Identification of high ionic conducting members (M(III) = Co, Ni) in the new lithium rich Li4.50M0.50TeO6 (M(III) = Co, Ni, In) series.

Many new lithium-excess compounds with rock-salt related structures have been extensively studied in recent years to discover high-capacity electrode materials for lithium-ion batteries. In the present work, lithium rich layered tellurates, Li4.50M0.50TeO6 (M(III) = Co, Ni, In), are added to the existing series of Li4.50M0.50TeO6 (M(III) = Cr, Mn, Fe, Al, and Ga) oxides. Structural investigations revealed their stabilization in the space group C2/m with a new cationic ordering. The structure consists of (Li1.50M0.50TeO6)3- honeycomb arrays along the ab plane by the edge sharing of TeO6 with (Li/M)O6 octahedra. The honeycomb arrays are separated by the intermediate layer of Li alone in Li4.50Co0.50TeO6. On the other hand, in the Ni and In analogues, the interlayer region consists of Li with Te, and Li with In ions, respectively. XPS studies confirmed the +3 oxidation state of Co and Ni ions. The appearance of a strong band at 680 nm resulting from LMCT (O → Co) in the UV-vis DRS data of the Li4.50Co0.50TeO6 sample further indicated the presence of Co3+ (d6, low spin) ions. The absence of characteristic Ni2+ bands at around ∼650 and 740 nm supported Ni3+ ions. Li4.50Co0.50TeO6 showed diamagnetic behaviour, while Li4.50Ni0.50TeO6 displayed paramagnetic nature. A negative θ (-14(2)) K has been obtained in the temperature region of 300-100 K for Li4.50Ni0.50TeO6 representing dominant antiferromagnetic interactions. At 2 K, Li4.50Ni0.50TeO6 unveiled a non-linear trend with no significant hysteresis and nearly saturation at 5 T field indicating the existence of additional interactions. Li4.50Co0.50TeO6 and Li4.50Ni0.50TeO6 exhibited significant conductivity values of 0.016 and 0.003 S cm-1, respectively, at 300 °C, thereby opening up further studies in this direction.