• S Uma

      Articles written in Journal of Chemical Sciences

    • Soft-chemical routes to synthesis of solid oxide materials

      J Gopalakrishnan S Uma K Kasthuri Rangan N S P Bhuvanesh

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      We describe three different families of metal oxides, viz., (i) protonated layered perovskites, (ii) framework phosphates of NASICON and KTiOPO4 (KTP) structures and (iii) layered and three-dimensional oxides in the H-V-W-O system, synthesized by ‘soft-chemical’ routes involving respectively ion-exchange, redox deintercalation and acid-leaching from appropriate parent oxides. Oxides of the first family, HyA2B3O10(A = La/Ca; B = Ti/Nb), exhibit variable Bronsted acidity and intercalation behaviour that depend on the interlayer structure. V2(PO4)3 prepared by oxidative deintercalation from Na3 V2(PO4)3 is a new host material exhibiting reductive insertion of lithium/hydrogen, while K0.5Nb0.5 M0.5OPO4 (M = Ti, V) are novel KTP-like materials exhibiting second harmonic generation of 1064nm radiation. HxVxW1-xO3 for x = 0–125 and 0.33 possessing α-MoO3 and hexagonal WO3 structures, prepared by acid-leaching of LiVWO6, represent functionalized oxide materials exhibiting redox and acid-base intercalation reactivity.

    • Interesting cationic (Li+/Fe3+/Te6+) variations in new rocksalt ordered structures

      Akanksha Gupta Vinod Kumar S Uma

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      A new series of layered oxides, Li3(Li1.5xFe3−(x+1.5x)Tex)O6, (0.1 ≤ 𝑥 ≤ 1.0) possessing rock-salt superstructures crystallizing in monoclinic (S.G. C2/m) symmetry is reported here. Investigations based on single crystal and powder X-ray diffraction studies for the 𝑥 = 1 member, Li3(Li1.5Fe0.5Te)O6, (a=5.1834(1); b=8.8858(2); c=5.16840(8) Å; 𝛽=110.660(1)°) confirmed the stabilization of (Li1.5Fe0.5Te1.0O6)3− honeycomb arrays with a very high amount of lithium ions. The structure for the x = 0.5 member (Li3.75Fe1.75Te0.5O6) has also been confirmed by the powder X-ray diffraction Rietveld refinements. Li3(Li1.5Fe0.5Te)O6 and Li3(Li0.75Fe1.75Te0.5)O6 oxides exhibited Curie–Weiss behaviour in the temperature range of 50–300 K with negative 𝜃 values. Their respective ionic conductivities were found to be 6.76 × 10−5 S cm−1 and 2.21 × 10−6 S cm−1 at 573 K. The UV-visible diffuse reflectance measurements for the differentmembers of the series Li3(Li1.5xFe3−(x+1.5x)Tex)O6, (0.1 ≤ 𝑥 ≤ 1.0) show the expected shifts in their absorptionedges based on the increasing amount of Fe3+ions starting from 𝑥 = 1.0 member to 𝑥 = 0.1 member

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