Chemistry and superconductivity of intercalated metal nitride halides

Titanium, zirconium and hafnium nitride halides show layered structures based on double slabs with compositions [X–M–N–N–M–X] (X = Cl, Br or I; M = Ti, Zr or Hf) that are separated by van der Waals gaps. Intercalation of electron donor species between the double layers induces superconductivity with critical temperatures up to 25.5 K. This review discusses the synthesis and structural chemistry of the hosts and intercalation compounds in this group of materials, as well as the influence of the structural and chemical features of the intercalates on the superconducting properties. (paper)