Studies of the Crystal Structure of Ammonium Sulfate in Connection with Its Ferroelectric Phase Transition

Abstract

The crystal structure of ammonium sulfate is studied at -140, -90, -64 and -53.5°C in the ferroelectric phase, and at -48.5 and -40°C in the paraelectric phase, by the use of the data collected by a four-circle diffractometer. The difference Fourier synthesis at -53.5°C shows a clear indication of the disordering of the ammonium ion I but does not show the disordering of the sulfate ion. The configuration of the sulfate ion below T c can be derived from that above T c by a condensation of one of the librational B 1 u symmetry modes. The order parameters of two kinds of ammonium ions are determined by the use of the reflection data below T c . From the temperature dependence of the displacement of the sulfate ion and order parameters of two kinds of ammonium ions below T c , a pseudospin-phonon coupled model is proposed.