Modification in the local atomic spin configuration drives an insulator to metallic phase transition in disordered double perovskites: the case of Nd₂CoFeO₆

The double perovskite (DP) Nd₂CoFeO₆ was recently synthetized presenting short-range magnetic interactions in temperatures below 540K, and a classic paramagnetic behavior above that temperature. In the paramagnetic phase, and for temperatures around 900 K, electrical resistivity measurements show a peculiar insulator to metallic transition, which could suggest a modification in the spin state of Co or Fe. Using DFT calculations, we address this curious phenomenon in the disordered Nd₂CoFeO₆. To model the complex structural and magnetic state of this DP, we used a supercell with 80 atoms and the quasi-random structure method to distribute the atoms and the magnetic moment of Co/Fe. We demonstrate a direct relation between the local magnetic moment of Co and the band gap of Nd₂CoFeO₆. In the most stable paramagnetic configuration, Co ions have a low spin configuration resulting in Nd₂CoFeO₆ with opened band gap. However, when the average Co magnetic moment increases, the band gap decreases. The modification in the spin state of Co ions spreads the unoccupied Co d orbitals in the band gap, resulting in the insulator to metallic phase transition. The energy difference from the insulator to the metallic configuration is around 70 meV, which agree very well to the experimental results.