ESR Study of NH₃⁺ and SeO₃⁻ Radicals in (NH₄)₂SO₄

Abstract

Ferroelectric phase transition of ammonium sulfate containing a small amount of ammonium selenate was studied by means of ESR. All the lines of the radicals, NH₃⁺ and SeO₃⁻, split discontinuously at the transition temperature (T_c). The analysis of NH₃⁺ spectra showed that the radical rapidly jumped among two equivalent configurations above T_c, while it mainly stayed one of them below T_c. This fact indicates that the NH₄⁺ ion in (NH₄)₂SO₄ undergoes an order-disorder type of transition. Two different species of NH₃⁺ which were respectively located at the non-equivalent sites of NH₄⁺ showed very different temperature dependences below T_c. Furthermore the analysis of SeO₃⁻ spectra indicated that SeO₄²⁻ tetrahedron deformed below T_c. These facts support the proposal by Unruh that there are two or more kinds of dipoles which have different temperature dependence.