A Microscopic Model for the Ferroelectric Phase Transition in (NH4)2SO4

The behaviours of two kinds of radicals, NH 3 + (α) and NH 3 + (β) in the γ-irradiated ammonium sulfate crystals were examined in more detail. Above T c each radical flipped between two stable configurations around the a - b plane with the frequency higher than 10 8 sec -1 , but below T c it tilted from the a - b plane with a certain angle. One half of the flipping angle was two or three times as large as the tilt angle. Based on the results a microscopic model for the ferroelectric phase transition of ammonium sulfate was considered. In the model it was assumed that at T c the SO 4 2- tetrahedron was newly deformed owing to the correlated movement in the NH 4 + (α)-SO 4 2- -NH 4 + (β) bonding and an abrupt increase of the dipole-dipole interaction energy occured. The small Curie constant of ammonium sulfate can then be explained satisfactorily by the model.