Low-frequency Raman-scattering experiments on tetramethylammonium tetrachlorozincate crystals have been performed in the temperature range from 310 to 80 K, which includes the five successive phase transitions. As the temperature decreased, an 11-${\mathrm{cm}}^{\mathrm{-}1}$ mode appeared in the (aa) scattering geometry in the incommensurate phase II and lockin phases III and IV, which have modulation wave vectors ${\mathbf{q}}_0$=(2/5+δ)${\mathbf{a}}^{\mathrm{*}}$, 2/5${\mathbf{a}}^{\mathrm{*}}$, and 1/3${\mathbf{a}}^{\mathrm{*}}$ respectively, disappeared in phase V (${\mathbf{q}}_0$=0), and reappeared in phase VI (${\mathbf{q}}_0$=1/3${\mathbf{a}}^{\mathrm{*}}$). The temperature dependence of the observed frequency shift demonstrates that this mode is the folded ${\mathrm{\Sigma }}_3$ acoustic mode scattering via a second-order process. A weak 15-${\mathrm{cm}}^{\mathrm{-}1}$ mode observed in phase VI can be interpreted as the folded ${\mathrm{\Sigma }}_4$ acoustic mode scattering via a third-order process.

• Received 10 May 1990

DOI:https://doi.org/10.1103/PhysRevB.42.8465