Debdutta Lahiri

Structural understanding of the spectral characteristics of SnO2:Eu:Y2O3, using extended x-ray absorption fine structure

SnO2:Eu is a well-known luminescent material, emitting red and orange lines. The intensity ratio of red to orange emission, being sensitive to the deviation of Eu3+ ions from symmetric location, finds wide application as sensor. The luminescence intensity of such lanthanide-doped sensors is generally optimized by high temperature annealing. However, for the present system (SnO2:Eu) it had been found that the red emission suddenly disappears while annealing beyond 900 °C, which can however be recovered by dispersing the system in a secondary host matrix of Y2O3. Understanding the mechanism of this recovery has important implication for designing of phosphor. In this work, we structurally explain this spectral evolution, by employing x-ray absorption fine structure technique. The initial disappearance of the red line is realized to be due to the formation of Eu2Sn2O7 and the recovery, to the intercalation of the Eu3+ ions from the SnO2 surface into Y2O3. Oxygen vacancy in Y2O3 creates the asymmetric environ...

XAFS investigation of the role of orientational disorder in the stabilization of the ferromagnetic metallic phase in nanoparticles of La0.5Ca0.5MnO3

The inclusion of the contribution of Jahn-Teller distortion of MnO(6) units, in addition to double-exchange, has been largely successful in explaining the magneto-transport behavior of manganites. However, our recent experiments on La(0.5)Ca(0.5)MnO(3) demonstrated the limitation of these factors in explaining the radical difference between the magneto-transport properties of bulk and nanocrystalline forms. While bulk La(0.5)Ca(0.5)MnO(3) exhibits insulator character (4-300 K) and an anti-ferromagnetic-ferromagnetic transition at 200 K, the nanocrystalline form stabilizes in a metallic ferromagnetic phase (4-300 K). This is counter-intuitive since large Jahn-Teller distortion, which promotes anti-ferromagnetism or insulator character, exists in the nanocrystals too (as indicated by x-ray diffraction results). In this work, we resolve this paradox by considering the role of structural disorder. Employing x-ray absorption spectroscopy, we establish that the disorder in inter-octahedral coupling is enhanced by 57% in the nanocrystals, as the octahedral units are randomly oriented with respect to each other. This orientational disorder promotes metallic ferromagnetism by destroying the stringent orbital ordering that is needed for anti-ferromagnetism and the co-operative nature of the orbital order.

Investigating structural aspects to understand the putative/claimed non-toxicity of the Hg-based Ayurvedic drug Rasasindura using XAFS.

XANES- and EXAFS-based analysis of the Ayurvedic Hg-based nano-drug Rasasindura has been performed to seek evidence of its non-toxicity. Rasasindura is determined to be composed of single-phase α-HgS nanoparticles (size ∼24 nm), free of Hg(0) or organic molecules; its structure is determined to be robust (<3% defects). The non-existence of Hg(0) implies the absence of Hg-based toxicity and establishes that chemical form, rather than content of heavy metals, is the correct parameter for evaluating the toxicity in these drugs. The stable α-HgS form (strong Hg-S covalent bond and robust particle character) ensures the integrity of the drug during delivery and prevention of its reduction to Hg(0) within the human body. Further, these comparative studies establish that structural parameters (size dispersion, coordination configuration) are better controlled in Rasasindura. This places the Ayurvedic synthesis method on par with contemporary techniques of nanoparticle synthesis.

Understanding temperature and magnetic-field actuated magnetization polarity reversal in the Prussian blue analogue Cu0.73Mn0.77[Fe(CN)6].zH2O, using XMCD

We have investigated the microscopic origin of temperature and magnetic-field actuated magnetization reversal in Cu0.73Mn0.77[Fe(CN)6].zH2O, using XMCD. Our results show a fair deviation from the mean-field-theory in the form of different ordering temperatures of Fe and Mn sublattices. A preferential sign reversal of Mn spin under magnetic field and different spin cant angles for the two sublattices have also been observed. An antiferromagnetic coupling between the Fe and Mn sublattices along with different ordering temperatures (sublattice decoupling) for these sublattices explain the temperature-dependent magnetization reversal. Whereas, Mn spin reversal alone (under external magnetic field) is responsible for the observed field-dependent magnetization reversal. The dissimilar magnetic behavior of Fe and Mn sublattices in this cubic 3d-orbital system has been understood by invoking disparity and competition among inter-sublattice magnetic control parameters, viz. magnetic Zeeman energy, exchange coupling constant and magnetic anisotropy constant. Our results have significant design implications for future magnetic switches, by optimizing the competition among these magnetic control parameters.

High pressure experiments at the XAFS Beamline, INDUS-2

The dispersive XAFS beamline BL-08 at the INDUS-2 synchrotron radiation source, RRCAT, Indore uses a bent Si (111) crystal as dispersive-cum-focusing element and a position sensitive CCD detector to enable instantaneous measurement of the whole XAFS spectrum around the absorption edge of interest. This beamline is ideal for characterisation of materials under high pressure using Diamond Anvil Cell with ~50 ?m spot size. For this setup, the theoretically determined spot size (Horizontal ? Vertical) varies between 17 ? 137 ?m and 37 ? 142 ?m for the x-ray energy range 5 keV-20 keV. To reduce the vertical spot size to <50 ?m, we have designed an additional focusing mirror between the polychromator and sample position. The mirror, procured from SESO (France), will be installed shortly. Meanwhile, we have developed a dummy mirror bender setup at CDM (B ARC) and have carried out feasibility tests to confirm reduction in spot size using the same. We have also conducted preliminary XAFS experiments (at BL-08) on SrRuO3 at ~16 keV, under ambient conditions and inside diamond anvil cell, in order to assess the signal intensity and quality. We have obtained reasonably good signal.

Probing the involvement of non-magnetic Sr ion in the ferromagnetic transition of SrRuO3, using XAFS

Recent calculations by Maiti et al. have shown that non-magnetic ions like Sr/Ca in Sr/Ca ruthenates may not be mere spectators in the magnetic transitions but their strong hybridization with O atoms may modify the magnetic ground state by distorting the Ru-O-Ru bond angle. Our XAFS results at Sr and Ru K-edges on SrRuO3 indeed demonstrate that the Debye Waller factor for the Sr-coupled bonds viz. Sr-Sr evolve most significantly across the ferromagnetic transition, almost replicating the magnetization curve while the Ru-octahedra evolves only gradually. This clearly establishes the definite role of Sr disorder in the ferromagnetic transition.

Ascertaining the nanocluster formation within an ion-irradiated Pt/Ni/C multi-trilayer with X-ray absorption spectroscopy

In this work nanoclusters formed in a Pt/Ni/C multi-trilayer by the ion-irradiated method of synthesis are characterized. In particular, an attempt to understand the role of interfaces in the synthesis is made. With this objective, ion-irradiation-induced structural changes in a Pt/Ni/C multi-trilayer using X-ray absorption spectroscopy (at the Ni K-edge) in conjunction with the X-ray standing-wave technique are investigated. The XANES analysis identifies chemical binding at pristine Ni/C and Ni/Pt interfaces, in contrast with physical adsorption at the Pt/C interface. The chemical nature of the interfaces determines their relative stability with respect to irradiation and controls the extent of metallic diffusion. The most interesting structural change, upon irradiation, is the disruption of the Pt/C interface and subsequent migration of Pt atoms towards pre-diffused Ni atoms within the C layer, leading to the formation of Ni-centered Ni-Pt bimetallic nanoclusters (with Ni:Pt = 60:40). These clusters are highly disordered beyond their nearest neighbor and find wide-scale applications as, for example, magnetic devices etc. The implications of these findings on the design goals are discussed.

XSW-XAFS characterization of ion-irradiated Pt/Ni/C multilayer

We employ XSW assisted XAFS at Ni K edge to characterize ion-irradiated Pt/Ni/C multilayer, particularly the nanoclusters formed within C layer, revealed by X-TEM and angle-resolved fluorescence studies. Retrieving the structural model from XAFS coordination results involved intriguing steps such as accounting for the intensity variation across the layers, determination of extra pre-diffused Ni into C layer (beyond the reflectivity determined roughness) and decoupling interfacial, layer and cluster coordination. The clusters are determined to be Ni centered Ni-Pt bimetallic nanoclusters (Ni:Pt = 60:40), formed due to irradiation induced diffusion of Pt atoms from the disrupted Pt/C interface towards pre-diffused Ni atoms present in C layer. They are highly disordered beyond nearest neighbor and resemble glassy structure which could find wide-scale applications in magnetic devices.

Focusing Properties of Energy Dispersive Exafs Beamline at Indus-2 SRS Using Additional Optical Elements

An Extended X-ray Absorption Fine Structure (EXAFS) beamline using a bent crystal polychromator is being developed at INDUS-2 Synchrotron Radiation Facility. One of the proposed activities with this beamline is to record the EXAFS spectra of materials under high pressure. For this purpose x-rays originating from the synchrotron source are to be perfectly focused on the experimental samples having sizes of the order of few micrometers. Extensive ray tracing work has been carried out for studying the imaging properties of this beamline using the software program SHADOW. It has been found that the desired focal spot size can he achieved when some additional mirrors in the form of elliptic cylinders are included in the beamline lay-out.

Image Evaluation of a DCM Based XAFS Beamline at Indus-2 SRS Facility

A double crystal monochromator (DCM) based beamline is proposed to be set up at 2.5 GeV INDUS-2 SRS for XAFS studies in the fluorescence mode. With this beamline it will be possible to record XAFS spectra of diverse samples, such as, semiconductor with low metal doping, liquids, metal nanoparticles dispersed in matrix, thin films, etc. In this paper we have discussed the detailed optical layout of the beamline. Imaging properties of the beamline have been evaluated by actual ray-tracing. During ray tracing we have considered several options for the optical layout of the beamline in which focusing is done by a toroidal mirror or by two concave mirrors in the form of meridonial cylinders. It has been found that the system consisting of two concave mirrors and a bent crystal shows better focusing properties with a very small focal spot size at the final image position where the experimental sample is to be placed.