Structural and magnetic properties of antifluorite-type Li${}_{5+x}$Fe${}_{1-x}$Co${}_x$O${}_4$

We report on the structural and magnetic properties of Li${}_{5+x}$Fe${}_{1-x}$Co${}_x$O${}_4$ (0≤x≤1).< The system assumes two distinct crystal structures at room temperature (α and β), and shows a transition in preference from α to β between x=0.18 and x=0.25. This tendency is supported by DFT-calculations. Upon heating, Li${}_5$FeO${}_4$ undergoes a reconstructive transition from α→β at ∼1060 K, while Li${}_6$CoO${}_4$ shows an order-disorder transition from β→ϵ at ∼885 K, where ϵ is a cubic cation-disordered antifluorite structure. Li${}_{5.50}$Fe${}_{0.50}$Co${}_{0.50}$O${}_4$ is stabilised towards the ordered β-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 Li${}_5$FeO${}_4$ is described on the basis of powder neutron diffraction at 1.5 K, while the magnetic structures of Li${}_{5.50}$Fe${}_{0.50}$Co${}_{0.50}$O${}_4$ and Li${}_6$CoO${}_4$ are found to be slightly incommensurate below the ordering temperature. (© 2023 The Authors. Zeitschrift für anorganische und allgemeine Chemie published by Wiley-VCH GmbH)