The ordered vacancy Pm3m (''NbO'') structure was identified by Treece
et al. as the phase of their pulsed laser deposited thin film samples
of niobium nitride. However, the stoichiometric Pm3m phase of NbN is n
ot easily reconciled with published local density functional results,
which predict a 5% smaller lattice constant than measured and a high e
nergy compared with the competing rocksalt phase (which is common and
therefore expected to be the stable phase). Here we investigate a poss
ible tetragonal instability of the Pm3m structure that might complicat
e the interpretation of xray data on the thin films. We also consider
the consequences of incorporation of hydrogen into the films. Neither
of these aspects is able to resolve the apparent discrepancy. Copyrigh
t (C) 1996 Published by Elsevier Science Ltd