Structural and magnetic properties of antifluorite-type Li5+xFe1−xCoxO4

Abstract

Abstract 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.