Abstract
In this review article, we describe in detail the temperature-induced intermetallic charge transfer between A-site Cu and B-site Fe ions in the A-site-ordered double perovskites RCu3Fe4O12 (R=La, Bi). In these compounds, a very rare Cu3+ valence state at the square-planar-coordinated A sites was stabilized by high-pressure synthesis. By increasing the temperature, a Cu–Fe intermetallic charge transfer producing a high Fe3.75+ valence state occurred. This charge transfer gave rise to a first-order isostructural phase transition with unusual volume contraction, as well as to antiferromagnetism-to-paramagnetism and insulator-to-metal transitions. The substitution of Bi for La stabilized the low-temperature phase containing Cu3+ and increased the charge transfer transition temperature from 393 K for LaCu3Fe4O12 to 428 K for BiCu3Fe4O12.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. An A-site-ordered double perovskite with a general chemical formula A'A3B4O12 contains transition metal ions at both the A and B sites. As a result, A–B and A–A interactions emerge in addition to the conventional B–B interaction seen in simple ABO3 perovskite, leading to a wide variety of intriguing physical properties.
Main results. We found an unusual A–B site interaction, i.e., Cu–Fe intermetallic charge transfer, in A-site-ordered double perovskites (La/Bi)Cu3Fe4O12. A very rare Cu3+ ground state was stabilized by high pressure synthesis, and the charge combination of the compounds was (La/Bi)Cu3+3e3+4O12. With increasing temperature, a first-order charge-transfer transition occurred between the A-site Cu and the B-site Fe, and the charge formation changed to (La/Bi)Cu2+3Fe3.75+4 O12. This charge transfer gave rise to antiferromagnetism-to-paramagnetism and insulator-to-metal transitions, accompanied by an isostructural phase transition with unusual volume contraction.
Wider implications. The present A-site-ordered perovskites showed a new interesting phenomenon in correlated electronic systems, where charge, spin and lattice degrees of freedom are strongly coupled. The unusual high valence states in Fe and Cu and the intermetallic charge transfer between these ions also revealed the importance of ligand holes.
Figure. Crystal structure of A-site-ordered perovskite LaCu3Fe4O12. Cu (A site)–Fe (B site) intermetallic charge transfer takes place at 393 K, resulting in the charge combination change from low-temperature LaCu3+3Fe3+4 O12 to high-temperature LaCu2+3Fe3.75+4 O12.