Abstract
Optical measurements were carried out by infrared spectroscopy on AA′3B4O12 A-site ordered quadruple perovskite EuCu3Fe4O12 (microscopic sample) as function of temperature. At 240 K (=TMI), EuCu3Fe4O12 undergoes a very abrupt metal to insulator transition, a paramagnetic to antiferromagnetic transition and an isostructural transformation with an abrupt large volume expansion. Above TMI, optical conductivity reveals a bad metal behavior and below TMI, 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 Cu3dxy, Fe3d and O2p orbitals as principal actors of the spectral weight transfer. The present work constitutes a first step to shed light on EuCu3Fe4O12 electronic properties with optical measurements and ab-initio calculations.
Highlights
Among A-site ordered quadruple perovskites, lanthanides composed LnCu3Fe4O12 present a strong interest insofar as they undergo rare valence state transitions accompanied by intriguing functional properties
We report on low-energy electrodynamics of ECFO investigated by infrared spectroscopy
We found that ECFO undergoes an abrupt metal to insulator transition (MIT) at 240 K associated with a change of optical properties over an energy scale larger than 1 eV
Summary
Among A-site ordered quadruple perovskites, lanthanides composed LnCu3Fe4O12 (noted LnCFO in the following) present a strong interest insofar as they undergo rare valence state transitions accompanied by intriguing functional properties. Large size rare earth ions family (Ln =La, Pr, Nd, Pm, Sm, Eu, Gd and Tb) undergoes the following intermetallic charge transfer 3Cu2+ + 4Fe3.75+ → 3Cu3+ + 4Fe3 + leading to a paramagnetic-metal to an AFM-insulator phase transition between 240 K and 350 K9. These properties result from the transition between the high valence state of Cu3+ at the square-coordinated A sites and the Fe3.75+ at the octahedron-coordinated B sites. Both Fe3d and Cu3d bands are strongly modified at the transition, in relation with the charge transfer and the magnetic transition
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