Journal Article

Temperature-induced A–B intersite charge transfer in an A-site-ordered LaCu3Fe4O12 perovskite

Azuma, M., Muranaka, S., Long, Y. W., Shimakawa, Y., Hayashi, N., Saito, T.

2009

Overview

Transition metal takes charge The introduction of 'foreign' elements into transition-metal oxides (called chemical doping) can change the valence state of the metal's cations and hence modify the physical properties of the material as a whole. These changes can be dramatic, for example causing high-temperature superconductivity in copper oxides and colossal magnetoresistance in manganese oxides. Youwen Long et al . have identified an oxide system, the perovskite LaCu 3 Fe 4 O 12 , in which changes in valence state occur when charge is shuffled between different cations (iron and copper) in the host structure, rather than via doping. As a result, the material can be reversibly transformed from one possessing iron in an unusually high Fe 3.75+ state (partnered with fairly common Cu 2+ ions) to one possessing rare Cu 3+ ions. These changes are reflected in the magnetic and electronic properties of the material and, intriguingly, the material contracts slightly on being warmed through the transition. The temperature sensitivity of this effect makes it a strong candidate for technological applications. This paper identifies an oxide system where changes in valence state occur as a result of charge being shuffled between different cations in the host structure, rather than via doping, this charge transfer being sensitive to temperature. As a result, the material can be reversibly transformed from one possessing iron in an unusually high Fe3.75+ state to one possessing rare Cu3+ ions. These changes are reflected in the magnetic and electronic properties of the material and, intriguingly, are accompanied by negative thermal expansion. Changes of valence states in transition-metal oxides often cause significant changes in their structural and physical properties 1 , 2 . Chemical doping is the conventional way of modulating these valence states. In ABO 3 perovskite and/or perovskite-like oxides, chemical doping at the A site can introduce holes or electrons at the B site, giving rise to exotic physical properties like high-transition-temperature superconductivity and colossal magnetoresistance 3 , 4 . When valence-variable transition metals at two different atomic sites are involved simultaneously, we expect to be able to induce charge transfer—and, hence, valence changes—by using a small external stimulus rather than by introducing a doping element. Materials showing this type of charge transfer are very rare, however, and such externally induced valence changes have been observed only under extreme conditions like high pressure 5 , 6 . Here we report unusual temperature-induced valence changes at the A and B sites in the A-site-ordered double perovskite LaCu 3 Fe 4 O 12 ; the underlying intersite charge transfer is accompanied by considerable changes in the material’s structural, magnetic and transport properties. When cooled, the compound shows a first-order, reversible transition at 393 K from LaCu 2+ 3 Fe 3.75+ 4 O 12 with Fe 3.75+ ions at the B site to LaCu 3+ 3 Fe 3+ 4 O 12 with rare Cu 3+ ions at the A site. Intersite charge transfer between the A-site Cu and B-site Fe ions leads to paramagnetism-to-antiferromagnetism and metal-to-insulator isostructural phase transitions. What is more interesting in relation to technological applications is that this above-room-temperature transition is associated with a large negative thermal expansion.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: Condensed matter: electronic structure, electrical, magnetic, and optical properties / Electron states / Exact sciences and technology / Humanities and Social Sciences / letter / Magnetic phase boundaries (including magnetic transitions, metamagnetism, etc.) / Magnetic properties and materials / Magnetic resonances and relaxations in condensed matter, mössbauer effect / Magnetically ordered materials: other intrinsic properties / Metal-insulator transitions and other electronic transitions / multidisciplinary / Mössbauer effect; other γ-ray spectroscopy / Physics / Science / Science (multidisciplinary)