Intermetallic charge transfer between A-site Cu and B-site Fe in A-site-ordered double perovskites

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.