Structural changes of (NH4)2SO4 crystals

Abstract

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

Soft-mode driven polarity reversal in ferroelectrics mapped by ultrafast x-ray diffraction

“…Integrating over all pixels with identical scattering angle 2 θ yields 1D powder diffraction patterns as shown in Fig. 2(b) for an unexcited ferroelectric AS sample at T = 200 K. The result is in good agreement with literature and allows for an assignment of the 15 Bragg peaks to sets of lattice planes 22 …”

“…1(a)]. 22 The dimensions of the ferroelectric unit cell are a  = 0.78566(3) nm, b  = 1.05813(4) nm, and c  = 0.59530(2) nm. The experimentally observed ferroelectric polarization has a value of P fe (T = 222 K) = 6 mC m −2 slightly below the ferro- to paraelectric phase transition temperature T C  = 223 K 23,24 .…”

“…The sample temperature was set using a cooled nitrogen gas flow (Oxford Instruments CryojetHT). At the beginning of a measurement series, we slowly cooled down the sample under optical excitation and repeatedly recorded the diffraction pattern at different temperatures until the phase transition of the entire sample (at T C  = 223 K) was indicated by sudden changes of the angular position of selected Bragg reflections 22 . The sample was then cooled down by another 20 K to achieve a uniform sample temperature of 200 ± 5 K under the experimental conditions.…”

Solid-state 33S MAS NMR of inorganic sulfates

“…al. [43] showed that the lattice parameters and unit cell volume of (NH 4 ) 2 SO 4 change as a function of temperature and that it undergoes a transition from a paraelectric phase to a ferroelectric phase at 223 K. Thus, it was necessary to repeat the X-ray crystallography measurements under the same conditions as the NMR experiments. Columns 2 and 3 of Table 4 contain the S-O bond distances and bond angles of (NH 4 ) 2 SO 4 obtained at 298 and 100 K. It shows that the D(S-O) m at 100 K is 0.013 Å resulting in a calculated C q of 0.84 MHz (see Table 3).…”

Photoinduced structural dynamics of polar solids studied by femtosecond X-ray diffraction

“…At T = 300 K AS crystallizes in an orthorhombic structure belonging to the space group Pnam, with four (NH 4 ) 2 SO 4 entities per unit cell ( Fig. 6a; Schlemper & Hamilton, 1966;Ahmed et al, 1987). The dimensions of the unit cell are a = 7.78, b =10.64 and c = 5.99 Å and the material is paraelectric.…”