• A K Ganguli

      Articles written in Journal of Chemical Sciences

    • Magnetic properties of Ca2F2-xMnxO5

      A K Ganguli J Gopalakrishnan

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      Magnetic susceptibility of Ca2F2-xMnxO5 members crystallizing in two different structures, one having octahedral (O), tetrahedral (T) and square-pyramidal (SP) coordination of transition metal atoms (OTSP structure) and the other having octahedral and tetrahedral coordination (OT structure), has been investigated. Susceptibility behaviour of the oxides with OTSP structure is different from that of the oxides with OT structure. Ca2Fe1-33Mn0-67O5 with OTSP structure shows an antiferromagnetic ordering while the corresponding oxide with OT structure shows weak ferromagnetism.

    • Phase analysis and dielectric properties of oxides obtained in the MgO–(1−x)Nb2O5–(x)Ta2O5 system

      M Thirumal A K Ganguli

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      MgNb2-xTaxO6 (0 ≶x ≶ 2) phases can be obtained as the major phase (75 to 90%) by solid state reactions starting from oxides. These oxides crystallize in the orthorhombic columbite structure tillx = 1.75 and the tetragonal trirutile structure for MgTa2O6 (x = 2.0). For all the compositions there exist secondary phases like Nb2O5 or Ta2O5 in addition to the major AB2O6 phase. Sintered disks (1200°C) show dielectric constants varying between 14.8 and 16.0 for the entire range of composition at a frequency of 500 kHz. The dielectric loss is nearly constant around 0025 to 003 between 0 ≶x ≶ 1 but increases to 017 for the MgTa2O6 phase (x = 2.0). Scanning electron micrographs reveal a gradual decrease in grain size with increase in Ta concentration with a size of 3 micron forthe x=0 composition (sintered at 1200°C) while thex = 2 phase shows a grain size of approximately 0.5 microns. The microwave dielectric constant at ∼14 GHz is found to be 20.9 for thex = 0 composition and 17.7 for thex = 2 composition.

    • Superconductivity at 31.3 K in Yb-doped La(O/F)FeAs superconductors

      J Prakash S J Singh S Patnaik A K Ganguli

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      The effect of ytterbium substitution at the lanthanum site on the superconducting properties of La$_{1-x}$Yb$_x$O0.8F0.2FeAs ($x = 0.10$, 0.20 and 0.30) oxypnictides has been investigated. Powder X-ray diffraction studies show the presence of Yb2O3 and LaOF as secondary phases. The superconducting transition temperature ($T_c$) of 31.3 ($\pm \, 0.05$) K has been observed in $x = 0.1$ composition which is the maximum $T_c$ so far in the La(O/F)FeAs superconductor family at ambient pressure. Further increase in 𝑥 leads to suppression and broadening of superconducting transition. The resistive transition curves under different magnetic fields were investigated, leading to determination of upper critical field $H_{c2}$ (𝑇) of this new superconductor. The value of $H_{c2}$ at zero temperature is estimated to be about 46 T corresponding to coherence length ∼ 27 Å.

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