Investigation of the structure, optical and magnetic properties of the honeycomb layered Na3(MnIIM)SbO6 (M (II) = Fe, Co, Ni, Zn, Mg) and Na3(FeIIM′)SbO6 (M′ (II) = Co, Ni, Zn, Mg) oxides

Abstract

Rock salt superstructured oxides synthesized through conventional high temperature methods have been shown to accommodate Mn2+ and Fe2+ ions leading to the formation of Na3(MnM)SbO6 (M (II) = Fe, Co, Ni, Zn, Mg) and Na3(FeM')SbO6 (M′ (II) = Co, Ni, Zn, Mg) oxides. The layered structures (in S. G. C2/m) consist of honeycomb arrays formed by the 2:1 ordering of (Mn, M)2+, Sb5+ and (Fe, M′)2+, Sb5+ cations and are interleaved by the Na+ cations. PXRD measurements do not show any evidence for additional superstructure formation, and the unit cell volumes confirm a linear variation with the average ionic radii of the divalent cations. Diffuse reflectance measurements exhibit the characteristic d-d transitions from the optically active combination of transition metal ions in the solid solution members. Dilution of Mn2+ (d5) ions by M = Co, Ni, Zn resulted in typical paramagnetic behavior (300–100 K), with definite AFM ordering (below 8 K) and negative Weiss (θ) constants as shown by the magnetic susceptibility measurements of Na3(MnM)SbO6 oxides. The divergence between the ZFC and FC values is absent (at 0.1 T) in the solid solution members, unlike the parent Na3Mn2SbO6. Na3(MnMg)SbO6 alone differed and registered only a negative θ without any AFM ordering. All of the corresponding Na3(FeM')SbO6 (M′ = Co, Ni, Zn, Mg) oxides exhibited definite AFM ordering with negative θ values indicating a ground state with dominant AFM interactions. The exception has been the Co2+ substituted sample showing a positive θ (20 K) value from the Curie-Weiss plot, suggesting ferromagnetic interactions. The reported low dimensional oxides derived by combining divalent metal ions in the honeycomb geometry provide scope for studying the exciting aspects of solid state chemistry and physics. Similarly, the green emission observed from the PL measurements in the oxides containing Mn2+ reveals the prospects of developing novel phosphor materials.