Structural changes of (NH4)2SO4 crystals

X-ray, DTA, and TMA are used to study the temperature dependence of structural channges of (NH4)2SO4. The measurements are made from room temperature down to 5 K. The lattice parameters and unit cell volume as a function of temperature, the DTA and MTA show a first order transition from the paraelectric phase to the ferroelectric phase at 233 K. The phase transformation is neither of order-disorder type nor ferrielectric type, and involves a symmetry change from the orthorhombic space group Pnam-D in the paraelectric phase to the orthorhombic space group Pna21-C in the ferroelectric phase where all atoms are in general positions. The unit cell contains four formula units consisting of two nonequivalent kinds of ammonium ions, namely, NH(I) and NH(II), and one kind of sulphate ion in both the phases. The tetrahedral molecules of NH and SO are distorted from regular tetrahedra, being accompanied with permanent dipoles.