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

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 Cu 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 Fe valence state occurred. This charge transfer gave rise to a first-order isostructural phase transition with unusual volume contraction, as well as to antiferromagnetismto-paramagnetism and insulator-to-metal transitions. The substitution of Bi for La stabilized the low-temperature phase containing Cu and increased the charge transfer transition temperature from 393K for LaCu3Fe4O12 to 428K for BiCu3Fe4O12. 1 Present address: Multiferroics Project, ERATO, JST, c/o Cross-Correlation Materials Research Group (CMRG), ASI, RIKEN, Wako, Saitama 351-0198, Japan. 2 Authors to whom any correspondence should be addressed. New Journal of Physics 12 (2010) 063029 1367-2630/10/063029+17$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft