Papers by Author: P.M.G. Nambissan

Abstract: CoCr_xFe_2-xO₄ samples with x = 0.0 to 2.0 were prepared by air oxidation of aqueous suspension containing Co²⁺, Cr³⁺ and Fe³⁺ ions and characterized by x-ray diffraction. Positron lifetime and coincidence Doppler broadening (CDB) measurements indicated three distinct stages of positron trapping in defects. Initially the vacancy-type defects located at the A-sites (tetrahedral) trapped positrons but, with the substitution of Fe by Cr in low concentrations (x <= 0.7), positrons are trapped by defects at the B-sites (octahedral). Mossbauer spectroscopic results indicated the cationic distribution at B-sites to be stoichiometry-dependent and, till x = 0.7, the deficiency of Fe³⁺(B) ions was compensated by interchange of Fe³⁺(A) ions with Co²⁺(B) ions. Between x = 0.9 and 1.7, the substitution resulted in continued decrease of Fe³⁺(B) ions and the structure got fully transformed into a normal spinel configuration during x = 1.8 to 2.0.

Abstract: We report in this work about the inversion of the spinel structure of ZnFe₂O₄ induced by the substitution of Zn²⁺ by Ni²⁺ ions. Positron lifetimes were measured in Zn_1-xNi_xFe₂O₄ with different concentrations (x) of doped Ni²⁺ ions and a drastic change across x = 0.4 – 0.6 was observed, which is attributed to this transformation. The interchange of positions of the cations on doping leaves a fraction of them unoccupied and these vacancies act as positron trapping centres. Since Ni²⁺ is smaller in size than Zn²⁺, defects due to non-stoichiometry are less in NiFe₂O₄ than in ZnFe₂O₄. The increase in positron lifetime implies the trapping of positrons being shifted from A- to B-sites and is an indication of the transformation from inverse to normal spinel configuration. Coincidence Doppler broadening measurements supported these findings.