K. R. Priolkar

ZnGa 2 O 4 :Cr 3+ : a new red long-lasting phosphor with high brightness

ZnGa2O4:Cr3+ is shown to be a new bright red UV excited long-lasting phosphor potentially suitable for in vivo imaging due to its 650 nm-750 nm emission range. Photoluminescence and X-ray excited radioluminescence show the 2E → 4A2 emission lines of both ideal Cr3+ and Cr3+ distorted by a neighboring antisite defect while long-lasting phosphorescence (LLP) and thermally stimulated luminescence (TSL) almost exclusively occur via distorted Cr3+. The most intense LLP is obtained with a nominal Zn deficiency and is related to a TSL peak at 335K. A mechanism for LLP and TSL is proposed, whereby the antisite defect responsible for the distortion at Cr3+ acts as a deep trap.

Red persistent luminescence in MgGa2O4?:?Cr3+; a new phosphor for in vivo imaging

A new red-emitting long-lasting phosphorescence (LLP) material useful as biomarker for small animal in vivo imaging is presented. X-ray irradiated MgGa2O4 : Cr3+ is shown to be a suitable persistent phosphor emitting in the range 650–770 nm. In vivo re-excitation of its persistent luminescence is also possible with 580 nm excitation. MgGa2O4 : Cr3+ has almost 44% cationic site inversion. Cr3+ ion occupies octahedral site resulting in a broad emission peaking at 707 nm corresponding to Cr3+ 2E(2G) → 4A2(4F) transition. LLP is most intense for the compound with nominal Mg deficiency which also has the highest cation inversion. It is proposed that structural defects occurring due to cation inversion are responsible for LLP.

Role of crystal defects in red long-lasting phosphorescence of CaMgSi2O6:Mn diopsides

CaMgSi2O6:Mn diopsides are used for in vivo long-lasting phosphorescence (LLP) imaging. Trapping defects involved in the LLP mechanism were investigated. On annealing Mn-doped diopsides at 1100 °C in an Ar–H2 atmosphere, Ca EXAFS/XANES and electron paramagnetic resonance (EPR) evidenced paramagnetic oxygen vacancies while X-ray diffraction, Mn XANES and EPR revealed SiO2 formation and significant evaporation of the Mn dopant. A thermally stimulated luminescence (TSL) peak at 475 K ascribed to electron trapping at oxygen vacancies was found responsible for LLP at RT. Most intense red LLP suitable for in vivo imaging was achieved by a trade-off between a high MnII content (favourable to MnIIMg red over MnIICa orange luminescence) and the formation of oxygen vacancies favourable to LLP and luminescence light yield. Mn XANES revealed an effective charge larger than 2+ for MnII, in line with the role of hole trap in the LLP mechanism. Compounds annealed at lower temperatures (800 °C and 900 °C) in Ar–H2 showed smaller particle size (60–70 nm) and maximum Mn content but poor luminescence and LLP due to surface quenching defects.

Local structure of Pt and Pd ions in Ce1−xTixO2: X-ray diffraction, x-ray photoelectron spectroscopy, and extended x-ray absorption fine structure

Ce(1-x-y)Ti(x)Pt(y)O(2-delta) (x=0.15; y=0.01) and Ce(1-x-y)Ti(x)Pd(y)O(2-delta) (x=0.25; y=0.02 and 0.05) are found to be good CO oxidation catalysts [T. Baidya et al., J. Phys. Chem. B 110, 5262 (2006); T. Baidya et al., J. Phys. Chem. C 111, 830 (2007)]. A detailed structural study of these compounds has been carried out by extended x-ray absorption fine structure along with x-ray diffraction and x-ray photoelectron spectroscopy. The gross cubic fluorite structure of CeO(2) is retained in the mixed oxides. Oxide ion sublattice around Ti as well as Pt and Pd ions is destabilized in the solid solution. Instead of ideal eight coordinations, Ti, Pd, and Pt ions have 4+3, 4+3, and 3+4 coordinations creating long and short bonds. The long Ti-O, Pd-O, and Pt-O bonds are approximately 2.47 A (2.63 A for Pt-O) which are much higher than average Ce-O bonds of 2.34 A.

Study of local environment of Ag in Ag/CeO2 catalyst by EXAFS

The combustion synthesized Ag/CeO2 catalysts have been characterized by Extended Xray Absorption Fine Structure (EXAFS) spectroscopy at the Ag K-edge. It has been found that Ag+ like species is present in 1% Ag/CeO2 catalyst, whereas mostly Ag metal clusters are found in 3% Ag/CeO2. The analysis of EXAFS spectra indicates that about one oxygen atom is coordinated to Ag central atom at a distance of 2.19 Angstrom in 1% Ag/CeO2 catalyst along with eight coordinated Ag-Ag bond at 2.86 Angstrom. The Ag-O bond is absent in 3% Ag/CeO2

Order and disorder around Cr(3+) in chromium doped persistent luminescent AB2O4 spinels.

The X-ray absorption near edge structure (XANES) spectroscopy technique is used to better understand the charging and decharging processes of the persistent luminescence in the Cr(3+)doped AB2O4 spinels (A = Zn, Mg and B = Ga and Al) with low photon energy excitation by visible light. Cr K edge XANES spectra have been simulated for different near neighbour environments around the Cr(3+) recombination centres and compared with the experimental curve. In the Cr(3+):ZnGa2O4 compound, the Cr(3+) local structure corresponds mostly to that of a normal spinel (∼70%), while the rest comprises of a distorted octahedral environment arising from cationic site inversion and a contribution from chromium clustering. This local structure is considerably different in Cr(3+):MgGa2O4 and Cr(3+):ZnAl2O4, where, for both cases, chromium clustering represents the main contribution. The strong correlation between the intensity of persistent luminescence and the percentage of Cr in clusters leads us to infer that the presence of Cr clusters is responsible for the decrease of the intensity of the visible light induced persistent luminescence in the Cr(3+) doped AB2O4 spinels.

Effect of local structural distortions on magnetostructural transformation in Mn3SnC

In this paper we attempt to understand the different nature of first order magnetic transformation in Mn3SnC as compared to that in Mn3GaC. The transformation in Mn3SnC is close to room temperature ( K) and is associated with a large change in magnetic entropy that makes it a suitable candidate for applications in ferroic cooling. Using a combination of x-ray and neutron diffraction and x-ray absorption fine structure spectroscopy we infer that the magnetic ground state consisting of antiferromagnetic and ferromagnetic Mn atoms is due to structural distortions present in Mn6C octahedra.

Persistent luminescence of ZnGa2O4: Cr, an outstanding biomarker for in vivo imaging

ZnGa2O4 (ZGO) is a normal spinel. When doped with Cr3+ ions, ZGO:Cr becomes a high brightness persistent luminescence material with an emission spectrum perfectly matching the transparency window of living tissues. It allows in vivo mouse imaging with a better signal to background ratio than classical fluorescent NIR probes. The most interesting characteristic of ZGO:Cr lies in the fact that its LLP can be excited with red light, well below its band gap energy and in the transparency window of living tissues. A mechanism based on the trapping of carriers localized around a special type of Cr3+ ions namely CrN2 can explain this singularity. The antisite defects of the structure are the main responsible traps in the persistent luminescence mechanism. When located around Cr3+ ions, they allow, via Cr3+ absorption, the storage of not only UV light but also all visible light from the excitation source.

Optical properties and storage capabilities in AB2O4:Cr3+ (A=Zn, Mg, B=Ga, Al)

Red emitting long-lasting phosphorescence (LLP) material, are useful biomarker for small animal in vivo imaging. We report here our investigations on the optical features of chromium doped AB2O4 spinels (A=Zn, Mg and B=Ga, Al…) suitable for such applications. It is possible to tune the emission wavelengths of Cr3+ by a crystal field variation to be well centered in the biological window and it is also possible to adjust the traps depth in order to better control the release of the traps. These traps are therefore stable at room temperature and could be emptied by thermal or near infrared source making this material a potential new photostimulated/optically compound. Photoluminescence (PL) and thermally stimulated luminescence (TSL) studies are reported.