R.M. Kadam

Reactions of superoxide radicals with curcumin: probable mechanisms by optical spectroscopy and EPR.

Reactions of superoxide-crown ether complex with curcumin have been studied in acetonitrile. Optical absorption spectra showed that curcumin on reaction with superoxide forms a blue color intermediate absorbing at 560 nm, which subsequently decayed in a few hours with the development of the absorption band corresponding to the parent curcumin. The regeneration was 100% at low superoxide concentrations (1:1, or 1:2 or 1:3 of curcumin:superoxide) but reduced to 60% at high superoxide concentration (>1:5). The regeneration of curcumin is confirmed by HPLC analysis. Stopped-flow studies in acetonitrile following either the decay of parent curcumin at 420 nm or formation of 560 nm absorption have been used to determine the rate constant for the reaction of superoxide with curcumin. EPR studies confirmed the disappearance of characteristic superoxide signal in presence of curcumin with the formation of new featureless signal with g=2.0067. Based on these studies it is concluded that at low superoxide concentrations curcumin effectively causes superoxide dismutation without itself undergoing any chemical change. At higher concentrations of superoxide, curcumin inhibits superoxide activity by reacting with it.

Chemical incorporation of fluorine into YBa2 Cu3O6.5+δ

Abstract Evidence of incorporation of fluorine into the lattice of 1,2,3,7 orthorhombic superconducting oxide by a reaction of this compound with NH4HF2 at 600K is presented. While TG/DTA data give clue to the reaction between the two, XRD of the product shows a broad peak around 2Θ=26° which disappears on heating to 1000K giving BaF2 peaks. The product with four F atoms in place of two O atoms shows Tc close to that in the virgin orthorhombic oxide.

Low-field microwave absorption in Gd2CuO4: similarity and contrast with high-temperature superconducting materials

Abstract A low-field microwave absorption signal observed in Gd 2 CuO 4 below 260 K in EPR spectrometric measurements has features strickingly similar to those reported for high-temperature superconductors below T c . It has opposite phase compared to that of EPR absorption of Gd 3+ and is found to exhibit hysteresis effects on magnetic field cycling. Furthermore, the difference between zero-field cooled and field cooled samples is found to be similar to that expected for magnetization in spin glasses. The only contrasting feature, as compared to superconducting samples, is in the relative position of the signal during field increasing and decreasing cycles and this feature is shown to be most important in distinguishing the genuine signature of superconductivity. The possible consequences of the present observation on the mechanism of superconductivity are discussed.

Luminescence Properties of SrZrO3/Tb3+ Perovskite: Host-Dopant Energy-Transfer Dynamics and Local Structure of Tb3+

SrZrO3 perovskite (SZP) was synthesized using gel-combustion route and characterized systematically using X-ray diffraction and time-resolved photoluminescence techniques. A detailed analysis of the optical properties of Tb(3+) ions in SrZrO3 was performed to correlate them with the local environment of the lanthanide ions in this perovskite. Photoluminescence (PL) spectroscopy showed that emission spectrum consists of host as well as Tb(3+) emission indicating the absence of complete host-dopant energy transfer. On the basis of emission spectrum and PL decay study it was also observed that Tb(3+) is not homogeneously distributed in SrZrO3 perovskite; rather, it is occupying two different sites. It is corroborated using extended X-ray absorption fine structure studies that Tb(3+) is stabilized on both six-coordinated Zr(4+) and eight-coordinated Sr(2) site. The energies calculated using density functional theory (DFT) indicates that Tb occupation in Sr site is energetically more favorable than Zr site. The analysis of valence charge distribution also substantiated our structural stability analysis of site-selective Tb doping in SrZrO3. Time-resolved emission spectroscopy is employed to elucidate the difference in the spectral feature of Tb(3+) ion at Sr(2+) and Zr(4+) site. DFT-calculated density of states analysis showed that energy mismatch of Tb-d states with Zr-d and O-p states of SZP makes the energy transfer from host SZP to Tb(3+) ion difficult.

E.p.r. studies on dichloromono(1-phenylamidino-o-alkylurea)copper(II) complexes. Evidence for field induced partial ordering in the solid state and some unusual features in solution

E.p.r. and optical absorption studies have been conducted on dichloromono(1-phenylamidino-o-alkylurea)copper(II) complexes (Alkyl = Me, Et, Pr, Bu or Pe). The e.p.r. spectra of solids, recorded at 300 K, confirmed the square-planar geometry with the unpaired electron in the dx2−y2 orbital of copper. Magnetic field-induced partial molecular alignment has been observed in some of the polycrystalline samples when cooled in a magnetic field of 1 T at 77 K. E.p.r. spectra at 77 K in pyridine and DMF has shown axial ligation of solvent molecules (pyridine and DMF), whereas in MeOH and DMSO at least three structurally different CuII species have been identified. These features are consistent with differences in electronic absorption spectra in the powder and in solution.

Photoluminescence and EPR studies on Fe3+ doped ZnAl2O4: an evidence for local site swapping of Fe3+ and formation of inverse and normal phase

Considering that ZnAl2O4 spinel has two different sites (octahedral and tetrahedral) and its properties change with dopant ion distribution among these two sites; ZnAl2O4 doped with varied concentrations of Fe(3+) was synthesized by a low temperature sol-gel combustion method. Phase purity and structural investigations were carried out using Rietveld refined X-ray diffraction which shows a decrease in the value of cell parameters at higher doping levels. Photoluminescence (PL) and electron paramagnetic resonance (EPR) studies have shown that on doping, Fe(3+) ions were distributed in both tetrahedral and octahedral sites. At octahedral sites, Fe(3+) exhibited a broad red emission around 745 nm while at tetrahedral sites it exhibited well-defined vibronic sidebands at 665, 674, 684 and 693 nm along with a broad blue band with a maxima at 445 nm at room temperature. EPR studies have shown a broad spectrum at g ≈ 2.2 which corresponds to the Fe(3+) in octahedral sites, while the broad signal at g ≈ 4.2 belongs to Fe(3+) in tetrahedral sites. It was also inferred from these studies that Fe(3+) prefers to occupy octahedral sites at higher concentrations and at higher annealing temperatures. The PL decay behavior of Fe(3+) in ZnAl2O4 has also shown that two different types of Fe(3+) ions were present in this matrix. The first type was a long lived species (τ ≈ 170 μs) present at octahedral sites and the other was a short lived species (τ ≈ 40 μs) present at the tetrahedral sites; the fraction of the long lived species predominate at higher concentrations. Thus the present work is mainly focused on understanding the tuning of local site occupancy of the dopant ion among those sites with varying concentration and annealing temperature, using the dopant ion itself as a spectroscopic probe, which further helps in understanding the phase (inverse and normal) of the spinel.

Diglycolamide-functionalized task specific ionic liquids for nuclear waste remediation: extraction, luminescence, theoretical and EPR investigations

A 3.6 × 10−2 M solution of a diglycolamide-functionalized task specific ionic liquid (DGA-TSIL) in [C4mim][NTf2] was used for the extraction of actinides (mainly Am) and other elements present in high level nuclear waste. The extraction of Eu3+ was relatively higher than that of Am3+ conforming to the mechanism displayed by other diglycolamide extractants such as TODGA (tetraoctyl diglycolamide). The distribution ratio values decreased in the presence of simulated high level waste (SHLW) as compared to those obtained with pure tracers. The nature of the extracted species was established by the slope analysis method which suggested 1:2 species for the extraction of Am3+ and Eu3+. Calculation of the Judd–Ofelt parameters from the luminescence data of the Eu3+ complexes indicated that the structure of the extracted complexes exhibits S4 symmetry. Theoretical calculations showed virtually no difference between the structures of the complexes of Am3+ and Eu3+. The nature of the radiolytic degradation products was analyzed by electron paramagnetic resonance (EPR) spectroscopic measurements revealing the presence of alkyl imidazolium and methyl radicals.

Probing local site environments and distribution of manganese in SrZrO3:Mn; PL and EPR spectroscopy complimented by DFT calculations

In order to understand the local environment, valence state and cationic distribution of manganese ions in gel-combustion derived SrZrO3 (SZO), a combined experimental and theoretical approach was formulated based on photoluminescence (PL), electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations. An attempt was also made to investigate the same as a function of manganese ion concentration. The phase identification of the samples was confirmed using powder X-ray diffraction technique (PXRD). In all the doped compounds, manganese was found to be stabilized as divalent Mn2+ and preferentially occupying the 8-coordinated Sr2+ ion site. However, the proportion of manganese ions residing at zirconium sites was enhanced at higher concentrations. The cohesive energies from DFT calculations explained the stability of Mn2+ ions at different sites. It was also observed from the density of states (DOS) that the substitution of manganese at strontium sites leads to the generation of shallow defect states, whereas that at zirconium site generates both shallow and deep defect states within the band gap of the material. A change in the host emission due to these defect states with varied concentrations of Mn2+ was also observed, which further supported the observed cationic distributions trend. The decrease in the band gap energy explained the red shift of the emission spectra. PL decay study also suggested the existence of shallow and deep trap states. The intensity of the EPR signal at g ≈ 1.976, due to paramagnetic oxygen vacancies, was found to increase at higher Mn-concentration because of more substitution at Zr4+ sites. Two additional EPR sextets with g ≈ 1.993 and 2.013 in Mn doped SZO compounds were attributed to lattice and surface bound Mn2+ ions, respectively, which disappeared at higher Mn-concentrations, giving a broad signal.

Luminescence of undoped and Eu3+ doped nanocrystalline SrWO4 scheelite: time resolved fluorescence complimented by DFT and positron annihilation spectroscopic studies

SrWO4 (SWO) and Eu3+ doped SrWO4 (SEWO) scheelite samples were synthesized using a polyol method. Crystallite sizes of the as prepared samples annealed at 300 and 500 °C are in a similar range (<20 nm) whereas those annealed at 700 and 900 °C are about ∼40–50 nm and 80–100 nm, respectively. Photoluminescence (PL) spectra of SWO samples show a broad peak corresponding to the oxygen to tungsten charge transfer transition. Along with the enhancement in emission intensity in the samples annealed at 700 and 900 °C, there is a blue shift in peak maxima. Our first principles quantum mechanical calculations showed that the break in symmetry of the unit cell of SrWO4 creates inherent defects in the lattice which are responsible for the reduction of the electronic band gap in the SrWO4 sample with decrease in size. On europium doping; energy transfer from the O2− → W6+ charge transfer band to Eu3+ takes place and the reason behind this is explained using density functional theory (DFT) calculations. Based on time resolved PL measurements, it is suggested that Eu3+ ions occupy two sites in SWO; a regular symmetric Sr2+ site and an asymmetric site (closer to charge compensating defects). With increase in annealing temperature, emission intensity as well as asymmetry around europium increased. The changes in asymmetry around europium and defect densities as determined from positron annihilation lifetime spectroscopy (PALS) suggest that though the overall vacancy concentration is reduced with an increase in annealing temperature, it is likely that vacancies closer to europium are slowly annealed out than the others.

Superconducting transition temperature of single-phase Tl-2223: Crucial role of Ca-vacancies and Tl-content

Abstract Detailed investigations on the synthesis and phase-indentification of a number of Tl-Ca-Ba-Cu oxides show that single-phase Tl-2223 superconductor with the best transition temperature ( R = 0 at 130 K) can be prepared from a thallium-deficient starting composition, namely Tl ≈1.0 Ca 2 Ba 2 Cu 3 O x . A number of other starting compositions containing Tl: Ca T c -values between 115 K and 130 K. It is argued that the existence of vacancies at Ca-sites is crucial to the production of pure 2223-phase and the highest T c (130 K) is obtained when the number of such vacancies reaches an optimum. Starting compositions deficient in Tl lead to such a possibility by aiding the migration of some Ca-ions to vacant Tl-sites. Under standardised preparation conditions single-phase Tl-2223 structure is obtained for a combined nominal stoichiometry 3 ⩽ (Tl + Ca) ⩽ 4.