X-ray, DTA, and TMA are used to study the temperature dependence of structural changes of (NH4)2S04. 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 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-Dii in the paraelectric phase to the orthorhombic space group Pna2,-C,PV 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. Rontgenbeugung, DTA und TMA werden benutzt, um die Temperaturabhangigkeit von Strukturanderungen in (NH,),SO, zu untersuchen. Die Messungen werden zwischen Zimmertemperatur und 5 K durchgefuhrt. Die Gitterparameter und das Volumen der Einheitszelle in Abhangigkeit von der Temperatur, DTA und MTA zeigen einen Ubergang erster Ordnung von der paraelektrischen Phase zur ferroelektrischen Phase bei 223 K. Der Phasenubergang ist weder vom Typ Ordnung-Pehlordnung noch vom ferrielektrischen Typ und schliel3t eine Symmetrieanderung von der rhombischen Raumgruppe Pnam-Dii in der paraelektrischen Phase zur rhombischen Raumgruppe Pna2,-C& in der ferroelektrischen Phase, in der alle Atome sich in allgemeinen Positionen befinden. Die Einheitszelle enthalt vier Formeleinheiten und besteht aus zwei nichtaquivalenten Arten von Ammoniumionen, NHI(I) und NH:(II) und einer Art von Sulfation in beiden Phasen. Die Tetraedermolekiile von NH: und sind gegeniiber regularen Tetraedern verzerrt und r o n permaiiricnten Dipoleri begleitet. IiitruductioiiThe dielectric anirnoniurii sulphate (NH,),SO, has been studied siiice 1956 when Matthias and Iterneika [l] discovered that i t becomes ferroelectric below 225 K.The crystal structure of paraelectric and ferroelectric phases was extensively studied \)y Schleinper arid Hamilton [Z], by Hsebe 131, and by O'Reilly and 'hang 141. Several models were proposed for the rnechariism of the phase trarisition in this structure; Schlemper and Hamilton [ 2 ] pointed out that the transition is not of the order-disorder type but rather involves a change in hydrogen bonds in the ferroelectric phase, on the other hand, O'Reilly and Tsang [4] stated that the transition is of the order-disorder

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((NH4)1–xKx)2SO4 powdered crystals with x = 0.0, 0.1, 0.15, 0.25, 0.32, 0.6, 0.7, and 1.0 are studied by X‐ray diffraction from room temperature down to 73 K. It is found that the mixed crystals are isomorphous to (NH4)2SO4 and K2SO4. The phase transition temperature decreases with the increase of the substituted ions (K+). The phase transition completely disappeares when x > 0.6. Lattice parameters as a function of composition are presented, as well as their variation with temperature for each composition. A suggested phase diagram for ((NH4)1–xKx)2SO4 mixed crystal systems is presented.

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