Origin of high Neel temperature in the low coordination number system AFeO2 (A=K and Rb)

To explore the origin of high Neel temperature (T-N=930 K) of KFeO2, which has only 4 coordination number, we have investigated electronic structures and magnetic properties of AFeO(2) (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 AFeO(2) (A=K and Rb) have the strong covalent-bonding nature between O 2p and Fe 3d states, which produces the large kinetic superexchange interaction between Fe ions and high T-N. The calculated T-N 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 T-N of RbFeO2 (821 K) is predicted to be even higher than that of KFeO2 due to the larger Fe-O-Fe bond angle in RbFeO2.