Origin of high Néel temperature in the low coordination number system A FeO₂ (A=K and Rb)

To explore the origin of high Néel temperature (TN =930 K) of KFeO₂, which has only 4 coordination number, we have investigated electronic structures and magnetic properties of A FeO₂ (A=K and Rb) by employing the first-principles band-structure method and the Monte Carlo simulation. We have confirmed the observed antiferromagnetic (AFM) ground state of KFeO₂, and obtained nearly identical electronic structures for KFeO₂ and RbFeO₂. We have found that A FeO₂ (A=K and Rb) have the strong covalent-bonding nature between O2p and Fe3d states, which produces the large kinetic superexchange interaction between Fe ions and high TN. The calculated TN of KFeO₂ (804 K) is in good agreement with experiment, suggesting that the strong AFM superexchange interaction compensates the low coordination number in KFeO₂. The estimated TN of RbFeO₂ (821 K) is predicted to be even higher than that of KFeO₂ due to the larger Fe-O-Fe bond angle in RbFeO ₂.