Esr study of (SO4)2? dynamics in relation to the ferroelectric phase transition in ammonium sulfate

Ferroelectric phase transition in ammonium sulfate has been studied by ESR of CrO₄^3? radical substituting for SO₄^2? ion in (NH₄)₂SO₄. In addition to discontinuous changes at T_c, certain continuous changes are observed in ESR parameters of this probe below T_c, which reflect the role of the sulfate ion in the phase transition. A microscopic mechanism of the phase transition is proposed and discussed in terms of the change of orientation of the sulfate tetrahedron through a finite angle. The degree of the change of orientation below T_c is thought to be the possible order parameter of the phase transition.