A Raman scattering study of the solid phases of HCl, DCl, HBr, and DBr

Raman scattering studies were carried out on the ferroelectric and paraelectric phases of HCl, DCl, HBr, and DBr at temperatures from ~72 to ~160 K. Changes in the lattice region spectra at temperatures near Tc confirm the order–disorder nature of the ferroelectric transitions and the importance of anharmonic effects in these materials. The structural transition occurring in HCl near 120 K was observed by internal mode depolarization measurements which change irreversibly; these changes are attributed to macroscopic disorder in the sample below this temperature. Observations of the internal modes of HBr show a thermal hysteresis around an intermediate cubic phase and analogous results obtained for DBr imply the existence of a new phase which is similar to the intermediate phase in HBr.