Modulus Spectroscopy and Dielectric Dispersion Studies in Alkali Metal Perchlorates

Alkali metal perchlorates (KClO4, RbClO4, and CsClO4) undergo a structural phase transition from the orthorhombic to the cubic phase at elevated temperatures. A detailed dielectric study of these crystals across the phase transition is carried out at different frequencies. The crystals are found to exhibit pronounced dielectric dispersion in the kHz frequency range. The results support the view that these transitions are of order–disorder type. The dielectric behaviour at temperatures above Tc is discussed in terms of modulus spectroscopy. An estimate of conductivity relaxation times above the phase transition temperatures made from modulus spectroscopy data gives values of 3.1, 12.2 and 17.7 μs for KClO4, RbClO4, and CsClO4, respectively.

Nature of nitrogen adsorbed on transition metal surfaces as revealed by electron spectroscopy and cognate techniques

The nature of the chemisorbed states of nitrogen on various transition metal surfaces is discussed comprehensively on the basis of the results of electron spectroscopic investigations augmented by those from other techniques such as LEED and thermal desorption. A brief discussion of the photoemission spectra of free N2, a comparison of adsorbed N2 and CO as well as of physisorption of N2 on metal surfaces is also presented. We discuss the chemisorption of N2 on the surfaces of certain metals (e.g. Ni, Fe, Ru and W) in some detail, paying considerable attention to the effect of electropositive and electronegative surface modifiers. Features of the various chemisorbed states (one or more weakly chemisorbed gamma-states, strongly chemisorbed alpha-states with bond orders between 1 and 2. and dissociative chemisorbed beta-states) on different surfaces are described and relations between them indicated. While the gamma-state could be a precursor of the alpha-state, the alpha-state could be the precursor of the beta-state and this kind of information is of direct relevance to ammonia synthesis. The nature of adsorption of N2 on the surfaces of some metals (e.g. Cr, Co) deserves further study and such investigations might as well suggest alternative catalysts for ammonia synthesis.

On the models for deuterium long-range isotope effects in13C NMR spectroscopy

The long-range deuterium isotope effects on13C nuclear shielding are physically not yet completely understood. Two existing models for explaining these effects, vibrational and substituent, are compared here. The vibrational model is based on the Born-Oppenheimer approximation, but it can explain only one-bond deuterium effects. To the contrary, the substituent model may explain many long-range isotope effects, but it is controversial due to the assumption of some distinct electronic properties of isotopes. We explain how long-range deuterium isotope effects may be rationalized by the subtle electronic changes induced by isotope substitution, which does not violate the Born-Oppenheimer approximation.

Reduction In Metal-Mold Reactivity In Titanium Castings Through The Use Of Sodium Aluminate As Binder In Zircon Sand Molds

The present work is aimed at evaluating an alternative moulding system, namely, sodium aluminate bonded zircon sand mould and assess its suitability in relation to the much studied sodium silicate bonded zircon sand moulding system. It is described in the study presented here that with regard to metal - mould reaction, sodium aluminate bonded zircon sand mould system is a superior viable system as compared to sodium silicate bonded zircon moulding system at mould firing temperatures of 873 - 1473 K.

Superconducting gap in Nb seen by point contact spectroscopy

The nonlinear current voltage characteristics of a point contact convey information about various excitations in the metal. We have made a poin~ contact study on a superconductor to see the band gap and on a normal metal to see Ihe transport characteristics.

Metal-insulator transitions in Cr-doped V2O3 by x-ray and ultraviolet photoelectron spectroscopy

X-ray and ultraviolet photoelectron spectroscopy have been employed to investigate the high temperature metal-insulator transitions in V2O3 and (V0.99Cr0.01)2O3. The high temperature transitions are associated with more gradual changes in the 3d bands compared to the low-temperature transitions

A Study of copper-compounds by x-Ray Absorption-spectroscopy

Chemical shifts, ΔE, of the K-absorption discontinuity in several compounds of copper possessing formal oxidation states between 0 and III have been measured. The shifts show a parabolic dependence on the formal oxidation state as well as on the effective atomic charge, q, on copper. Anomalous chemical shifts shown by some of the compounds are discussed in terms of the bonding in these compounds. The ΔE values have also been correlated with the core electron binding energies obtained from X-ray photoelectron spectroscopy.

Single‐gate deep level transient spectroscopy technique

A new deep level transient spectroscopy technique is suggested which allows the deep level parameters to be obtained from a single temperature scan. Using large ratio t2/t1 of the measurement gate positions t1 and t2 and analyzing the steep high‐temperature side of the peak, it is demonstrated that the deep level activation energy can be determined with high accuracy.

Raman induced phase conjugation spectroscopy

Raman induced phase conjugation (RIPC) spectroscopy is a relatively new coherent Raman spectroscopic (CRS) technique using optical phase conjugation (OPC), with which complete Raman spectra of transparent media can be obtained. It is a non-degenerate four-wave mixing technique in which two pulsed laser beams at Ω1 and Ω1 ± Δ where A corresponds to a vibrational frequency of a nonlinear medium mix with a third laser beam at Ω1 to generate a fourth beam Ω1 ± Δ, which is nearly phase conjugate to one of the beams at Ω1. With this technique one can measure the ratio of the resonant and nonresonant components of the third-order nonlinear susceptibilities of the nonlinear media. We have used this technique to get Raman spectra of well-known organic solvents like benzene etc., using pulsed Nd: YAG -dye laser systems. We have also studied the effect of delaying one of the interacting beams with respect to the others and the phase conjugate property of RIPC signals.

An Investigation Of Alkali Aluminosilicate Glasses By Si-29 And Al-27 Magic-Angle-Spinning Nmr-Spectroscopy

Alkali aluminosilicate glasses prepared by the gel and the melt routes have been investigated by Si-29 and Al-27 MAS NMR spectroscopy. It is found that Al has a tetrahedral coordination in the gel glasses modified with equivalent proportions of alkalis unlike in a pure aluminosilicate glass where Al has both four and six coordinations. Silicon is present as Q4 units in all the 5M2O 5Al2O3 9OSiO2 ( M = Li, Na and K) gel glasses studied whereas it is present in Q2 or Q3 species in the lithium aluminosilicate glasses of compositions 40Li2O x Al2O3 (1-x)SiO2 (1 less-than-or-equal-to x less-than-or-equal-to 15) and xLi2O 10Al2O3 (1-x)SiO2 (20 less-than-or-equal-to x less-than-or-equal-to 40). The combination of Q2 and Q3 is also found in certain sodium aluminosilicate glasses, but they change to Q2 and Q1 as the concentration of SiO2 decreases.

Electrical transport in magnesium aluminate

The conductivity of MgAl2O4 has been measured at 1273, 1473 and 1673 K as a function of the partial pressure of oxygen ranging from 105 to 10−14 Pa. The MgAl2O4 pellet, sandwiched between two platinum electrodes, was equilibrated with a flowing stream of either Ar + O2, CO + CO2 or Ar + H2 + H2O mixture of known composition. The gas mixture established a known oxygen partial pressure. All measurements were made at a frequency of 1 kHz. These measurements indicate pressure independent ionic conductivity in the range 1 to 10−14 Pa at 1273 K, 10−1 to 10−12 Pa at 1473 K and 10−1 to 10−4 Pa at 1673 K. The activation energy for ionic conduction is 1·48 eV, close to that for self-diffusion of Mg2+ ion in MgAl2O4 calculated from the theoretical relation of Glyde. Using the model, the energy for cation vacancy formation and activation energy for migration are estimated.

Circular dichroism and 13C nuclear magnetic resonance spectroscopy of pennisetin from pearl millet

The conformation and stability of pearl millet prolamin (pennisetin) were examined by using circular dichroism and C-13 nuclear magnetic resonance spectroscopy. The far UV spectrum of pennisetin in 70% (v/v) aqueous ethanol showed the presence of predominant alpha-helical structure and its occurrence in the alpha + beta class of protein. The far and near UV spectra of pennisetin in ethanol: trifluoroethanol also supported this observation. However pennisetin showed the presence of some helical structure in 8 M urea which is known to be a highly unordered structure forming solvent. A decrease in alpha helical content of native pennisetin was observed with rise in temperature from 5-75-degrees-C and this effect of temperature was found to be reversible. A C-13 NMR spectrum of pennisetin in 70% ethanol suggested a high degree of molecular mobility in ethanol. Comparison of the cross polarization spectrum with the single pulse excitation spectrum suggested pennisetin to be a heterogeneous protein.