Spectroscopic properties of ZnWO4 single crystal doped with Fe and Li impurities

The influence of Fe and Li co-doping on optical properties of ZnWO₄ crystals have been investigated by optical spectroscopy and electron paramagnetic resonance (EPR). The iron ions were added to the ZnWO₄ composition in the form of Fe⁰, FeO or Fe₂O₃. EPR has been used for determination of the Fe³⁺ concentration. The luminescence and excitation spectra obtained at different temperatures are presented. Under excitation 325 nm, two bands were observed: blue-green emission peaked at 490 nm and red peaked at 650 nm. The blue- green emission is related to Fe³⁺ and their shape depends on the doping method. The red emission is not related to the Fe³⁺.     

On the Nature of Manganese Ions in the β-Ca2 SiO4 Structure Detected by High Field ESR

The stabilization effect of dopants of the structure of β-dicalcium silicate using manganese in different oxidation states was investigated in detail by ESR at 3.2 cm (X-band), 8mm(Q-band) and by high field ESR at 2mm(W-band). The assignment of the signals was supported by corresponding single crystal measurements using X-Band and Q-band. If there are only low manganese concentrations on forming the microcrystallites the incorporated Mn²⁺ will be localized on less distorted sites of the lattice. Increasing the manganese concentration the Mn²⁺ spectra are superimposed by the fine structure of Mn²⁺ and of Mn⁴⁺ ions occupying distorted lattice sites with axial and rhombic symmetry. This pattern of the cation substitution represents the typical transition metal behaviour and obviously favours the formation of regular crystalline units from the melt on compensating of non-equivalent distances and charges.     

Isoperimetric inequalities and identities fork-dimensional cross-sections of convex bodies

Supported in part by a grant from the National Science foundation     

Control of Luminescence by Tuning of Crystal Symmetry and Local Structure in Mn4+‐Activated Narrow Band Fluoride Phosphors

Mn⁴⁺‐doped fluoride phosphors have been widely used in wide‐gamut backlighting devices because of their extremely narrow emission band. Solid solutions of Na₂(Si_xGe_1?x)F₆:Mn⁴⁺ and Na₂(Ge_yTi_1?y)F₆:Mn⁴⁺ were successfully synthesized to elucidate the behavior of the zero‐phonon line (ZPL) in different structures. The ratio between ZPL and the highest emission intensity υ₆ phonon sideband exhibits a strong relationship with luminescent decay rate. First‐principles calculations are conducted to model the variation in the structural and electronic properties of the prepared solid solutions as a function of the composition. To compensate for the limitations of the Rietveld refinement, electron paramagnetic resonance and high‐resolution steady‐state emission spectra are used to confirm the diverse local environment for Mn⁴⁺ in the structure. Finally, the spectral luminous efficacy of radiation (LER) is used to reveal the important role of ZPL in practical applications.     

Nanostructured cathodic catalysts for direct methanol fuel cells

The high activity of catalysts based of nanodisperse Pt-P system and their tolerance to the poisoning effect of methanol are demonstrated for the working potentials of cathodes in methanol-air fuel cells. The catalysts’ activity in the oxygen reduction reaction in the presence of methanol is nearly hundred times that of catalysts based on mixed metal-chalcogenide systems.     

Padé approximants to the evolution operator through the Lippmann Schwinger variational principle

Using the connection between the evolution operator and the stationary value of the Lippmann–Schwinger functional, approximations to this operator are obtained using diagonal Padé approximants. A harmonic oscillator with a non-hermitean perturbation proportional to powers of the bosonic creation operator is considered and its evolution operator is evaluated. The poles of the spectral representation obtained by this method are compared to both: the ones of the usual perturbative expansion and those of the exact solution. Extensions to Hermitian Hamiltonians are discussed, involving the necessity of inverting more complex operators in the calculation of the Fourier transform. However, the approximation obtained by this procedure becomes exactly unitary. © 1995 John Wiley & Sons, Inc.     

Fluoroaliphatic esters of fluorosulfonic acid. 1. Reactions of higher fluoroolefins with electrochemically generated peroxydisulfuryl difluoride

The electrolysis of fluorosulfonic acid in the presence of perfluoro-2-alkenes in an electrolyzer without a diaphragm results in the formation of a mixture of products, whose composition is determined by both the anodic and cathodic processes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2048–2056, September, 1990.