Silicon nitride is a mechanically tough and light-weight ceramic material able to withstand high temperatures. It is therefore becoming increasingly attractive for use in specific technical applications. The material exists in two distinct phases under normal conditions, alpha and beta, which are both stable. The alpha phase may only be transformed into the beta phase under specific conditions, which involve a combination of high temperature, high pressure and/or significant inclusion of impurities. The reverse process (beta to alpha) has never been observed. In order to understand this behaviour we propose to model the material based on DFT calculations. In order for the model to be accurate, the basic vibrational motions of atoms (phonons) need to properly included. Inelastic netutron measurements would provide an experimental benchmark to assess the quality of the models.