Changes of valence states in transition-metal oxides often cause significant changes in their structural and physical properties. 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. 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. 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.

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