Raman study of theα→βstructural phase transition of solidN2

Raman spectra of solid ${\mathrm{N}}_2$ have been obtained at small temperature intervals in the range 4 to 40 K. Relative intensities of both the low-frequency lattice modes and the higher-frequency vibrational stretching modes have been measured with a spectral resolution of approximately 2 ${\mathrm{cm}}^{\mathrm{-}1}$. In addition to changes associated with the structural transition between a low-temperature ordered cubic (α phase) and a high-temperature hcp lattice (β phase) at $T_c$=35.6 K, we report a dramatic evolution of the Raman intensity with temperature within the α phase, and present evidence for a partial relaxation of the orientational ordering approximately 10 K below $T_c$. The results support the existence of a ‘‘plastic phase’’ within the α phase, near the α-β phase boundary.