Raman spectrum and phase transition in sodium azide

The Raman spectra of both the $\alpha$ and $\beta$ phases of Na${\mathrm{N}}_3$ have been obtained and interpreted. The Raman-active $E_g$ mode of the $\beta$ phase splits into $A_g$ and $B_g$ modes as Na${\mathrm{N}}_3$ undergoes a phase transition at approximately 20 °C at atmospheric pressure. These phonons have been used to study the phase transition as a function of pressure and temperature. The temperature studies at atmospheric pressure were made over a range of 20 to 650 °K. The pressure studies were made for pressures ranging from 0 to 2.9 kbar for nine temperatures covering the range of 258 to 360 °K. The observed data are used to support an order-disorder transition model with a slightly disordered $\beta$ phase. The contribution of the induced lattice strain to the phonon-frequency changes is shown by these data to dominate any contribution due to phonon coupling.