Preparation and electronic states of Na16Ba8Si136 clathrate

A Si136 compound with an endohedral accommodation of Ba is newly made and its electronic states are compared to those of pure silicon-network superconducting Ba8Si46. The resulting Na16Ba8Si136 has indeed a small density of states at the Fermi level and does not superconduct down to 2 K. Rietveld analyses show that Ba is accommodated in a Si28 cage with 0.98 occupancy and Na in Si20 with 0.61. The disappearance of superconductivity could be explained in terms of the little hybridization of Ba 5d/6s with Si28 polyhedral cluster orbitals, leading to a very low density of states at the Fermi level.