Evidence of cationic mixing and ordering in the honeycomb layer of Li4MSbO6 (M(III) = Cr, Mn, Al, Ga) (SG C2/c) oxides

We report the synthesis of the rock salt derived structures of Li4MSbO6 (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 alpha-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 approximate to 5.11; b approximate to 8.85; c approximate to 9.84 angstrom; beta approximate to 100 degrees) compared to Li4FeSbO6 (C2/m: a = 5.165(6); b = 8.928(13); c = 5.155(7) angstrom; beta = 109.47(2)degrees). 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 Li4CrSbO6 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.