Intercalation and superconductivity in ternary layer structured metal nitride halides (MNX: M = Ti, Zr, Hf; X = Cl, Br, I)

The ternary metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) contain two types of layer structured polymorphs having different types of two-dimensional metal nitride networks. Both are band insulators, and changed into superconductors with moderately high transition temperatures T(c)s up to 25.5 K upon electron-doping by means of intercalation through the interlayer space. The structural characteristics, electron doping by intercalation, and the unconventional superconductivity are reviewed.