Crystal Structure of the Host Lattices of the Superconductors LixMNX (M = Zr, Hf; X = Cl, Br)

The crystal structure of the host lattice for the new family of layered superconductors LixHfNCl, LixZrNCl, and LixZrNBr is derived from the ZrCl polytype structure. As in the superconducting halide carbides Ln2C2X2 (Ln = La, Y, Lu; X = Br, I), the new structures can be considered as a result of the intercalation of nonmetal atoms (N3- in the nitrides, C24- in the carbides) in the interstices between the metal layers of the ZrCl structure. This suggests the existence of a new class of layered covalent superconductors with critical temperatures higher than for Nb3Ge that opens novel perspectives in the search for new non-oxide superconducting materials.