Intersite charge transfer in the $A$-site-ordered LaCu$_{3}$Fe$_{4}$O$_{12}$ perovskite

A novel transition metal oxide LaCu$_{3}$Fe$_{4}$O$_{12}$ was prepared at 10 GPa and 1400 K. It crystallizes an $A$-site-ordered perovskite structure, which has a general chemical formula A$^{\prime}$A$_{3}$B$_{4}$O$_{12}$, with space group \textit{Im}-3. Bond valence sum calculations and M\"{o}ssbauer spectra confirm the change of charge combination at 393 K from a high-temperature La$^{3+}$Cu$^{2+}_{3}$Fe$^{3.75+}_{4}$O$_{12}$ with unusually high oxidation Fe$^{3.75+}$ ions at the $B$ site to a low-temperature La$^{3+}$Cu$^{3+}_{3}$Fe$^{3+}_{4}$O$_{12}$ with exceptional Cu$^{3+}$ ions at the $A$ site. The results strongly suggest an unusual intermetallic charge transfer between the $A$-site Cu and $B$-site Fe ions. The simultaneous valence change (3Cu$^{2+}$-3$e^{-}\to $3Cu$^{3+}$ and 4Fe$^{3.75+}$+3$e^{-}\to $4Fe$^{3+})$ caused by the $A-B$-site charge transfer leads to a first-order and reversible isostructural phase transition accompanied by an anomalous volume contraction as large as 1.0{\%}. Meanwhile, a paramagnetic metal to antiferromagnetic insulator transition is also induced by the $A-B$-site charge transfer.