Ferroelectric phase transition in triglycine selenate: An interpretation based on its structure and its comparison with triglycine sulphate

Triglycine selenate (TGSe) is a hydrogen-bonded ferroelectric, which undergoes a structural phase transition at T-c = 295 K. It is isomorphous to triglycine sulphate (TGS) which is a very well studied order-disorder ferroelectric extensively used in infrared detection. The crucial molecular unit from the point of view of ferroelectric phase transition in these crystals is the -NH3+ group of one of the three glycine ions GI, which has two equivalent positions in an asymmetric unit. This group gets disordered between its two equivalent positions for temperatures above T-c. The potential energy of this group as a function of the distance between its equivalent positions was modelled, and the phase transition in TGSe was interpreted using "the coupled anharmonic oscillator model" of ferroelectrics proposed by Y. Onodera. Similarities as well as differences between the ferroelectric phase transition in TGS and TGSe are discussed.