Origin of Metallic Nature of NaN.

Among inorganic clathrates, the inner cavity space rarely affects the electronic structure of the framework. We report that the anti-ReO-type compound NaN has a metallic nature irrespective of the stoichiometric chemical composition of simple representative elements and that this unusual nature originates from the collapse of the bandgap owing to the presence of a crystallographic cavity. We synthesized NaN by the plasma-assisted nitridation of alkali metals, and diffuse reflectance measurements indicated a metallic nature.