The ferroelectric phase transition and dielectric anomaly in tri-glycine sulfate

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Abstract
The ferroelectric phase transition of tri-glycine sulfate (TGS) at 49°C has been studied by Raman spectroscopy. The spectrum of the external lattice mode region has been followed as a function of temperature. It is found that the modes corresponding to the symmetry A(y), y being the ferroelectric axis, show interesting temperature behavior. The vibrations corresponding to other symmetries have very little temperature dependence. In the X(ZX)Z configuration, nine modes appear at low temperature. At room temperature, only three out of these nine modes are observed. They are shifted from the laser frequency by 63, 75 and 111 cm-1. The line originally at 63 cm-1 undergoes a sharp increase in frequency and merges into the mode at 75 cm-1 at Tc. This behavior is different from that predicted by the soft mode model of ferroelectric phase transitions.The dielectric constant of TGS has been analyzed on the basis of a Mason-Debye type relaxation formula. From this, the phonon contribution to the dielectric constant is found to be small (about 10%) below Tc and becomes important (about 90%) only after the phase transition is completed. A microscopic model consistent with the x-ray determined structure is proposed wherein the nitrogen atom in one of the glycine groups has two equilibrium positions. The jumping of this nitrogen atom between these two positions results in a dipolar mode which is associated with the dielectric anomaly of TGS at the transition temperature.