Preparation and superconductivity of chlorine-deintercalated crystals β-MNCl1-x (M = Zr, Hf)

The layer structured crystals beta-MNCl (M = Zr, Hf) consist of rhombohedrally stacked six-layer [Cl-M-N-N-M-Cl] slabs, each of which is composed of a strongly bonded honeycomb-like double MN layer sandwiched by two close-packed chlorine layers. A part of the chlorine atoms was deintercalated by the reaction with alkali metals (A) formed by the thermal decomposition of the azides AN(3) (A = Na, K, Rb). Upon the partial deintercalation of chlorine atoms, electrons were doped into the MN layers, and the semiconducting P-MNCl changed into superconductors with the transition temperatures (Tc's) of 13-14 and 23-24 K for M = Zr and Hf, respectively. The thermal stability and the structure of the deintercalated phase beta-MNCl1-x are discussed.