Soft acoustic modes inNaN3

Brillouin scattering measurements in the high-temperature phase of ${\mathrm{NaN}}_3$ are reported. A five-pass Fabry-Perot interferometer was used to analyze the scattered radiation. Soft modes whose velocity extrapolated to zero at $T_C-2\mathrm{}°\mathrm{C}$ were observed. Here $T_C$ is trigonal to monoclinic transition temperature. No appreciable line broadening in the Brillouin spectra nor changes in intensity of the central scattering peak were observed. The measurements provide the first data on these soft modes. They indicate that the transition is almost a prototype displacive rather than an order-disorder transition.