V. Grover

Sm2−xDyxZr2O7 Pyrochlores: Probing Order−Disorder Dynamics and Multifunctionality

The present work involves the synthesis of a series of Sm(2-x)Dy(x)Zr(2)O(7) compounds (0.0 ≤ x ≤ 2.0) by a controlled gel combustion process. The powders were thoroughly analyzed by powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, and diffuse-reflectance UV-visible spectroscopy. The powder XRD studies revealed the system to be single-phasic throughout with retention of pyrochlore-type ordering until 40 mol % of Dy(3+), beyond which the pyrochlore lattice gives way to the defect fluorite structure. Interestingly, Raman spectroscopic studies (as against XRD studies) showed retention of pyrochlore-type ordering throughout the homogeneity range of the compositions studied. This is perhaps the first study that reports retention of a weak pyrochlore-type superstructure in the Dy(2)Zr(2)O(7) system, which was otherwise known to crystallize in the defect fluorite system. The ionic conductivity measurements showed an increase in the activation energy (E(a)) with an increase in the mole percent of Dy(3+) owing to the decreased mobility with an increase in the degree of disorder. The system possesses a tunable band gap with varying amounts of Dy(3+). First-principles calculations were performed to support a decrease in the band gap of the doped system with an increase in the Dy(3+) content. The potential as photocatalysts of some of these compositions was explored, and they exhibited high photocatalytic activity for degradation of xylenol orange, with t(1/2) increasing from pure Sm(2)Zr(2)O(7) to pure Dy(2)Zr(2)O(7).

X-ray diffraction, μ-Raman spectroscopic studies on CeO2−RE2O3 (RE=Ho, Er) systems: Observation of parasitic phases

The phase relations in CeO2−Ho2O3 and CeO2−Er2O3 systems have been established under the slow-cooled conditions. As per x-ray diffraction (XRD), in both the series a single-phasic solid solution forms up to the nominal composition Ce0.6RE0.4O1.8 (RE=Ho, Er) retaining the F-type structure of parent ceria. In Ce1−xErxO2−x/2 system the presence of microdomains of C-type phase have been revealed by Raman spectroscopy for composition x=0.4, which has been identified as single phasic by XRD. Photoluminescence studies also show that biphasic region commences from x=0.4 for Ce1−xHoxO2−x/2 series. The biphasicity continues until x=0.7 for both the series. From x=0.8 the solid solutions exist as C-type single phasic, which is isotypic to another end member RE2O3 (RE=Ho, Er) and as revealed by both XRD and Raman spectroscopy. High temperature XRD studies show that no temperature induced phase change has been observed in either of the series until 1273 K. In this work photoluminescence data was used to delineate the ...

Solid state white light emitting systems based on CeF3: RE3+ nanoparticles and their composites with polymers.

A series of doped CeF(3): RE(3+) (RE(3+): Tb(3+), Eu(3+) and Dy(3+)) nanoparticles were synthesized, with the aim of obtaining a white light emitting composition, by a simple polyol route at 160°C and characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), Fourier transform infrared spectroscopy (FT-IR) and photoluminescence. Uniformly distributed and highly water-dispersible rectangular nanoparticles (length ~15-20 nm, breadth ~5-10 nm) were obtained. The steady state and time resolved luminescence studies confirmed efficient energy transfer from the host to activator ions. Lifetime studies revealed that optimum luminescence is observed for 2.5 mol% Dy(3+) and 7.5 mol% Tb(3+). The energy transfer efficiencies (Ce(3+) to activators) were found to be 89% for CeF(3): Tb(3+) (7.5 mol%) nanoparticles and 60% for CeF(3): Dy(3+) (2.5 mol%) nanoparticles. Different concentrations of Tb(3+), Eu(3+) and Dy(3+) were doped to achieve a white light emitting phosphor for UV-based LEDs (light emitting diodes). Finally CeF(3), triply doped with 2.0 mol%Tb(3+), 4.5 mol% Eu(3+) and 3.5 mol% Dy(3+), was found to have impressive chromaticity co-ordinates, close to broad day light. The colloidal solutions of doped CeF(3) nanoparticles emitted bright green (Tb(3+)), blue (Dy(3+)) and white (triply doped) luminescence upon host excitation. Composites of poly methyl methacrylate (PMMA) and poly vinyl alcohol (PVA) were made with CeF(3): 5.0 mol%Tb(3+), CeF(3): 5.0 mol% Dy(3+) and triply doped white light emitting composition. The CeF(3)/PMMA (PVA) nanocomposite films, so obtained, are highly transparent (in the visible spectral range) and exhibit strong photoluminescence upon UV excitation.

Experimental and theoretical investigations on the polymorphism and metastability of BiPO4

In this work we report the metastability and the energetics of the phase transitions of three different polymorphs of BiPO4, namely trigonal (Phase-I, space group P3(1)21), monoclinic monazite-type (Phase-II, space group P2(1)/n) and SbPO4-type monoclinic (Phase-III, space group P2(1)/m) from ambient and non-ambient temperature powder XRD and neutron diffraction studies as well as ab initio density functional theory (DFT) calculations. The symmetry ambiguity between P2(1) and P2(1)/m of the high temperature polymorph of BiPO4 has been resolved by a neutron diffraction study. The structure and vibrational properties of these polymorphs of the three polymorphs have also been reported in detail. Total energy calculations have been used to understand the experimentally observed metastable behavior of trigonal and monazite-type BiPO4. Interestingly, all of the three phases were found to coexist after heating a single phasic trigonal BiPO4 to 773 K. The irreversible nature of these phase transitions has been explained by the concepts of the interplay of the structural distortion, molar volume and total energy.

Structural analysis of excess-anion C-type rare earth oxide: a case study with Gd1−xCexO1.5+x/2 (x = 0.20 and 0.40)

Structural analysis of anion-rich C-type Gd2O3 was carried by the Rietveld refinement of the powder X-ray diffraction data for compositions Gd0.8Ce0.2O1.60 and Gd0.6Ce0.4O1.70. Both compounds have a body-centred cubic lattice (space group Ia\bar{3}, No. 206, Z = 32) with unit-cell parameters of 10.8488 (1) and 10.8542 (1) A, respectively. Both of these compounds are iso-structural with the C-type rare earth oxides, with excess anions as required for charge balance. The structural analysis reveals that there are two different kinds of metal ion site, namely 8b (M1) and 24d (M2), and two different kinds of anion sites, namely 48e (O1) and 16c (O2). The excess anions occupy the 16c (xxx) sites. The two metal ions each form an approximately eightfold-coordination polyhedron with O1 and O2. The details of these two compositions are explained and compared with both the CeO2 structure and the Gd2O3 structure, i.e. the end member.

Color tunable YF3: Ce3+/Ln3+ (Ln3+: Eu3+, Tb3+, Dy3+, Sm3+) luminescent system: role of sensitizer and energy transfer study

Nano-fluorides with attractive optical properties have been proposed as potential phosphors for solid state lighting and bio-applications. In the present work, redispersible YF3: Ln3+ and YF3: Ce3+/Ln3+ nanocrystals were obtained by a simple one step ethylene glycol mediated soft chemical synthesis. Uniformly distributed nanoparticles with a leaf-like morphology and particle size of 10–15 nm were observed. X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transformed infrared spectroscopy (FT-IR), photoluminescence (PL) and decay studies were employed to characterize the samples. The highly intense emissions of various activators (Dy3+, Tb3+, Eu3+ and Sm3+) in the presence of Ce3+, their long lifetimes (∼ms) and significant reduction in the lifetime of Ce3+ proved an efficient energy transfer operating in the nanocrystals. The time resolved emission studies indicated towards the presence of two different sites for the activator ions. Under ultraviolet irradiation, the emission color of YF3: Ce3+/Ln3+ nanophosphors can be easily tuned by changing the Ln3+ ions. Bright blue (Dy3+), green (Tb3+), red (Eu3+) and purple (Sm3+) emissions were observed from corresponding YF3: Ce3+, Ln3+ nanocrystals under ultraviolet light.

Nano-cerium vanadate: a novel inorganic ion exchanger for removal of americium and uranium from simulated aqueous nuclear waste.

Cerium vanadate nanopowders were synthesized by a facile low temperature co-precipitation method. The product was characterized by X-ray diffraction and transmission electron microscopy and found to consist of ∼25 nm spherical nanoparticles. The efficiency of these nanopowders for uptake of alpha-emitting radionuclides (233)U (4.82 MeV α) and (241)Am (5.49 MeV α, 60 keV γ) has been investigated. Thermodynamically and kinetically favorable uptake of these radionuclides resulted in their complete removal within 3h from aqueous acidic feed solutions. The uptake capacity was observed to increase with increase in pH as the zeta potential value decreased with the increase in pH but effect of ionic strength was insignificant. Little influence of the ions like Sr(2+), Ru(3+), Fe(3+), etc., in the uptake process indicated CeVO4 nanopowders to be amenable for practical applications. The isotherms indicated predominant uptake of the radioactive metal ions in the solid phase of the exchanger at lower feed concentrations and linear Kielland plots with positive slopes indicated favorable exchange of the metal ions with the nanopowder. Performance comparison with the other sorbents reported indicated excellent potential of nano-cerium vanadate for removing americium and uranium from large volumes of aqueous acidic solutions.

High-pressure x-ray diffraction study of bulk and nanocrystalline PbMoO4

We studied the effects of high-pressure on the crystalline structure of bulk and nanocrystalline scheelite-type PbMoO4. We found that in both cases the compressibility of the materials is highly nonisotropic, being the c-axis the most compressible one. We also observed that the volume compressibility of nanocrystals becomes higher that the bulk one at 5 GPa. In addition, at 10.7(8) GPa we observed the onset of an structural phase transition in bulk PbMoO4. The high-pressure phase has a monoclinic structure similar to M-fergusonite. The transition is reversible and not volume change is detected between the low-pressure and high-pressure phases. No additional structural changes or evidence of decomposition are found up to 21.1 GPa. In contrast nanocrystalline PbMoO4 remains in the scheelite structure at least up to 16.1 GPa. Finally, the equation of state for bulk and nanocrystalline PbMoO4 are also determined.

Multicolored and white-light phosphors based on doped GdF3 nanoparticles and their potential bio-applications

Rare-earth-doped gadolinium fluoride nanocrystals were synthesized by a single step synthesis employing ethylene glycol as solvent. Based on X-ray diffraction studies, stabilization of hexagonal modification of GdF(3) has been inferred. The microscopic studies show formation of uniformly distributed nanocrystals (~15 nm). The nanoparticles are readily dispersible in water and show bright luminescence in colloidal solution. The luminescence properties have been investigated as a function of activator concentrations, and enhanced optical properties have been attributed to efficient energy transfer from the Gd(3+) to the activator RE(3+) ions, which has further been confirmed by steady-state and time-resolved optical studies. It has been demonstrated that on doping appropriate amount of activators in host GdF(3), a novel white-light-emitting phosphor is obtained with CIE co-ordinates and correlated color temperature (CCT) very close to broad daylight. This can have promising applications as phosphor for white-light ultraviolet-light-emitting diodes (UV-LEDs). Our experiments showed efficient labeling of human breast carcinoma cells (MCF-7) by Tb(3+)-doped GdF(3) nanoparticles. The fluorescence intensity was found to be dependent on the surface modifying/coating agent, and the results were validated using confocal microscopy in terms of localization of these functionalized nanoparticles.

Synthesis and characterization of M1−xNdxF2+x (M = Sr2+, Ca2+; 0.00 ≤ x ≤ 1.00)

Series of the mixed fluorides with the general composition M{sub 1-x}Nd{sub x}F{sub 2+x} (0.00{<=}x{<=}1.00; M=Sr{sup 2+} and Ca{sup 2+}) were prepared by a vacuum heat treatment of the appropriate mixtures of MF{sub 2} (M=Ca and Sr) and NdF{sub 3}. The products obtained were analyzed by powder X-ray diffraction, which revealed the phase relations in these systems. At the MF{sub 2}-rich compositions a fluorite-type solid solution was observed in both systems. The typical solid solution limits of NdF{sub 3} in the SrF{sub 2} and CaF{sub 2} lattice are about 40 and 45 mol%, respectively. The incorporation of NdF{sub 3} in the fluorite lattice of CaF{sub 2} causes an expansion of the unit cell volume. The unit cell volume of the fluorite lattice in Sr{sub 1-x}Nd{sub x}F{sub 2+x} system surprisingly remains constant throughout the solid solution range. The NdF{sub 3} (tysonite) type phase separates out beyond the solid solution limit, in both systems. Also, it was observed that about 15 and <10 mol% of CaF{sub 2} and SrF{sub 2}, respectively, could be retained in the NdF{sub 3}, maintaining the tysonite lattice. No fluorite or tysonite-related ordered phases were observed in these systems.