The dynamical properties of β-${\mathrm{Si}}_3$${\mathrm{N}}_4$ (${\mathit{C}}_{6\mathit{h}}^2$) have been investigated within the framework of a simple phenomenological model, based on a short-range pair potential. In developing the model, we have discussed some structural peculiarities of the lattice, which argue for the possibility of its pressure-induced destabilization, caused by interatomic bond tensions arising as a response to external influence. Thus, the nature of the instability can be attributed to a purely mechanical effect whose description does not require any ad hoc model parameters. The simulation of the hydrostatically compressed lattice has demonstrated the vanishing of a low-frequency mode inside the Brillouin zone (k=0,0,0.36) at a pressure of 72 kbar, thus predicting a second-order phase transition to a structure with incommensurate modulation along the z direction. Since the destabilization of β-${\mathrm{Si}}_3$${\mathrm{N}}_4$ has never been observed, in contrast to the α (${\mathit{C}}_{3\mathit{v}}^4$)→β (${\mathit{C}}_{6\mathit{h}}^2$) conversion, this theoretical result has been considered in detail, including aspects of its reliability and of the possibility of the β-α structural transformation proceeding via an intermediate incommensurate phase.

• Received 10 May 1993

DOI:https://doi.org/10.1103/PhysRevB.48.13326