Superconductivity in the α-Form Layer Structured Metal Nitride Halide

Layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) have two polymorphs, including α- and β-forms, which have the FeOCl and SmSI structures, respectively. These compounds are band insulators and become metals and show superconductivity after electron doping by intercalating alkali metals between the layers. The superconductivity of β-form had been extensively characterized from decades ago, but it is not easy to consistently interpret all experimental results using conventional phonon-mediated Bardeen–Cooper–Schriefer mechanisms. The titanium compound TiNCl crystallizes only in the α-form structure. TiNCl also exhibits superconductivity as high as ~16 K after electron doping by intercalating metals and/or organic basis. It is important to compare the superconductivity of different M–N networks. However, α-form compounds are vulnerable to moisture, unlike β-form ones. The intercalation compounds are even more sensitive to humid air. Thus, there are few experimental studies on the superconducting mechanism of α-form, although it has been discussed for exotic Cooper-pairing mechanisms. This short review gathers the recent progress in experimental studies of TiNCl.