Evidence of cationic mixing and ordering in the honeycomb layer of Li ₄MSbO₆ (M(iii) = Cr, Mn, Al, Ga) (S.G. C2/c) oxides

We report the synthesis of the rock salt derived structures of Li ₄MSbO₆ (M(iii) = Cr, Mn, Al, Ga) oxides. These layered oxides are shown to have the formation of preferred cationic mixing in the (LiMSbO₆)^3- layers based on single crystal and powder XRD studies, in addition to the cation- (Li⁺ ions versus (Li ⁺/M³⁺/Sb⁵⁺) ions) ordering observed in several α-NaFeO₂ type oxides. The additional ordering found in the honeycomb layer is justified by the oxygen octahedra formed by more Sb/less Li, more Li/less M and more M/less Li/less Sb. This preferential cation mixing is proven structurally for the first time in these oxides and is evident from the superstructure observed by the doubling of the c axis (C2/c: a ≈ 5.11; b ≈ 8.85; c ≈ 9.84 Å; β ≈ 100°) compared to Li ₄FeSbO₆ (C2/m: a = 5.165(6); b = 8.928(13); c = 5.155(7) Å; β = 109.47(2)°). The driving force seems to be the minimization of the various cation-cation (Sb⁵⁺-Sb⁵⁺, Sb⁵⁺-M³⁺, M³⁺-M³⁺) interactions expected in the edge shared octahedral structures. The magnetic susceptibility of the oxides with magnetic metal ions in a triangular lattice follows the Curie-Weiss law in the temperature range of 300 K-75 K for Li ₄CrSbO₆ and in the temperature range of 300 K-50 K for Li₄MnSbO₆, with negative Weiss constants at 67 K and 68 K respectively. At low temperatures (<7 K) antiferromagnetic interactions are expected because of the interaction between the layers. Ionic conductivity measurements of Li₄MSbO₆ and ion-exchange experiments with Ag⁺ ions resulting in delafossite based oxides confirmed the mobility of interlayer lithium ions in these oxides. This journal is