Structural, thermodynamic and optical properties of MgF2 studied from first-principles theory

A detailed theoretical study of structural, electronic, elastic, thermodynamic and optical properties of rutile type MgF₂ has been carried out by means of first-principles Density Functional Theory (DFT) calculations using plane wave pseudo-potentials within the local density approximation and generalized-gradient approximation for the exchange and correlation functionals. The calculated ground state properties and elastic constants agree quite well with experimental values. From the calculated elastic constants we conclude that MgF₂ is relatively hard when compared to other alkaline-earth fluorides and ductile in nature. The thermodynamic properties such as heat capacity, entropy, free energy, phonon density of states and Debye temperatures are calculated at various temperatures from the lattice dynamical data obtained through the quasi-harmonic Debye model. From free energy and entropy it is found that the system is thermodynamically stable up to 1200 K. The imaginary part of the calculated dielectric function ε₂(ω) could reproduce the six prominent peaks which are observed in experiment. From the calculated ε(ω), other optical properties such as refractive index, reflectivity and electron energy-loss spectrum are obtained up to the photon energy range of 30 eV.     

Pressure-induced polymorphism in Al3BC3: A first-principles study

Al₃BC₃, an isostructural phase to Mg₃BN₃, experienced no pressure-induced phase transformation that occurred in the latter material (J. Solid State Chem. 154 (2000) 254-256). The discrepancy is not clear yet. Using the first-principles density functional calculations, we predict that Al₃BC₃ undergoes a hexagonal-to-tetragonal structural transformation at 24 GPa. The predicted phase equilibrium pressure is much higher than the previously reported pressure range, i.e., 2.5-5.3 GPa, conducted on phase stability of Al₃BC₃. A homogeneous orthorhombic shear strain transformation path is proposed for the phase transformation. The transformation enthalpy barrier is estimated to yield a low value, i.e., 0.129 eV/atom, which ensures that the transformation can readily take place at the predicted pressure.     

The molecular geometry of the addition compound Cl3Ga.NH3 as studied by electron diffraction

The molecular geometry of the complex of gallium trichloride with ammonia, Cl₃Ga.NH₃, has been studied by electron diffraction. The most important internuclear distances in terms of r_a parameters are as follows: r(Ga-Cl) = 2.142±0.005Å, r(Ga-N) = 2.057±0.011Å, r(Cl?Cl) = 3.642±0.010Åand r(Cl?N) = 3.242±0.012Å. As in the case of the aluminium analogue, the flat pyramidal configuration of the GaCl₃ part of the complex suggests a planar equilibrium structure for free GaCl₃. The distance between the donor and acceptor parts may indicate a somewhat weaker interaction than is the case in the aluminium analogue.     

On the molecular structure of (CH3)2NSO2N(CH3)2 as studied by gas electron diffraction

The following bond lengths and bond angles have been deduced from a vapour phase electron diffraction study of (CH₃)₂NSO₂N(CH₃)₂: r(C-H) 1.114 ± 0.005 Å, r(S-O) 1.432 ± 0.010 Å, r(N-C) 1.475 ± 0.013 Å, r(S-N) 1.651 ± 0.003 Å, ∠N-C-H 109.3 ± 2.0°, ∠C-N-C 118.0 ± 302°, ∠S-N-C 115.2 ± 1.1°, ∠N-S-N 110.5±1.3° and ∠O-S-O 114.7±2.5°. The sulphur bond configuration and the prevailing conformation, which was identical to that in the crystal, are discussed in relation to analogous sulphide and sulphoxide derivatives.     

Exploring the mechanism of Ta3N5/KTaO3 photocatalyst for overall water splitting by first-principles calculations

The rational fabrication of heterostructures is one of efficient strategies for improving photocatalytic performance of semiconductor photocatalysts.Very recently,Domen and co-workers found that Ta₃N₅ single crystals grown on the surface of KTaO₃ can accomplish photocatalytic overall water splitting for the first time.In order to comprehend the underlying mechanism of this photocatalytic system,we have performed a systematic study based on density functional theory first-principles calculations.Ta₃N₅(010)/KTaO₃(110)slab models have been built according to experimental observations by considering two common terminations of KTaO₃(110)surface,named as Ta₃N₅/O₂ and Ta₃N₅/KTaO.The formations of interfacial bonds are thermodynamically stable,showing a covalent interaction between two components of a heterostructure.Ta₃N₅/O₂ has a higher mobility of photogenerated charge carriers and lower recombination rate of charge carriers than Ta₃N₅/KTaO.The light absorption of heterostructures displays the feature of KTaO₃ in the short wavelength region and the characteristic of Ta₃N₅ in the long wavelength region.The calculated band offsets show that Ta₃N₅/O₂ and Ta₃N₅/KTaO have distinct Type-II band alignments,with Ta₃N₅ as the accumulator of photoinduced electrons in the former and the collector of photogenerated holes in the latter,respectively.The difference in charge density and electrostatic potential between two components acts as a driving force to promote the transfer of electrons and holes to different domains of the interface,which is beneficial to extend the lifetime of photoinduced carriers.Our results demonstrate that the function of Ta₃N₅ in Ta₃N₅/KTaO₃ photocatalytic system is determined by the termination property of KTaO₃(110)surface,which provides a likely reason of the observed photocatalytic activity of overall water splitting achieved by Ta₃N₅ synthesized by using KTaO₃ as a precursor for the nitridation reaction.     

Optical properties of CeBO3 and CeB3O6 compounds: first-principles calculations and experimental results

The cerium borates o-CeBO₃, m-CeBO₃ and CeB₃O₆ have been shown to be isostructural to their lanthanum derivatives. From diffuse reflectance, electron energy loss spectroscopy (EELS) and band structure calculations, it has been evidenced that a Ce³⁺ 4f-5d transition is responsible for weak absorption peaks around 3.5 eV while the O2p-Ce5d charge transfer gives rise to a strong absorption around 7 eV. Starting from self-consistent full potential LAPW calculations, the dielectric tensors of the three compounds were computed and compared to experimental data. It results in a satisfactory fit between the observed and the calculated extinction coefficient k and the index of refraction n.     

Cathodic electrochemiluminescence at double barrier Al/Al2O3/Al/Al2O3 tunnel emission electrodes

Double insulating barrier tunnel emission electrodes were fabricated by adding a new pure aluminum layer upon oxidized aluminum electrodes by vacuum evaporation and thermally oxidizing the new aluminum layer in air at room temperature. Resulting Al/Al₂O₃/Al/Al₂O₃ electrodes allow the use of various aluminum alloys in the electrode body necessary for hardness or shaping ability of the electrode while obtaining the luminescence properties of pure aluminum oxide. During electrical excitation of luminescent labels by cathodic hot electron injection into aqueous electrolyte solution, the background noise is mainly based on high-field-induced solid-state electroluminescence and F-center luminescence of the outer aluminum oxide film. The more defect states and/or impurity centers the outer oxide film contains, the higher is the background emission intensity. The present electrode fabrication method provides a considerable improvement in signal-to-noise ratio for time-resolved electrochemiluminescence (TR-ECL) measurements when the original native oxide film of the electrode body contains luminescence centers displaying long-lived luminescence. The excellent performance of the present electrodes is demonstrated by extremely low-level detection of Tb(III) chelates, luminol, Pt(II) coproporphyrin and Tb(III) labels in an immunometric immunoassay by time-resolved electrochemiluminescence.     

New Alq3 derivatives with efficient photoluminescence and electroluminescence properties for organic light-emitting diodes

Two new ligands prepared under solvent free conditions and five aluminum complexes derived from 8-hydroxyquinoline have been synthesized and characterized. The majority of the new aluminum tris(8-hydroxyquinoline) derivatives have nitrogen functionalities at position-4 of the quinolate ligand. The photoluminescence emission wavelengths of the new Alq₃ derivatives are shifted according to the electronic properties of the substituents at position-4. Results from differential scanning calorimetry (DSC) investigations of the new Alq₃ derivatives indicate that the complexes 9 and 10 are non crystalline and have higher transition glass temperatures than the parent Alq₃. The EL measurements of OLED devices with complexes 7, 9, and 10 as emitters revealed that complexes 7, 9, and 10 are efficient emitters in organic light emitting diodes.     

自组装铝/氧化铜和铝/氧化铁及其性能评估

本文分别采用模板法制备氧化铜纳米花,水热法制备氧化铁纳米环,并自组装制备了铝/氧化铜和铝/氧化铁2种铝热剂。自组装增大了异相材料之间的接触,分别使得铝/氧化铜的反应放热量和压力由523 J.g-1、1 858 kPa增加至1 069 J.g-1、4 389kPa;铝/氧化铁的反应放热量和压力由1 448 J.g-1、749 kPa增加至2 039 J.g-1、2 280 kPa。两种铝热剂的放热量和压力差别较大,且铝/氧化铜的静电感度高于大多数含能材料,铝/氧化铁的撞击感度特别低,显示出不同的应用特点。     

The role of acidity of MoO3?SiO2 and WO3?SiO2 catalysts

The acidity of various MoO₃–SiO₂ and WO₃ catalysts has been measured by means of ammonia adsorption. Whereas the acidity of silica and pure metal oxides is very low, an increase of surface acidity results from a reaction between silica support and metal oxides. A significant correlation between surface acidity and catalytic activity for aromatization of lower olefines could be found.

---

MoO₃–SiO₂ WO₃ . SiO₂ . , SiO₂ . .

     

Transient SIMS depth-profiles at the interface Si3N4/GaAs

Summary A transient secondary ion signal enhancement during sputtering through the interface of a two-layer target of Si₃N₄/GaAs could be demonstrated to be an artefact caused by the bombardment with oxygen ions, which were used as a primary beam. A simple model is established which describes ion-induced composition changes during sputtering (sputter-emission and recoil implantation from the surface, cascade mixing, and implantation of projectiles). The application of this model permits the simulation of internal profiles of a trace element, the matrix atoms and of implanted primary ions, and consequently the simulation of the emitted particle flux during sputtering. These calculations indicate an accumulation of implanted primary ions at the GaAs-side of the interface which considerably enhances the yield of positive secondary ions in this zone. The calculated response fits experimental data within a factor of less than 2.

---

Quantitative SIMS-Analyse an der Grenzfläche Si₃N₄/GaAs

Zusammenfassung Bei der Analyse von Verunreinigungen (Chrom) in Si₃N₄-Schichten auf GaAs mit der Sekundärionen-Massenspektrometrie (SIMS) tritt am Schichtübergang ein transienter Anstieg der Intensität positiver Sekundärionen von Chrom und Arsen auf. Es wird experimentell gezeigt, daß dieser Anstieg nicht auf das Vorhandensein einer parasitären Zwischenschicht aus Chrom oder eines Oxides, sondern auf den Beschuß mit einem Sauerstoff-Primärstrahl zurückzuführen ist. Es wurde ein einfaches Modell erstellt, das die ioneninduzierten Veränderungen der gesputterten Probe beschreibt (Sputter-Emission und Recoil-Implantation aus der Oberfläche, Vermischung der Matrixatome durch Kaskadenmischung, Implantation der Primärionen). Die numerische Simulation der SIMS-Messung durch Anwendung dieses Modelles auf die untersuchten Proben zeigt eine Anhäufung implantierter Primärionen knapp hinter dem Schichtübergang. Im Fall von Sauerstoff erhöht dieser lokal die Ausbeute an positiven Sekundärionen. Der erechnete Verlauf dieser Signalüberhöhung stimmt innerhalb eines Faktors < 2 mit den Experimenten überein.

     

ESR spectra of O−3 and ClO3 exchange-coupled to molecular oxygen

ESR spectra of two paramagnetic defects in X-irradiated KClO₄ are reported. It is shown that the defects can be described in terms of complexes of molecular oxygen with the ClO₃ radical and the O_?3 radical-ion, the interaction being well represented as an isotropic exchange coupling.     

Ethylene-butadiene copolymerization with three-component catalyst systems Al[N(CH3)2]3, Al(C2H5)Cl2 and vanadium chlorides

The ethylene butadiene copolymerization with systems of three components Al[N(CH₃)₂]₃, Al(C₂H₅)Cl₂ and VOCl₃ or VCl₄, is described. In the resulting copolymers, the butadiene units are substantially in trans-1,4 configuration. Although it is possible to obtain copolymers with a wide range of composition, attention was paid to products with a low content of unsaturation (less than 2% mole of butadiene). These copolymers are highly homogeneous. They show high crystallinity of the polyethylene type and they can be crosslinked with conventional sulphur recipes.     

Molecular structures of (SiCl3)2CH2 and (SiCl3)2CCl2 as studied by electron diffraction

An electron diffraction analysis of the molecular structures of 1,1,1,3,3,3-hexachloro-1,3-disilapropane and octachloro-1,3-disilapropane has been carried out. Deviations from the staggered conformation are indicated. The data may be approximated by models with C₂ symmetry and a small tilt of the SiCl₃ groups. The main bond lengths (r_g) and bond angles obtained for (SiCl₃)₂ CH₂ are: SiCl, 202.7(4); SiC, 186.6(6); CH, 109.8(24) pm, ClSiCl, 107.9(1); SiCSi, 118.3(7)°; and for (SiCl₃)₂CCl₂: SiCl, 202.0(4); SiC, 190.2(9); CCl, 179.6(9) pm; ClSiCl, 109.5(1); SiCSi, 120.6(9); ClCCl, 110.9(16); SiCCl, 106.3(3)°.     

Synthesis and molecular structure of [Au3Ru4(μ3-H)(CO)12(PPh3)3]

The title compound can be prepared in good yield by heating either [Ru₄(μ-H)₄(CO)₁₂] or [Au₂Ru₄(μ₃-H)₂(CO)₁₂(PPh₃)₂] with [AuMe(PPh₃)] in toluene. The related compound [Au₃Ru₄(μ₃-H)(μ-dppm)(CO)₁₂(PPh₃)] has also been prepared. Both trigoldtetraruthenium clusters undergo dynamic behaviour in solution, involving intramolecular rearrangement of the metal core, as revealed by variable temperature NMR studies. The crystal structure of [Au₃Ru₄(μ₃-H)(CO)₁₂(PPh₃)₃] has been established by an X-ray diffraction study. The metal atom core comprises a trigonal bipyramidal AuRu₄ unit with two AuRu₂ faces capped by gold atoms.     

Photoinduced formation of phosphido-bridged clusters derived from Ru3(CO)9(PPh2H)3. Crystal and molecular structure of Ru3(μ-H)(μ-PPh2)3(CO)7

The new complex Ru₃(CO)₉(PPh₂H)₃ (I) was prepared by the direct thermal reaction of Ru₃(CO)₁₂ with PPh₂ H and was spectroscopically characterized. Irradiation of I with λ ≥ 300 nm leads to the formation of Ru₂(μ-PPh₂)₂(CO)₆ (II) and three new phosphido-bridged complexes, Ru₃(μ-H)₂(μ-PPh₂)₂(CO)₈ (III), Ru₃(μ-H)₂(μ-PPh₂)₂(CO)₇(PPh₂H) (IV) and Ru₃(μ-H)(μ-PPh₂)₃(CO)₇ (V). These complexes have been characterized spectroscopically and Ru₃ (μ-H)(μ-PPh₂)₃(CO)₇ by a complete single crystal X-ray structure determination. It crystallizes in the space group P2₁/n with a 20.256(3), b 22.418(6), c 20.433(5) Å, β 112.64(2)°, V 8564(4) ų, and Z = 8. Diffraction data were collected on a Syntex P2₁ automated diffractometer using graphite-monochromatized Mo-K_α radiation, and the structure was refined to R_F 4.76% and R_wF 5.25% for the 8,847 independent reflections with F₀ > 6σ(F₀). The structure consists of a triangular array of Ru atoms with seven terminal carbonyl ligands, three bridging diphenylphosphido ligands which bridge each of the RuRu bonds, and the hydride ligand which bridges one RuRu bond. Complex IV was also shown to give V upon photolysis and is thus an intermediate in the photoinduced formation of V from I.     

Theoretical study of the molecular ions SiH−5 and SiH−3

Ab initio HF and Cl calculations were performed to determine the equilibrium geometry of SiH^?₅ and SiH^?₃, the barrier for internal rotation (SiH^?₅) and inversion (SiH^?₃) and the stability of SiH^?₅ and further to study the effect of electron correlation on reaction energies. The gaussian-type basis included d and f functions on Si and a p set on II. The D_3h structures of SiH^?₅ is lower in energy than the C_4v structure by 2.9(3.2) kcal/mol (corresponding HF results in parentheses). SiH^?₃ has C_3v structure, the inner-ion barrier computed is 26.2 (27.3) kcal/mol. SiH^?₅ turns out to be stable with respect to SiH₄ + H^? by 20.3 (13.8) kcal/mol, but it is unstable with respect to SiH^?₃ ← H₂ by 6.3 (5.6) kcal/mol. These results show that electron correlation has a small effect on barriers of inversion (SiH^?₃) or pseudorotation (SiH^?₅), but may have a pronounced effect on reaction energies even if all systems involved have closed shells. The correlation energy contributions are analyzed in terms of intrapair and interpair terms in order to get a better understanding of the influence of correlation on reaction and activation energies.     

HNbO3 and HTaO3: New cubic perovskites prepared from LiNbO3 and LiTaO3 via ion exchange

The synthesis of HNbO₃ and HTaO₃ from LiNbO₃ and LiTaO₃ via ion exchange in hot aqueous acid solutions is reported. This reaction is accompanied by a topotactic structural transformation from the rhombohedral LiNbO₃ structure to the cubic perovskite structure; cell constants are a = 3.822(1) Å for HNbO₃ and 3.810(2) Å for HTaO₃. These new compounds have been characterized by powder X-ray diffraction, thermogravimetric analysis, and solid-state NMR. They are electronic insulators and have low ionic conductivity. Evidence of partially proton-exchanged phases Li_1?xH_xMO₃ was also seen. The possible significance of this ion exchange reaction for devices using LiNbO₃ or LiTaO₃ is discussed.     

Luminescence properties of efficient X-ray phosphors of YBa3B9O18, LuBa3(BO3)3, α-YBa3(BO3)3 and LuBO3

The samples of YBa₃B₉O₁₈, LuBa₃(BO₃)₃, α-YBa₃(BO₃)₃ and LuBO₃ powders have been synthesized by the solid-state reaction methods at high temperature and their X-ray excited luminescent properties were investigated. All the studied materials show a broad emission band in the wavelength range of 300-550 nm with the peak centers at about 385 nm for YBa₃B₉O₁₈ and LuBa₃(BO₃)₃, 415 nm for α-YBa₃(BO₃)₃ and 360 nm for LuBO₃ powders, respectively. Even though those compounds have the different atomic structures, they have the common structural feature of each yttrium or lutetium ion bonded to six separate BO₃ groups, i.e., octahedral RE(BO₃)₆ (RE=Lu or Y) moiety. This octahedral RE(BO₃)₆(RE=Lu or Y) moiety seems to be an important structural element for efficient X-ray excited luminescence of those compounds, as are the edge-sharing octahedral TaO₆ chains for tantalate emission.