LATTICE-DYNAMICS AND PREDICTION OF PRESSURE-INDUCED INCOMMENSURATE INSTABILITY OF A BETA-SI3N4 LATTICE WITHIN A SIMPLE MECHANICAL MODEL

The dynamical properties of beta-Si3N4 (C6h2) 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 beta-Si3N4 has never been observed, in contrast to the alpha (C3v4) double-line arrow pointing right beta (C6h2) conversion, this theoretical result has been considered in detail, including aspects of its reliability and of the possibility of the beta-alpha structural transformation proceeding via an intermediate incommensurate phase.