Intermetallic Charge Transfer in A-Site-Ordered Double Perovskite BiCu₃Fe₄O₁₂

An A-site-ordered double perovskite BiCu(3)Fe(4)O(12) was synthesized at high-pressure and high-temperature conditions. Similar to the isostructural LaCu(3)Fe(4)O(12), the temperature-induced intermetallic charge transfer between the A'-site Cu and the B-site Fe ions occurs but at higher temperature (428 K) than LaCu(3)Fe(4)O(12) (393 K) does. This charge transfer causes an isostructural phase transition with volume contraction by 0.6% as well as semiconductor-to-metal and antiferromagnetism-to-paramagnetism transitions. Although the Bi cation at the A site does not take part in the charge transfer, it appears to enhance the charge-transfer temperature by stabilizing the square-planar coordinated Cu(3+).