On the energy scale involved in the metal to insulator transition of quadruple perovskite EuCu 3 Fe 4 O 12: Infrared spectroscopy and ab-initio calculations

Optical measurements were carried out by infrared spectroscopy on AA′ 3 B 4 O 12 A-site ordered quadruple perovskite EuCu 3 Fe 4 O 12 (microscopic sample) as function of temperature. At 240 K (=T MI), EuCu 3 Fe 4 O 12 undergoes a very abrupt metal to insulator transition, a paramagnetic to antiferromagnetic transition and an isostructural transformation with an abrupt large volume expansion. Above T MI, optical conductivity reveals a bad metal behavior and below T MI, an insulating phase with an optical gap of 125 meV is observed. As temperature is decreased, a large and abrupt spectral weight transfer toward an energy scale larger than 1 eV is detected. Concurrently, electronic structure calculations for both high and low temperature phases were compared to the optical conductivity results giving a precise pattern of the transition. Density of states and computed optical conductivity analysis identified Cu 3dxy, Fe 3d and O 2p orbitals as principal actors of the spectral weight transfer. The present work constitutes a first step to shed light on EuCu 3 Fe 4 O 12 electronic properties with optical measurements and ab-initio calculations.