Pressure Effect on Intersite Charge Transfer in A-site-Ordered Double-Perovskite-Structure Oxide

The intersite charge transfer at room temperature was found to occur by applying pressure in the A-site-ordered double-perovskite-structure oxide LaCu 3 Fe 4 O 12 . The pressure-induced charge transfer between the A-site Cu and B-site Fe causes a first-order-type transition from low-pressure LaCu 3+ 3 Fe 3+ 4 O 12 to high-pressure LaCu 2+ 3 Fe 3.75+ 4 O 12 . The transition is accompanied by significant reduction of unit-cell volume, by unusual softening in bulk modulus, and by a change from an antiferromagnetic-and-insulating state to a paramagnetic-and-metallic state. The intersite charge-transfer transition in LaCu 3 Fe 4 O 12 is induced by changing pressure at ambient temperature as well as by changing temperature at ambient pressure. Besides, either physical pressure or chemical pressure, which is induced by cation substitution at the A′ site, decreases the intersite charge-transfer transition temperature, and reducing the unit-cell volume stabilizes Cu 2+ at the A site and unusual high-valence Fe 3.75+ at the B site in the A-site-ordered double-perovskite-structure oxides.