Phase transition of ammonium sulfate

The dielectric constant of ammonium sulfate along the a-, b-, and c-axis is measured; a small anomaly in the dielectric constant is found at 6 to 12 K below the well-known ferroelectric transition at −49.5 °C. It is proposed that this new anomaly is due to the appearance of a spontaneous polarization which is produced by the secondary order parameter and is in an anti-parallel direction to the spontaneous polarization due to the primary order parameter which produces the well-known ferroelectric transition. According to this model, ammonium sulfate is ferroelectric just below the transition at −49.5 °C, but it gradually changes into a weak ferrielectric below this new anomaly temperature.