Hypothetical phase transition, structural disorder and spontaneous polarization in ammonium sulfate crystals

EPR spectra of SeO3- radicals in ammonium sulfate crystal were studied in the high temperature phase. Two magnetically inequivalent SeO3- radicals exist in the crystal as a consequence of different coupling to two inequivalent ammonium groups: (NH4)(I) and (NH4)(II). It is estimated that the distinction between the two SeO3- radicals would disappear at 585 K, which is interpreted as the hypothetical ferroelastic phase transition. The intensity ratio of the two SeO3- radicals can be changed by appropriate heat treatment of the sample. It is inferred that the intensity ratio is related to the disorder of protons in the asymmetric potential minima in hydrogen bonds. The unusual temperature dependence of spontaneous polarization was studied in crystals grown from solutions with pH in the range 2.0 to 5.0 as well as in crystals after heat treatment.