The role of the double-well potential seen by the amino group in the ferroelectric phase transition in triglycine sulfate

The two most important molecular movements which bring about the order-disorder ferroelectric phase transition in the hydrogen-bonded ferroelectric triglycine sulfate (TGS) are the swinging of the amino group (-NH3+) of one of its three glycine ions, namely GI, and the tunnelling of hydrogen in the hydrogen bond between its other two glycine ions, GII and GIII (GII-H-GIII). The potential function for bent hydrogen bonds is used along with the structural parameters of the TGS crystal to model the double-well potential (U) seen by the amino group (-NH3+) of GI in TGS. The ferroelectric phase transition in TGS is investigated from the point of view of the double-well instability. Results obtained are in good agreement with those obtained earlier using the Ising-type theoretical model. Correlation between the two crucial molecular movements in TGS, namely swinging of the -NH3+ group of GI and tunnelling of hydrogen in the hydrogen bond GII-H-GIII of TGS, is established.