Abstract
Dielectric constant of the (001)-plate of (NH4)3H(SO4)2 was measured in a temperature range from room temperature to that of liquid helium. A λ-type anomaly was found at about 46 K and 78 K on cooling and heating, respectively. Measurements on partially-deuterated crystals show that the anomaly corresponds to a phase transition from non-ferroelectric V phase to a ferroelectric VII phase.