshold energy and significant differences in the spectral fine structures among the three compounds. In order to interpret the chemical shift and the fine structures of these spectra, first-principles calculations using the full-potential linearized augmented plane wave method within the density functional theory are carried out, taking into account the core-hole effect. The resultant theoretical spectra quantitatively reproduce both the chemical shift and the spectral fine structures of the experimental ones. The dependence of the theoretical spectra on the supercell size is also examined.

First principles calculations of L-3 XANES/ELNES of GaN and InN with both wurtzite and zinc-blende structures have been made using OLCAO (orthogonalized linear combinations of atomic orbitals) method. Supercells with more than 100 atoms were employed. A core-hole was rigorously included in the calculation, and the photo absorption cross section (PACs) between the initial and final states was computed. Quantitative reproduction of experimental spectrum that is available in literature can be found when the PACs was computed. Although spectral shapes of two phases took similar, characteristic differences are predicted to appear at the first peak of the L3 XANES/ELNES. The first peak is notably broader in the zinc-blende phases. The origin of the broadness is analyzed using partial density of unoccupied states (PDOS) and Mulliken charge. We then conclude that the broadness can be related to greater covalency of the zinc-blende phase as compared to the wurtzite phase.

First principles calculations of hydrated polymolybdates complexes have been made with an atomic orbital basis molecular orbital method. Hydration effects are taken into account by the conductor-like screening model (COSMO) using dielectric constant of water. Hydrated heptapolymolybdate, Mo7O246-, shows a low symmetry structure, which agrees well to experimental results, i.e., X-ray diffraction of crystalline salts and X-ray absorption fine structure of the hydrated complex. Contrary to that, the hydrated heteropolymolybdate, NiMo6O2410- prefer to exhibit the high symmetry structure. Inspection of the electronic states found that the Ni ion exhibits trivalent state or d(7) configuration in a formal sense. Jahn-Teller distortion around Ni is therefore evident. Such distortion cannot be found in CrMo6O249- or CoMo6O249-.

Silicon nitride thin film was fabricated by pulsed laser deposition using KrF excimer laser and a silicon nitride compact as a target. The deposition was carried out on Al2O3 (0001) at 1173 K in N-2 gas pressure of 0.27 Pa. The X-ray diffraction did not provide any structural information of the deposited thin films except that it is composed of amorphous and/or micro-crystalline structure. X-ray absorption near edge structures at Si-K edge revealed that local arrangement of Si is not random. It should be composed of SiN4 unit similar to the case of alpha-Si3N4 crystal. Metallic Si component cannot be found in XANES.

First principles calculations have been carried out to investigate the core-hole effects on the theoretical fine structures of the X-ray absorption spectra of MgF2 and ZnF2 at F K-edge. Significant differences are found between the calculated spectral fine structures with and without core-holes. Experimental profiles of the near-edge X-ray absorption fine structures are well reproduced by the theoretical ones when the core-hole effect is introduced. The dependence of supercell size on the theoretical fine structures is also examined.

First principles calculations of rutile-type TiO2:S have been performed to investigate the effect of sulfur solutes on the electronic structure. Plane-wave pseudopotentials method has been employed and atomic relaxations were fully taken into account. All possible geometric configurations for sulfur solutes within a 12-atoms supercell have been examined changing sulfur concentration of x = 0, 0.25, 0.5. 0.75 and 1. Theoretical direct band gap is found to decrease as sulfur concentration is increased. The dependence on the sulfur concentration is weaker than that was predicted in literature. Both the optimization of solute configuration and atomic relaxation are found to be essential for quantitative evaluation of the electronic structures in the alloy.

Theoretical calculations of electron energy-loss near-edge structure (ELNES) and x-ray absorption near-edge structure (XANES) of selected wide-gap materials including TiO2, AlN, GaN, InN, ZnO, and their polymorphs are performed using a first-principles method. Calculations of 39 K and L-3(L-2,L-3) edges are made using large supercells containing 72 to 128 atoms. A core hole is included in the final state, and the matrix elements of the electric dipole transition between the ground state and the final state are computed. Structures of some metastable crystals are optimized by a plane-wave basis pseudopotential method. Spectral differences in ELNES and XANES among polymorphs are quantitatively reproduced in this way. The origin of the spectral differences is pursued from the viewpoint of chemical bondings. Crystallographic orientation dependence of ELNES and XANES is also examined both by experiment and theory. The dependence is found to be much larger in K edges than that in L-3(L-2,L-3) edges.

A high pressure form of ZnO with a rock-salt structure can be synthesized as a thin film by a pulsed laser deposition technique when greater than or equal to 15 mol% of MgO is alloyed. The phase is identified both by x-ray diffraction and near edge x-ray absorption fine structure (NEXAFS) measurements. NEXAFS is interpreted with the aid of first principles calculations employing a supercell composed of 108 atoms with a Zn-1s core-hole. The rock-salt phase can be formed only when an MgO(100) substrate is used because of a favourable lattice matching. As a result of the heteroepitaxy, the crystal is distorted tetragonally by c/a = 1.02. The energy increase due to the tetragonal deformation is estimated to be 2 meV/ZnO by a first principles calculation.

High-resolution Si KL2.3V Auger spectra of Si, SiC and SiO2 are measured to investigate changes in the electronic structure of the valence band in these materials. Significant differences between the lineshapes of these spectra are observed. First-principles electronic structure calculations are also carried out, which reproduce the lineshapes of the KL2.3V Auger spectra of Si, SiC and SiO2 observed experimentally. The core-hole effects on the theoretical spectral lineshapes are also taken into account in the present calculations. (C) 2003 Elsevier B.V. All rights reserved.

The properties of ceramic materials are strongly influenced by the presence of ultradilute impurities (dopants). Near-edge X-ray absorption fine structure (NEXAFS) measurements using third-generation synchotron sources can be used to identify ultradilute dopants, provided that a good theoretical tool is available to interpret the spectra. Here, we use NEXAFS analysis and first-principles calculations to study the local environments of Ga dopants at levels of 10 p.p.m in otherwise high-purity MgO. This analysis suggests that the extra charge associated with substitutional Ga on a Mg site is compensated by the formation of a Mg vacancy. This defect model is then confirmed by positron lifetime measurements and plane-wave pseudopotential calculations. This powerful combination of techniques should provide a general method of identifying the defect states of ultradilute dopants in ceramics.

Large-scale first-principles density functional theory (DFT) calculations have been carried out to investigate how Al3+ can be incorporated into MgSiO3 perovskite under high pressure and to study the resultant change in the compressional mechanism Of MgSiO3 perovskite. We examined two types Of MgSiO3 models with 6.25 mol% Al2O3: charge-coupled substitution and oxygen-vacancy mechanisms. Five pressure points from 0 to 100 GPa have been considered. At each pressure point, we have calculated five models of the oxygen vacancy and five models of the charge-coupled mechanisms. We also change the internal positions of the substituted Al in the calculated cells, which have 80 atoms. Our free energy calculations show Al3+ replaces the nearest-neighbor cation pairs (Mg2+ and Si4+) at all pressures investigated. The calculated bulk modulus of the most energetically favorable model is 3.4% lower than that of the Al-free MgSiO3 perovskite. These results may have important implications for discriminating between thermal and compositional effects of I-D Earth models and the possible influence of aluminum perovskite. (C) 2002 Elsevier Science B.V. All rights reserved.

Soft X-ray emission and absorption spectra in the Si L region of a number of substituted polysilanes, (SiR2)n, have been obtained using synchrotron radiation to investigate their electronic structures. Studied polysilanes were substituted with methyl (R = CH3), ethyl (C2H5), propyl (n-C3H7), butyl (n-C4H9), pentyl (n-C5H11) and phenyl (C6H5) groups. Although similar spectral features in both X-ray emission and absorption are observed among alkyl-substituted polysilanes, slight differences are distinguished between alkyl- and phenyl-substituted ones. These spectral features are qualitatively reproduced by summing calculated density-of-state spectra for Si3s- and Si3d-orbital.s. Thus, spectral features are explained through the hybridization of electronic orbitals in both backbone Si atoms and substituent C atoms. (C) 2002 Elsevier Science B.V. All rights reserved.