Rattling in the Quadruple Perovskite CuCu 3 V 4 O 12

Of particular interest is a peculiar motion of guest atoms or ions confined to nanospace in cage compounds, called rattling. While rattling provides unexplored physical properties through the guest–host interactions, it has only been observed in a very limited class of materials. Herein, we introduce an A‐site‐ordered quadruple perovskite, CuCu 3 V 4 O 12 , as a new family of cage compounds. This novel AA′ 3 B 4 O 12 ‐type perovskite has been obtained by a high‐pressure synthesis technique and structurally characterized to have cubic Im $\bar 3$ symmetry with an ionic model of Cu 2+ Cu 2+ 3 V 4+ 4 O 12 . The thermal displacement parameter of the A‐site Cu 2+ ion is as large as U iso ≈0.045 Å 2 at 300 K, indicating its large‐amplitude thermal oscillations in the oversized icosahedral cages. Remarkably, the presence of localized phonon modes associated with rattling of the A‐site Cu 2+ ion manifests itself in the low‐temperature specific heat data. This work sheds new light on the structure–property relations in perovskites.