A. N. Radhakrishnan

National Institute for Interdisciplinary Science and Technology, Tiruvananantapuram, Kerala, India

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Publications (8)22.2 Total impact

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    ABSTRACT: New stannate pyrochlore based red emitting phosphor materials: CaLa1-xSnNbO7: xEu3+ were prepared by the conventional solid state route and characterized by Powder X-ray diffractometer, scanning electron microscope and fluorescence measurements. Optimization studies by way of varying the calcination temperatures show that CaLaSnNbO7: Eu3+ prepared at 1,400 �C displayed the maximum luminescence characteristics. The dominant emission peak of CaLaSnNbO7: xEu3+ phosphor is 5D0 - 7F2 forced hypersensitive electric dipole transition which indicates the deviation of pyrochlore A-site from inversion symmetry. The present quaternary pyrochlore phosphors show strong red luminescence (~612 nm) under near UV (394 nm) and blue light (465 nm) excitations.
    Journal of Materials Science Materials in Electronics 09/2012; DOI:10.1007/s10854-012-0636-6 · 1.97 Impact Factor
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    ABSTRACT: Quaternary pyrochlore-type solid solutions, CaGdZrNb(1-x)Ta(x)O(7) (x = 0, 0.2, 0.4, 0.6, 0.8, 1), were prepared by a high-temperature ceramic route. The pyrochlore phases of the compounds were confirmed by powder X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy. The crystallographic parameters of the pyrochlore compounds were accurately determined by Rietveld analysis of the powder XRD data. The isovalent substitution of Ta in place of Nb at the B site can reveal the effect of chemical bonding on lattice thermal expansion and oxide ion conductivity because both Nb and Ta have the same ionic radius (0.64 Å). Lattice thermal expansion coefficients of the samples were calculated from high-temperature XRD measurements, and it was found that the thermal expansion coefficient decreases with substitution of Ta. Oxide ion conductivity measured by a two-probe method also shows the same trend with substitution of Ta, and this can be attributed to the high bond strength of the Ta-O bond compared to that of the Nb-O bond. Microstructural characterization using scanning electron microscopy proves that the size of the grains has a small effect on the oxide ion conductivity. Our studies established the role of chemical bonding in deciding the conductivity of pyrochlore oxides and confirmed that the 48f-48f mechanism of oxide ion conduction is dominant in pyrochlore oxides.
    Inorganic Chemistry 02/2012; 51(4):2409-19. DOI:10.1021/ic202383f · 4.79 Impact Factor
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    ABSTRACT: A series of quaternary nonstoichiometric cubic pyrochlores with varying cerium content in Ca–Ce–Ti–Nb–O system has been synthesized through solid state route. The powder X-ray diffraction and Rietveld analysis indicate that smaller Ti cation occupies the A site by displacing the A and O′ ions causing displacive disorder in the cubic pyrochlore structure. The electrical conductivity measurements reveal that they exhibit semiconductor behavior in a broad temperature range (30–600°C) and the conductivity increases with increase in Ce content. The X-ray photoelectron spectroscopy (XPS) analysis corroborates the presence of Ce in 3+ state. The impedance analysis reveals two types of conduction processes from the frequency dependent conductivity plots, i.e. low frequency conduction due to short range hopping and high frequency conduction due to the localized reorientation hopping mechanism. The relaxational properties indicate that hopping frequency is dependent on cerium concentration and is a measure of degree of disorder in the system.
    Materials Chemistry and Physics 05/2011; 127(1):162-169. DOI:10.1016/j.matchemphys.2011.01.053 · 2.13 Impact Factor
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    ABSTRACT: The effect of simultaneous substitutions of Ca at A site and Nb or Ta at B site in pyrochlore-type solid solutions: (Ca(x)Gd(1-x))(2)(Zr(1-x)M(x))(2)O(7) (x = 0.1, 0.2, 0.3, 0.4, 0.5 and M = Nb or Ta) were studied by powder X-ray diffraction (XRD), FT NIR Raman spectroscopic techniques and transmission electron microscopy. The solid solutions were prepared by the conventional high-temperature ceramic route. The XRD results and Rietveld analysis revealed that the defect fluorite structure of Gd(2)Zr(2)O(7) progressively changed to a more ordered pyrochlore phase by simultaneous substitutions at A and B sites. Raman spectroscopy reveals the progressive ordering in the anion sublattice with simultaneous doping. High-resolution images and selected-area electron diffraction patterns obtained from TEM confirms the XRD and Raman spectroscopic results. High-temperature XRD studies show that the lattice expansion coefficient in these pyrochlore oxides is of the order of 10(-6) K(-1). Lattice thermal expansion coefficient increases with increase of disorder in pyrochlore oxides, and hence the variation of thermal expansion coefficient with composition is also a good indicator of disordering in pyrochlore-type oxides. The ionic conducting properties of the samples were characterised by impedance spectroscopy, and it was found that Nb-doped compositions show a considerable change in conductivity near the phase boundary of disordered pyrochlore and defect fluorite phases.
    Dalton Transactions 02/2011; 40(15):3839-48. DOI:10.1039/c0dt01688h · 4.10 Impact Factor
  • Mannarakoth Deepa, Padala Prabhakar Rao, Sreedevi Sumi, Ambiyil Narayana Pillai Radhakrishnan, Peter Koshy
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    ABSTRACT: A series of quaternary powellites with varying cerium contents in a Ca–Ce–Nb–M–O (where M=Mo or W) system has been synthesized through the solid-state route. Powder X-ray diffraction analysis shows that these compounds crystallize in a tetragonal powellite structure with the I41/a space group. The structure was further confirmed by Raman spectroscopic analysis. The electrical conductivity studies reveal that they exhibit negative temperature coefficient (NTC) thermistor characteristics. The characteristic parameter of the thermistor (β) is found to be in the range of 5000–7000 K, and these values can be tuned with the cerium concentration. The electrical conductivity in these compounds is due to the presence of Ce3+, which remains in the reduced state without being oxidized to Ce4+ due to the structural stabilization. The X-ray photoelectron spectroscopy analysis corroborates the presence of Ce in the 3+ state.
    Journal of the American Ceramic Society 02/2010; 93(6):1576 - 1579. DOI:10.1111/j.1551-2916.2010.03616.x · 2.61 Impact Factor
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    ABSTRACT: CaREZrNbO7 (RE = La, Nd, Sm, Gd and Y) system changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios, r(Ca2+–RE3+)av/r(Zr4+–Nb5+)av were larger than 1.34. Thus, the La, Nd, and Sm compounds have a cubic P-type structure and the Gd and Y ones have a defect F-type structure. The electrical conductivity was measured using complex-plane impedance analysis over a wide temperature (300–750 °C) and frequency (1 Hz–1 MHz) ranges. The conductivity relaxation phenomenon was observed in these compounds and the relaxation frequencies were found to show Arrhenius-type behavior and activation energies were in good agreement with those obtained from high temperature conductivity plots. These results support the idea that the relaxation process and the conductivity have the same origin. The ionic conductivity of CaREZrNbO7 (RE = La, Nd, Sm, Gd and Y) system showed the maximum at the phase boundary between the F-type and P-type phases. On the other hand, the activation energy for the conduction decreased in the F-type phase and increased in the P-type phase with increasing ionic radius ratio. Among the prepared compounds, CaGdZrNbO7 showed the highest ionic conductivity of 9.47 × 10− 3 S/cm at 750 °C which was about twice as high as that observed in Gd2Zr2O7 (4.2 × 10− 3 S/cm at 800 °C). The grain morphology observation by scanning electron microscope (SEM) showed well-sintered grains. AC impedance measurements in various atmospheres further indicated that they are predominantly oxide ion conductors at elevated temperatures (> 700 °C).
    Solid State Ionics 08/2009; 180(s 20–22):1164–1172. DOI:10.1016/j.ssi.2009.06.006 · 2.11 Impact Factor
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    ABSTRACT: Order–disorder transformations in a quaternary pyrochlore oxide system, Ca–Y–Zr–Ta–O, were studied by powder X-ray diffraction (XRD) method, transmission electron microscope (TEM) and FT-NIR Raman spectroscopic techniques. The solid solutions in different ratios, 4:1, 2:1, 1:1, 1:2, 1:4, 1:6, of CaTaO3.5 and YZrO3.5 were prepared by the conventional high temperature ceramic route. The XRD results and Rietveld analysis revealed that the crystal structure changed from an ordered pyrochlore structure to a disordered defect fluorite structure as the ratios of the solid solutions of CaTaO3.5 and YZrO3.5 were changed from 4:1 to 1:4. This structural transformation in the present system is attributed to the lowering of the average cation radius ratio, rA/rB as a result of progressive and simultaneous substitution of larger cation Ca2+ for Y3+ at A sites and smaller cation Ta5+ for Zr4+ at B sites. Raman spectroscopy and TEM analysis corroborated the XRD results.
    Journal of Solid State Chemistry 08/2009; 182(8):2312-2318. DOI:10.1016/j.jssc.2009.06.016 · 2.20 Impact Factor
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    ABSTRACT: A new series of pyrochlore type ceramic semiconducting oxides in Ca–Ce–Ti–M–O (M=Nb or Ta) system has been synthesized by the conventional ceramic route. The electrical conductivity measurements show that these oxides exhibit semiconducting behavior and the conductivity increases with the Ce content in the compound. Activation energy of the current carriers is in the range of 0.5–1.6eV. The electrical conductivity in these oxides is due to the presence of Ce3+, which remains in the reduced state without being oxidized to Ce4+ by structural stabilization. The photoluminescence and X-ray photoelectron spectroscopy analysis corroborate the presence of Ce in the 3+ state. Impedance spectral analysis is carried out to evaluate the transport properties and indicates that the conduction in these compounds is mainly due to electronic contribution. The X-ray powder diffraction and Raman spectroscopy analysis establishes that these oxides belong to a cubic pyrochlore type structure.
    Materials Research Bulletin 07/2009; 44(7):1481-1488. DOI:10.1016/j.materresbull.2009.02.014 · 2.29 Impact Factor