Temperature and pressure induced Cu-Fe intermetallic charge transfer in LaCu$_{3}$Fe$_{4}$O$_{12}$

An A-site ordered double perovskite LaCu$_{3}$Fe$_{4}$O$_{12}$ was prepared at high pressure and high temperature. At ambient condition, the charge combination was proved to be LaCu$^{3+}_{3}$Fe$^{3+}_{4}$O$_{12}$. When the temperature was increased to 393 K, however, an intermetallic charge transfer was found to occur between the A-site Cu and the B-site Fe ions, giving rising to the change of charge combination from the low-temperature LaCu$^{3+}_{3}$Fe$^{3+}_{4}$O$_{12}$ to the high-temperature LaCu$^{2+}_{3}$Fe$^{3.75+}_{4}$O$_{12}$.[1,2] Similarly, at room temperature, if high pressure was applied, the Cu-Fe intermetallic charge transfer could also be induced. This charge transfer led to a first-order isostructural phase transition with sharp reduction in unit cell, negative thermal expansion and unusual softening (decreased bulk modulus). Moreover, the material experienced antiferromagnetism-to-paramagnetism and insulator-to-metal transitions accompanying with the charge-transfer transition. \\[4pt] [1] Y. W. Long \textit{et al}, Nature, 458, 60-63 (2009). \\[0pt] [2] Y. W. Long and Y. Shimakawa, New J. Phys. 12, 063029 (2010).