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.