High-pressure synthesis and unusual metallic conductivity of the A-site-ordered perovskite CaCu$_{3}$Ir$_{4}$O$_{12}$

Recently, much attention has been paid to the A-site-order perovskites AA'$_{3}$B$_{4}$O$_{12}$ due to the observation of a large variety of intriguing physical properties. We have prepared an A-site-ordered perovskites CaCu$_{3}$Ir$_{4}$O$_{12}$ under 9 GPa and 1250\r{ }C with a Walker-type Multianvil module. Rietveld refinements to the room-temperature x-ray diffraction pattern confirmed the cubic structure with lattice parameter a = 7.47380(6) {\AA} in the space group Im-3. Magnetic and transport properties measurements show that CaCu$_{3}$Ir$_{4}$O$_{12}$ is a paramagnetic metal with unusual temperature dependence. The inverse magnetic susceptibility $\chi ^{-1}$(T) above 80 K follows the Curie-Weiss law with $\mu _{eff}$ = 4.23 $\mu _{B}$/f.u. and $\theta _{W}$ = --233 K, while an unusual should-like feature is observed below 80 K. At the same temperature, the resistivity $\rho $(T) deviates from a T-linear behavior and exhibits a strong downward temperature dependence down to 4 K.