Abstract
We report on new superconductors Ba${}_{n}$${}_{+2}$Ir${}_{4n}$Ge${}_{12n+4}$ ($n$ $=$ 1, 2) with critical temperatures ${T}_{c}$ $=$ 6.1 and 3.2 K, respectively, along with their crystal structures, electron transport, and specific heat. The compounds are composed of alternating Ba@Ir${}_{8}$Ge${}_{16}$ and Ba@Ir${}_{2}$Ge${}_{16}$ cages, both of which are larger in the $n$ $=$ 1 sample than in the $n$ $=$ 2 sample. The normal-state heat capacity reveals two low-lying vibration modes associated with guest Ba cations, and both characteristic temperatures in Ba${}_{3}$Ir${}_{4}$Ge${}_{16}$ are smaller than those in Ba${}_{4}$Ir${}_{8}$Ge${}_{28}$. Meanwhile, the density functional theory calculations reveal that the Ge-4$p$ bands dominated the Fermi level in both samples. We propose that the softening of localized phonons due to expansion of the cage strengthens the electron-phonon coupling between Ba cations and Ge anions, leading to the higher ${T}_{c}$ in Ba${}_{3}$Ir${}_{4}$Ge${}_{16}$.
Published Version
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