Raman and Dielectric Studies of Structurally Modulated Crystals: Thiourea and Tetramethylammonium Tetrachlorozincate

The complex dielectric constant of thiourea has been measured in the temperature range from 80 K to 300 K with different measuring-field levels and frequencies, with and without d.c. biasing fields. The temperature interval between the two peaks in the varepsilon *(T) curve near 177 K was found to extrapolate to 0.7 K at zero measuring field, which indicates that the 1over 8 ferroelectric lock-in phase is intrinsic in a non-zero temperature range. Slowing -down of dielectric relaxation processes was observed at both the upper and lower transition points of this phase. The E-T phase diagram near the 1over 8 lock-in region was surveyed by investigating the effects of biasing fields. D-E hysteresis loops were investigated in the vicinity of the 1over 8 ferroelectric phase with measuring frequencies from 0.5Hz to 5mHz. The different D-E loops observed clearly demonstrate the difference between the intrinsic and "field-induced" lock-in mechanisms. The spontaneous polarization and the coercive field in the 1over 8 phase were measured. The experimental results can be well explained by the phenomenological Landau theory of phase transitions. Low-frequency Raman-scattering experiments on tetramethylammonium tetra-chlorozincate crystals have been performed in the temperature range from 310 K to 80 K, which includes the five successive phase transitions. As the temperature decreased, an 11 cm^{ -1} mode appeared in the (aa) scattering geometry in the incommensurate phase II and lock-in phases III and IV, which have modulation wavevectors {bf q}_0=({2over 5}+delta) {bf a}*, {2over 5}a*, and {1over 3} a*, respectively, disappeared in phase V (q _0 = 0), and reappeared in phase VI (q _0={1over 3}a*). The temperature dependence of the observed frequency shift demonstrates that this mode is the folded Sigma_3 acoustic mode scattering via a second-order process. A weak 15 cm^{-1} mode observed in phase VI can be interpreted as the folded Sigma _4 acoustic mode scattering via a third-order process.