Structural and magnetic properties of antifluorite-type Li_5+xFe_1-xCo_xO₄

We report on the structural and magnetic properties of Li5+xFe1-xCoxO4(0 <= x <= 1).< The system assumes two distinct crystal structures at room temperature (alpha and beta), and shows a transition in preference from alpha to beta between x=0.18 and x=0.25. This tendency is supported by DFT-calculations. Upon heating, Li5FeO4 undergoes a reconstructive transition from alpha ->beta at similar to 1060 K, while Li6CoO4 shows an order-disorder transition from beta ->epsilon at similar to 885 K, where epsilon is a cubic cation-disordered antifluorite structure. Li5.50Fe0.50Co0.50O4 is stabilised towards the ordered beta-phase until at least 1170 K. XANES-measurements confirm that the oxidation states of Fe and Co remain unchanged throughout the series with charge compensation from additional Li+ on increasing x. Magnetic measurements show onset of low-temperature antiferromagnetic ordering around 3 K to 12 K. The magnetic structure of Li5FeO4 is described on the basis of powder neutron diffraction at 1.5 K, while the magnetic structures of Li5.50Fe0.50Co0.50O4 and Li6CoO4 are found to be slightly incommensurate below the ordering temperature.