Raman spectrum and phase transition in sodium azide

Abstract

The Raman spectra of both the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phases of Na${\mathrm{N}}_{3}$ have been obtained and interpreted. The Raman-active ${E}_{g}$ mode of the $\ensuremath{\beta}$ phase splits into ${A}_{g}$ and ${B}_{g}$ modes as Na${\mathrm{N}}_{3}$ undergoes a phase transition at approximately 20 \ifmmode^\circ\else\textdegree\fi{}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 \ifmmode^\circ\else\textdegree\fi{}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 \ifmmode^\circ\else\textdegree\fi{}K. The observed data are used to support an order-disorder transition model with a slightly disordered $\ensuremath{\beta}$ 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.