ZrN in the NaCl structure is a superconductor with a Tc of 10 K. With use of an ion-beam process, nitride with higher nitrogen composition than NaCl can be deposited, which is found to be an insulator. A similar situation is found for the Hf but not for the Ti nitride. For this insulator an ordered defect structure Zr3N4 is suggested. An octet rule could make this compound an insulator. Augmented-spherical-wave (ASW) band-structure calculations are performed and yield a significant lowering of the Fermi energy with respect to ZrN (NaCl), but a small overlap between the N p and Zr d bands remains. Although these calculations, which are based on the local-density approximation, yield conduction and valence bands that overlap slightly, the fundamental origin of the insulating behavior appears to be quite clear. If Ne is implanted in the void, the insulator NeZr3N4 has a lattice spacing which perfectly matches that of the superconductor ZrN (NaCl) as predicted by ASW calculations and confirmed experimentally. A Josephson junction built with these isochemical and isostructural materials should have a perfect match at the interface. © 1985 The American Physical Society.