Dielectric and Thermal Study of(NH4)2S

X-ray diffraction, thermal analysis, and Raman scattering study ofK2BeF4and comparison to other members of theSoláns1, González-Silgo2, Calvet3 et al. 1998Phys. Rev. B

“…The value obtained is 27ϫ10 Ϫ6 C/cm 2 . In order to confirm this value, the model was applied to the ͑NH 4 ͒ 2 SO 4 , using the atomic coordinates at 209 K. The value obtained with the ionic model was 0.76ϫ10 Ϫ6 C/cm 2 , while the experimental value 19 …”

A phenomenological theory of dielectric and mechanical properties of improper ferroelectric crystals

“…Recently remarkable anomalies have been discovered again in the electromechanical property of Fe,B,O,,I (Fe-Iboracite) [3], [4]. Meanwhile, attention has long been attracted to the unusual dielectric behaviour of some ferroelectrics such as (NH,),SO, [5], Cd,(NH,),(SO,), [S], and (NH,),BeF, [7], where the weak-field dielectric constants along the ferroelectric axis hardly depend on temperature in the paraelectric state but show abrupt and sharp peaks a t the ferroelectric transition temperatures. In other words, in this group of crystals ((NH,),SO,-group) the Curie-Weiss law is not valid except in the close vicinity of the transition temperatures with very small values of Curie constants (of the order of several tens O K ) .…”

First principles study of structural stability, electronic and related properties of (NH4)2SO4

“…Using the experimental data of the temperature dependence of the molar specific heat [2], extrapolated to T = 0 K (inset of figure 4). …”

“…Crystalline ammonium sulfate (NH 4 ) 2 SO 4 undergoes a first-order ferroelectric phase transition at 223 K [1,2]. Characteristic properties are a relatively small value of the CurieWeiss constant [3] and a peculiar temperature dependence of the spontaneous polarization, which vanishes by cooling within the ferroelectric phase and changes its sign at 85 -87 K [4,5].…”

“…Such a complicated temperature dependence of the spontaneous polarization has been assigned to the existence of two nonequivalent ammonium groups NH 4 (1) and NH 4 (2) , giving non equal temperature dependent contributions to the spontaneous polarization, which therefore can be regarded as of ferrielectric type [5,6]. In both phases, four formula units (NH 4 ) 2 SO 4 are in the unit cell [2,3,7,8]. The transition from the space group Pnam to Pna2 1 , accompanied with the loss of the mirror plane ab and of the center of symmetry, leads to the occurrence of a spontaneous polarization along the c-axes.…”