Mechanism of A-B intersite charge transfer and negative thermal expansion in A-site-ordered perovskite LaCu ₃Fe ₄O ₁₂

Temperature induced intermetallic charge transfer and negative thermal expansion in compounds hold promise for many applications. Here, we report, by the first-principles calculations, the mechanism behind these effects and the associated electrical and magnetic properties of an A-site-ordered perovskite LaCu ₃Fe ₄O ₁₂. We find that the sensitive expansion of Cu-O bonds to temperature can trigger a transformation from Cu ³⁺ to Cu ²⁺, which imposes a covalent state transition of B-site Fe from 3 to 3.75. The resultant shrinkage of the Fe-O bonds is demonstrated to play a pivotal role in the volume contraction of the oxide at high temperatures.