Rohan Phatak

The effect of vanadium substitution on photoluminescent properties of KSrLa(PO4)x(VO4)2−x:Eu3+ phosphors, a new variant of phosphovanadates

Eu3+ doped red emitting phosphors of formulae KSrLa1−y(PO4)x(VO4)2−x:yEu3+ (where x = 0, 1 and 2 and y = 0, 0.02, 0.05, 0.10 and 0.015) were synthesized by a high temperature solid state reaction route. The compounds were characterized by a powder XRD method, and a complete structure determination for KSrLa(PO4)(VO4) was carried out using the Rietveld refinement method. The compound crystallized in the rhombohedral system with space group (Rm). The structure of the compound shows that La3+ is occupied at two positions, namely, trigonal antiprism (six coordinated) and truncated hexagonal bipyramidal (10 coordinated) centres, with occupancy of 0.94 and 0.06, respectively. The effect of vanadium substitution on excitation and emission spectra of the compounds was investigated using time resolved photoluminescence spectroscopy (TRPLS). The lifetimes of 5D0 → 7F2 transition were determined for all the compounds, which are in agreement with emission spectroscopic results. Judd–Ofelt analysis of the compounds clearly shows that the more polarized environment around Eu3+ in the case of the vanadate and phosphovanadate in comparison to the phosphate leads to more colour purity of the emission, when excited by UV light.

A mechanistic study on the effect of a surface protecting agent on electrocrystallization of silver nanoparticles

Cyclic voltammetry and chronoamperometry at characteristic potentials were employed to unravel the mechanism of electrocrystallization of silver nanoparticles (AgNPs) from their aqueous solution in the presence and absence of surface protecting agent tetrabutylammonium tetrafluoroborate (TBABF4). The electrocrystallization parameters viz. initial current density (j0), decay constant (τ), diffusion coefficient (D) of Ag(I), number of active sites (N0) and nucleation rate (a) were calculated by fitting the experimentally obtained current transients with the calculated current transients using a hybrid genetic algorithm (HGA). The theoretical currents were calculated from three popular electrocrystallization models viz. Scharifker and Mostany (SM), Sluyters-Rehbach, Wijenberg, Bosco and Sluyters (SRWBS) and Heerman and Tarallo (HT). Each of the three models fitted well by the HGA with the potentiostatic current transients observed at different potentials, both in the absence and in the presence of TBABF4 with residual sum of squares (∼10−7) and reduced χ2 (∼10−10). However, the electrocrystallization parameters were distinctly different in each of the three models. Principal component analysis of the calculated parameters i.e. D, NS and aN0 showed the absence of any correlation among the electrocrystallization parameters derived from the Scharifker and Hills (SH), SM, SRWBS and HT models. Further, the actual nuclei densities of the AgNPs, both in the presence and the absence of TBABF4, were found to be significantly higher than the predicted values from any of these models. Since these models are based on different empirical assumptions, one needs be careful in attaching any extra significance to the numerical values of j0, τ, D, N0, “a” of any system only based on the quality of fitting. From the present data, it was conclusively proved that the surface protecting agent slowed down the kinetics of electrocrystallization due to introduction of a higher activation overpotential at the electrode–electrolyte interface and subsequent decreases in the number of nuclei on the electrode surface in the presence of TBA+ ions, irrespective of the model.

Preparation and characterization of new phases in Na-(U0.5Pu0.5)-Mo-O andNa-(ThxU1−x)-Mo-O (x = 0.5 and 0.8) systems by X-ray and thermal methods

Abstract Na2(ThxU1−x)(MoO4)3 and Na4(ThxU1−x)(MoO4)4 (x = 0.5 and 0.8) were synthesized, by reacting (ThxU1−x)(MoO4)2 with Na2MoO4 in appropriate molar ratios at 873 K in an evacuated quartz ampoule for 25 h. While Na2(U0.5Pu0.5)(MoO4)3 and Na4(U0.5Pu0.5)(MoO4)4 were synthesized, by reacting (U0.5Pu0.5)(MoO4)2 with Na2MoO4 in appropriate molar ratios at 873 K in argon atmosphere for 45 h. The crystal structure of these compounds were derived from powder XRD data in a tetragonal system (space group: I41/a) by Rietveld profile method. Thermogravimetric curves of Na2(ThxU1−x)(MoO4)3 and Na4(ThxU1−x)(MoO4)4 did not show any weight changes up to 973 K in helium atmosphere. High temperature X-ray diffraction (HTXRD) studies of Na2(ThxU1−x)(MoO4)3 and Na4(ThxU1−x)(MoO4)4 in vacuum showed positive thermal expansion in the temperature range of 298 to 873 K.

X-ray absorption near-edge structure (XANES) studies on Sb-doped Bi2UO6 at Bi and U edges

X-ray absorption spectroscopy (XAS) measurements at Bi and U LIII edges with synchrotron radiation have been carried out on Bi2−xSbxUO6 samples for x= 0.04, 0.08, 0.12, 0.16 and 0.40 which are possible by-products of Bi based coolant and Uranium based fuels in advanced high temperature nuclear reactors. The chemical shift of the Bi absorption edges in the samples have been determined accurately from the XANES region of the X-ray absorption spectra and have been explained in terms of the difference in electronegativity values of Sb and Bi. The chemical shift of absorption edges show systematic variation only upto x = 0.08 (i.e., 4% Sb doping), which shows that the Sb enter in the matrix properly up to 4% doping concentration. The local structure of U is found to remain unchanged on Sb doping indicating clearly that Sb dopants preferably replace Bi atoms.

Structural studies of Na4(ThxU1−x)(MoO4)4(x = 0.5,0.8)

Na4(ThxU1−x)(MoO4)4(x = 0.5,0.8) type of compounds were synthesized by reacting (ThxU1−x)(MoO4)2 and Na2MoO4 in appropriate molar ratio at 873 K in an evacuated quartz ampoule for 25 h. The crystal structure of Na4(ThxU1−x)(MoO4)4 was refined from X-ray powder diffraction data in the tetragonal system (space group: I41/a) by Rietveld method and has super scheelite structure.