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, NH + 3 and SeO - 3 , split discontinuously at the transition temperature ( T c ). The analysis of NH + 3 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 NH + 4 ion in (NH 4 ) 2 SO 4 undergoes an order-disorder type of transition. Two different species of NH + 3 which were respectively located at the non-equivalent sites of NH + 4 showed very different temperature dependences below T c . Furthermore the analysis of SeO - 3 spectra indicated that SeO 2- 4 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.
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Shibata et al. (1976) studied this question.
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