High-Tc Superconductivity in Electron-Doped Layer Structured Nitrides

A new series of superconductors based on layer structured nitrides has been developed. The general compositions of the nitrides are MNX (M = Zr, Hf; X = Cl, Br, I). The β-type polymorph consists of MN double layers sandwiched between close-packed halogen layers, which are characterized as semiconductors with a band gap of 3–4 eV. Electrons can be doped to the nitride layers by intercalation of alkali metals between the layers. Upon the intercalation, the compounds become superconductors with the transition temperatures (Tcs) as high as 13 and 25.5 K for β-ZrNCl and β-HfNCl systems, respectively. The Tc of the electron doped β-HfNCl is higher than that observed in any intermetallic compound and suggests that layered nitrides may exhibit Tcs comparable to those observed in layer structured complex copper oxide superconductors. The layer structured nitrides can be variously modified by the amounts of doping, the types of alkali metals, and the interlayer separation, which can be controlled by co-intercalation of organic molecules with alkali metals. This article dicusses topics including the synthesis and structure of the transition metal nitride halides, intercalation, superconductivity, and band structures.