Structural origin for the change of the order of ferroelectric phase transition in triglycine sulfate/selenate systems

Crystal structures of triglycine selenate (TGSe) and triglycine sulfate (TGS) obtained from single crystal neutron diffraction are compared. The double well single cell local potential experienced by the non-planar amino group of one of the three glycine ions (GI) of these two isostructural crystals is obtained using their crystal structure. It is suggested that the change in the nature of the ferroelectric phase transition as one goes from TGS to TGSe is due to the increase in the zero point energy resulting due to the change in the shape and height of the double well local potential of these crystals. Substitution of a selenate ion (SeO(4)(2-)) in TGSe by a sulfate ion (SO(4)(2-)) is considered as a source of an effective chemical pressure that can be utilized to tune the ferroelectric phase boundary in these crystals. The influence of alanine substitution on the ferroelectric phase transition in these crystals is investigated using differential scanning calorimetry.