Metastability of fcc-related Si-N phases

The phenomenon of superhardening in TiN/SiN(x) nanocomposites and the prediction of extreme hardness in bulk gamma-Si(3)N(4) have attracted a large interest to this material system. Attempts to explain the experimental findings by means of first-principles calculations have so far been limited to static calculations. The dynamical stability of suggested structures of the SiN(x) tissue phase critical for the understanding of the nanocomposites is thus unknown. Here, we present a theoretical study of the phonon-dispersion relations of B1 and B3 SiN. We show that both phases previously considered as metastable are dynamically unstable. Instead, two pseudo-B3 Si(3)N(4) phases derived from a L1(2)- or D0(22)-type distribution of Si vacancies are dynamically stable and might explain recent experimental findings of epitaxial SiN(x) in TiN/SiN(x) multilayers.