Superconductivity in Electron-doped Layered Metal Nitrides

New high-transition temperature (high-Tc) superconductors based on layered metal nitrides have been developed. Layer structured crystals s-MNCl (M = Zr, Hf) have the SmSI type structure; the M-N double layers occupy the SmS positions and are sandwiched between the close-packed chlorine layers. The layers with a sequence of [Cl-M-N-N-M-Cl] are stacked with each other by a weak van der Waals interaction. The crystals are characterized as semiconductors with a band gap of larger than 3 eV. The nitride layers can be electron-doped by intercalation of lithium between the chloride layers. On intercalation the layers gave a metallic behavior and became superconductors with Tc as high as 13 and 25.5 K for s-ZrNCl and s-HfNCl, respectively.