Hydrogen ammonium selenate (AHSe) crystals are grown having in the room temperature 12 symmetry with lattice constants: a = 4.601, b = 7.547, and c = 19.258 Å; α = γ = 90°C and β = 90.90°. Measurements of electric permittivity yield a transition in 251.3 K from the para‐ to ferroelectric phase. The temperature anomaly is observed in the direction of the a‐axis only. The temperature dependence of permittivity fulfills the Curie‐Weiss law in a region of a few Kelvins before phase transition. The character of a spontaneous polarization and the observation of double hysteresis loops over the temperature of phase transition testify that this transioton is of first order. Copyright © 1979 WILEY‐VCH Verlag GmbH & Co. KGaA
