• J Gopalakrishnan

      Articles written in Journal of Chemical Sciences

    • A study of Co3−xNixO4 (O≤x≤1) system

      N K Appandairajan J Gopalakrishnan

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      A series of solid solutions of composition Co3−xNixO4 has been studied. For 0≤x≤1·1 the members crystallize in the cubic spinel structure Measurements of dc electrical conductivity and Seebeck coefficient show that there is a transition from semiconducting state to semi-metallic state aroundx=0·5–0·6. All compositions withx>0·5 are semimetallic and ferrimagnetic. The results have been rationalised on the basis of the cation distribution (Co2+)t [NixIIICo2−xIII]0O4 for the solid solutions which permits formation of σ*(eg) band through strong covalent interaction between octahedral-site low spin CoIII/NiIII and oxide ion in the spinel structure.

    • Preparation and structure of some Ln2MoO5 oxides

      A Manthiram J Gopalakrishnan

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      Ln2MoO5 oxides where Ln=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho and Y have been prepared by hydrogen-reduction of the corresponding Ln2MoO6 at 800–900°C. The lighter Ln2MoO5 oxides (Ln=La, Pr and Nd) crystallize in a monoclinic Yb2ReO5 structure, while the heavier ones along with the yttrium compound are formed in an anion-deficient fluorite type structure. The latter on annealing at 1200° C in vacuum also transform to the Yb2ReO5 structure type.

    • Amorphous MoS3 and AxMoS3 (A=Li or Na; 0<x<4)

      T Murugesan J Gopalakrishnan

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      Amorphous AxMoS3 (A=Li or Na; 0<x<4) prepared by the reaction of MoS3 withn-butyllithium or sodium naphthalide in organic solvents have been characterized by x-ray photoelectron spectroscopy, infrared spectroscopy as well as electrical and magnetic measurements. The results indicate that sulphur exists as polysulphide species in MoS3 and mainly as monosulphide in AxMoS3 whenx∼4; there is no discernible change in the Mo(3d) binding energies of MoS3 and AxMoS3. Both MoS3 and AxMoS3 are diamagnetic and non-metallic at room temperature. The data suggest that MoS3 probably exists as Mo2+(S32−) with Mo−Mo bonds, incorporation of alkali metal atoms resulting in the reduction of proportion of polysulphide ions.

    • Mixed valency in the high-temperature phases of transition metal molybdates,AMoO4 (A=Fe, Co, Ni)

      R A Mohan Ram J Gopalakrishnan

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      Transition metal molybdates of the formulaAMoO4 whereA=Fe, Co or Ni exhibit a first-order phase transition between 670K–970K. An investigation of the lowtemperature (lt) and high-temperature (ht) phases by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, magnetic susceptibility and other physical methods shows that the phase transition is associated with a valence change of the typeA2++Mo6+αA3++Mo5+ in the cases of iron and cobalt molybdates.

    • Synthesis and structure of some interesting oxides of bismuth

      J Gopalakrishnan

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      Synthesis and structures of several new oxides containing bismuth are described. Three types of structures are common among the multinary oxides containing trivalent bismuth. They are the sillenite structure of γ-Bi2O3, the layered perovskite structure of Aurivillius phases and the pyrochlore structure. The influence of Bi3+∶6s2 lone pair electrons is seen in all the three structures. In transition metal oxides containing trivalent bismuth,do cations (Ti4+, Nb5+, W6+) stabilize the layered perovskite structure, while cations containing partially-filledd orbitals (V4+, Cr3+, Fe3+) favour pyrochlore-related structures. Ferroelectric distortion ofMO6 octahedra of thedo cations seems to play an important role in stabilizing layered perovskite structures.

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