STRUCTURAL-CHANGES OF (NH4)2SO4 CRYSTALS

X-ray, DTA, and TMA are used to study the temperature dependence of structural changes 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 to 223 K. The phase transformation is neither of order-disorder type nor ferrielectric type, and involves a symmetry change from the orthorhombic space group Pnam-D2h16 in the paraelectric phase to the orthorhombic space group Pna21-C2v9 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 NH4+(I) and NH4+(II), and one kind of sulphate ion in both the phases. The tetrahedral molecules of NH4+ and SO4= are distorted from regular tetrahedra, being accompanied with permanent dipoles.