Papers by Sreekanth Chakradhar
EPR and luminescence properties of LiGa5O8:Mn green emitting phosphor
Journal of Luminescence, 2009
Green emitting LiGa 5 O 8 :Mn powder phosphor has been prepared in a short time by solution combu... more Green emitting LiGa 5 O 8 :Mn powder phosphor has been prepared in a short time by solution combustion method. Powder X-ray diffraction pattern indicated a dominant phase of LiGa 5 O 8 with another secondary LiGaO 2 phase. Morphology aspects were studied by using ...
J Luminesc, 2011
A blue phosphor, BaMgAl 10 O 17 :Eu 2 þ , has been synthesized in the furnace at a temperature of... more A blue phosphor, BaMgAl 10 O 17 :Eu 2 þ , has been synthesized in the furnace at a temperature of 500 1C by solution combustion method. The formation of the as-prepared BaMgAl 10 O 17 :Eu 2 þ phosphor was confirmed by the powder X-ray diffraction technique. The EPR spectrum exhibited an intense resonance signal centered at g ¼ 4.63 at 150 mT along with a number of resonances in the vicinity of g 4 2.0 and g o 2.0. The number of spins participating in resonance (N) and the susceptibility (c) for the resonance signal at g ¼ 4.63 have been calculated as a function of temperature. The excitation spectrum of BaMgAl 10 O 17 :Eu 2 þ phosphor showed a strong peak near 336 nm (4f 7 (8 S)-5d 1 (t 2g) transition) with a staircase like structure in the region 376-400 nm owing to crystal field splitting of the Eu 2 þ d-orbital. The 336 nm excitation produced a broad blue emission at 450 nm corresponding to 4f 6 5d-4f 7 transition. PL studies reveal the two emission centers one at 450 nm and the other at 490 nm in this phosphor.
Mater Chem Phys, 2008
LiAl 5 O 8 :Mn 2+ phosphors have been synthesized by combustion process in a short time (<5 min).... more LiAl 5 O 8 :Mn 2+ phosphors have been synthesized by combustion process in a short time (<5 min). X-ray diffractogram confirms that the synthesized material consists of LiAl 5 O 8 as main phase. Scanning electron microscopy showed angular plate-like crystals with non-uniform shape. The EPR spectra of Mn 2+ ions exhibit resonance signals at g ≈ 4.28 and g ≈ 2.0. The signal at g ≈ 2.0 is due to Mn 2+ ions in an environment close to tetrahedral symmetry, whereas the resonance at g ≈ 4.28 is attributed to the rhombic surroundings of the Mn 2+ ions. The variation of zero-field splitting parameter (D) with temperature is measured and discussed. The PL spectra exhibit green emission peaks at 505 and 518 nm which are due to the 4 T 1 (G) → 6 A 1 (S) transition of Mn 2+ ions. The EPR study of LiAl 5 O 8 :Mn 2+ phosphor gives clear indication of the Mn 2+ ions occupying more than one site in this phosphor.
Characterization, EPR and luminescence studies of ZnAl 2O 4:Mn phosphors
J Luminesc, 2008
ZnAl 2 O 4 :Mn green light emitting powder phosphors have been prepared by urea combustion techni... more ZnAl 2 O 4 :Mn green light emitting powder phosphors have been prepared by urea combustion technique involving furnace temperatures about 500 °C in a short time (<5 min). The prepared powders were characterized by X-ray diffraction, scanning electron microscopy, Fourier-...
EPR, FTIR, optical absorption and photoluminescence studies of Fe 2O 3 and CeO 2 doped ZnO–Bi 2O 3–B 2O 3 glasses
J Alloys Compounds, 2010
Glasses containing heavy metal oxide of the composition (wt.%) 23B 2 O 3 5ZnO72Bi 2 O 3 xFe 2 ... more Glasses containing heavy metal oxide of the composition (wt.%) 23B 2 O 3 5ZnO72Bi 2 O 3 xFe 2 O 3 /CeO 2 (0 ≤ x ≤ 0.0058 at.% in excess) were prepared by melt quenching technique. The glass formation was confirmed by XRD. FTIR spectra exhibit characteristic ...
J Alloys Compounds, 2009
This paper describes the sound velocity and the elastic properties of Na 2 O-B 2 O 3-V 2 O 5 glas... more This paper describes the sound velocity and the elastic properties of Na 2 O-B 2 O 3-V 2 O 5 glasses over a wide range of composition. Sound velocities (transverse and longitudinal) have been measured using a pulse echo superposition method at a frequency of 10 MHz. The elastic moduli (L, G, K and E), Poisson's ratio () and Debye temperature (Â D) exhibit composition dependent trends. The variation of these parameters has been explained using a structural motifs present and the dimensionality of the glass network.
J Luminesc, 2009
Magnesium aluminate (MgAl 2 O 4) doped with trivalent chromium (Cr 3+) was synthesized by the com... more Magnesium aluminate (MgAl 2 O 4) doped with trivalent chromium (Cr 3+) was synthesized by the combustion method. The prepared sample was characterized by X-ray powder diffraction, Brunauer-Emmet-Teller (BET) adsorption isotherms and diffuse-reflectance UV-vis spectroscopy techniques. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies have been performed at room temperature and at 110 K. The EPR spectrum exhibit resonance signals at g ¼ 5.37, 4.53, 3.82, 2.26 and 1.96 characteristic of Cr 3+ ions. The luminescence of Cr 3+-activated MgAl 2 O 4 exhibits a red emission peak around 686 nm from the synthesized phosphor particles upon 551 nm excitation. The luminescence is assigned to a transition from the upper 2 E g-4 A 2g ground state of Cr 3+ ions. By correlating EPR and optical data the crystal field splitting parameter (D q), Racah inter-electronic repulsion parameter (B) and the bonding parameters have been evaluated and discussed. The bonding parameters suggests that the ionic nature of Cr 3+ ions with the ligands and the Cr 3+ ions are in distorted octrahedral environment.
ChemInform Abstract: Synthesis, Characterization, Photoluminescnece and EPR Investigations of Mn Doped MgAl 2 O 4 Phosphors
Cheminform, 2008
ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance t... more ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Synthesis, characterization and photoluminescence of Eu3+, Ce3+ co-doped CaLaAl3O7 phosphors
Phil Mag a, 2010
ABSTRACT
J Mol Struct, 2008
Glass system of the composition xK 2 OA(90 À x)B 2 O 3 A10V 2 O 5 (where x = 10, 20, 30, 40, and ... more Glass system of the composition xK 2 OA(90 À x)B 2 O 3 A10V 2 O 5 (where x = 10, 20, 30, 40, and 50 mol%) have been prepared by melt quenching technique and characterized for their densities, and glass transition temperatures. The structure of the glasses has been investigated using infrared and magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The structural changes occur due to the addition of modifier oxide. Modification is based on the Sanderson's electronegativity principle, leading to the formation of [B 2 V 2 O 9 ] 2À (type I and type II), [BO 4/2 ] À and [BO 2/2 O] À. This is followed by a structural rearrangement that is the ratio of B 4 /B 4 + B 3 = N 4 increases up to 40 K 2 O mol%, further increase in modifier oxide concentration levels off N 4. The nature of speciation is examined using structural model. The variation of different properties is consistent with this model.
Synthesis, characterization, photoluminescence and EPR investigations of Mn doped MgAl 2O 4 phosphors
J Solid State Chem, 2007
ABSTRACT
The mixed alkali borate xNa 2 O-(30 2 x)K 2 O-70B 2 O 3 (5 # x # 25) glasses doped with 1 mol% of... more The mixed alkali borate xNa 2 O-(30 2 x)K 2 O-70B 2 O 3 (5 # x # 25) glasses doped with 1 mol% of manganese ions were investigated using EPR and optical absorption techniques as a function of alkali content to look for 'mixed alkali effect' (MAE) on the spectral properties of the glasses. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of the Mn 2þ ions. The resonance signal at g ø 2.02 exhibits a six line hyperfine structure. In addition to this, a prominent peak with g ø 4.64, with a shoulder around g ø 4.05 and 2.98, was also observed. From the observed EPR spectrum, the spin-Hamiltonian parameters g and A have been evaluated. It is interesting to note that some of the EPR parameters do show MAE. It is found that the ionic character increases with x and reaches a maximum around x ¼ 20 and thereafter it decreases showing the MAE. The number of spins participating in resonance (N) at g ø 2.02 decreases with x and reaches a minimum around x ¼ 20 and thereafter it increases showing the MAE. It is also observed that the zero-field splitting parameter (D) increases with x, reaches a maximum around x ¼ 15 and thereafter decreases showing the MAE. The optical absorption spectrum exhibits a broad band around ,20,000 cm 21 which has been assigned to the transition 6 A 1g (S) ! 4 T 1g (G). From ultraviolet absorption edges, the optical bandgap energies and Urbach energies were evaluated. It is interesting to note that the Urbach energies for these glasses decrease with x and reach a minimum around x ¼ 15. The optical band gaps obtained in the present work lie in the range 3.28-3.40 eV for both the direct and indirect transitions. The physical parameters of all the glasses were also evaluated with respect to the composition.
EPR of Fe3+ ions in macro porous nanocrystalline wollastonite ceramics
Electron paramagnetic resonance and optical absorption studies of Cu2+ ions in alkali barium borate glasses
ABSTRACT
Transactions of the Indian Ceramic Society, 2005
Nanocrystalline porous CaSiO 3 ceramic powders have been synthesized by a novel low temperature i... more Nanocrystalline porous CaSiO 3 ceramic powders have been synthesized by a novel low temperature initiated self-propagating, gas producing solution combustion process and characterized by XRD, SEM, EDS, porosity, surface area and Thermoluminescence (TL) studies. The effect of temperature on crystalline phase formation, amount of porogens and particle size of porous CaSiO 3 has been investigated. Single phase β-CaSiO 3 and α-CaSiO 3 are formed at 950 and 1200 °C respectively. The phase transformation
EPR and optical investigations of manganese ions in alkali lead tetraborate glasses
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2005
Electron paramagnetic resonance (EPR) and optical absorption spectra of Mn2+ ions in different al... more Electron paramagnetic resonance (EPR) and optical absorption spectra of Mn2+ ions in different alkali lead tetraborate glasses 90R2B4O7+9.25PbO+0.75MnSO4 (R=Li, Na and K) and 90Li2B4O7+(10-x)PbO+xMnSO4 (x=0.1, 0.25, 0.5, 0.75, 1.0, 1.25, 1.5 and 2 mol%) have been studied. The EPR spectrum of all the glass samples exhibit three resonance signals at g=2.0, 3.3 and 4.3. The resonance signal at g=2.0 is attributed to the Mn2+ ions in an environment close to an octahedral symmetry. The resonance signals at g=3.3 and 4.3 have been attributed to the rhombic symmetry of the Mn2+ ions. The effect of temperature (123-433 K) and the composition dependence of EPR signals have been studied for Mn2+ ions in lithium lead tetraborate glasses. It is interesting to observe that the variation of paramagnetic susceptibility (chi) with temperature obeys Curie-Weiss law. From the slope of 1/chi versus T graph, the Curie constant (C) has been evaluated. The zero-field splitting (zfs) parameter D has been calculated for different alkali lead tetraborate glasses from the intensities of the allowed hyperfine lines. The optical absorption spectrum exhibits three bands. An intense and broad band at lower energy side has been assigned to the spin-allowed (5Eg--&amp;amp;amp;amp;amp;amp;amp;amp;gt;5T2g) transition of Mn3+ ions in an octahedral symmetry. The intense and sharp band and a broad band at higher energy side have been assigned to charge transfer bands. A red shift is observed with increase of alkali ion size. The optical band gap energy (Eopt) decreases, whereas the Urbach energies (DeltaE) increases with increase of Mn content. The theoretical values of optical basicity (Lambdath) of the glasses have also been evaluated.
Fe3+ ions in alkali lead tetraborate glasses—an electron paramagnetic resonance and optical study
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2005
Glass systems of composition 90R(2)B(4)O(7)+9PbO+1Fe(2)O(3) (R=Li, Na and K) and 90Li(2)B(4)O(7)+... more Glass systems of composition 90R(2)B(4)O(7)+9PbO+1Fe(2)O(3) (R=Li, Na and K) and 90Li(2)B(4)O(7)+(10-x)PbO+xFe(2)O(3) (x=0.5, 1, 3, 4, 5, 7 and 9 mol %) have been investigated by means of electron paramagnetic resonance (EPR) and optical absorption techniques. The EPR spectra exhibit three resonance signals at g approximately 6.0, 4.2 and 2.0. The resonances at g approximately 6.0 and 4.2 are attributed to Fe(3+) ions in rhombic and axial symmetry sites, respectively. The g approximately 2.0 resonance signal is due to two or more Fe(3+) ions coupled together with dipolar interaction. The EPR spectra of 1 mol % of Fe(2)O(3) doped in lithium lead tetraborate glass samples have been studied at different temperatures (123-433 K). The intensity of g approximately 4.2 resonance signal decreases and the intensity of g approximately 2.0 resonance signal increases with the increase of temperature. The line widths are found to be independent of temperature. The EPR spectra exhibit a marked concentration dependence on iron content. A decrease in intensity for the resonance signal at g approximately 4.2 with increase in iron content for more than 4 mol % has been observed in lithium lead tetraborate glass samples and this has been attributed to the formation of Fe(3+) ion clusters in the glass samples. The paramagnetic susceptibility (chi) is calculated from the EPR data at various temperatures and the Curie constant (C) has been evaluated from 1/chi versus T graph. The optical absorption spectrum of Fe(3+) ions in lithium lead tetraborate glasses exhibits three bands characteristic of Fe(3+) ions in an octahedral symmetry. The crystal field parameter D(q) and the Racah interelectronic repulsion parameters B and C have also been evaluated. The value of interelectronic repulsion parameter B (825 cm(-1)) obtained in the present work suggests that the bonding is moderately covalent.
Photoluminescence and EPR studies of Cr-doped hibonite (CaAl12O19) phosphors
Solid State Sciences, 2008
CaAl 12 O 19 :Cr powder phosphors have been prepared at 500 °C in less than 5 min by a combustion... more CaAl 12 O 19 :Cr powder phosphors have been prepared at 500 °C in less than 5 min by a combustion method and characterized by XRD, FT-IR, SEM, EPR and photoluminescence techniques. The g values evaluated from EPR spectroscopy indicate that the site symmetry around Cr 3+ ...
physica status solidi (b), 2005
Chromium ions doped 90 Na 2 B 4 O 7 + (9-x)PbO + 1TeO 2 and 90 K 2 B 4 O 7 + (9-x)PbO + 1TeO 2 (x... more Chromium ions doped 90 Na 2 B 4 O 7 + (9-x)PbO + 1TeO 2 and 90 K 2 B 4 O 7 + (9-x)PbO + 1TeO 2 (x = 3, 4, 5, 6, 7 and 8 mol%) glasses have been studied by using Electron Paramagnetic Resonance (EPR) and optical absorption techniques. The EPR spectra exhibit a broad resonance signal at g = 4.65 and a narrow resonance signal at g = 1.97. The resonance at g = 4.65 can be attributed to isolated Cr 3+ ions in rhombic symmetry subjected to strong crystal field effects and the resonance at g = 1.97 is due to Cr 5+ ions. The EPR spectra have also been studied at different temperatures (153-393 K). The number of spins (N) participating in the resonance is measured as a function of temperature and the activation energy is calculated. The exchange coupling constant J between chromium ions is calculated. The optical absorption spectrum exhibits bands characteristic of Cr 3+ ions in an octahedral symmetry. The optical band gap (E opt) and Urbach energies (∆E) have been calculated for all the glass samples and it is observed that E opt decreases and ∆E increases with increase of chromium content. This has been explained as due to the creation of additional localized states with chromium doping, which overlap and extend in the mobility gap of the glass matrix.
EPR, optical, infrared and Raman studies of VO2+ ions in polyvinylalcohol films
Physica B: Condensed Matter, 2007
Electron paramagnetic resonance (EPR), optical, infrared and Raman spectral studies have been car... more Electron paramagnetic resonance (EPR), optical, infrared and Raman spectral studies have been carried out on vanadyl ions doped in polyvinylalcohol (PVA) films. The spin-Hamiltonian parameters (g and A) and the molecular orbital coefficients (β2*2 and k) have been evaluated. The values of spin-Hamiltonian parameters confirm that the vanadyl ions are present in PVA films as VO2+ molecular ions in an
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Papers by Sreekanth Chakradhar