Dispersion and Spontaneous Current Phenomena in NH₄Cl Single Crystals

Abstract

A b s t r a c t . The effect of frequency and d.c. biasing on the dielectnic constant of NH,Cl single crystal samples was studied. A dispersion phenomenon was observed in the frequency range lo5-lo8 Hz. This was attributed t o a piezoelectric effect.The present study revealed that a spontaneous electric current of the order 10-l2 A was found to accompany the order -disorder process. This current showed a clear singnal in the phase transition region of the order-disorder transition 111-11 of NH,CI. This singnal was well controlled by the application of a polarizing electric field so that its amplitude could be increased, decreased, vanish or even reversed by the application of an appropriate external electric field. Dispersion und spontan fliegender Strom in NaHC1-EinkristallenEs werden die Frequenzabhangigkeit der Dielektrizitiitskonstante und der EinfluS einer Vorpolarisation a n NH,Cl-Einkristallproben untersucht. Bei den Frequenzen von lo5-los Hz wird ein Dispersionsphiinomen beobachtet, was auf einen piezoelektrischen Effekt zuriickgefiihrt wird.Die Untersuchung zeigt, daB der ubergang in den ungeordneten Zustand bei NH,CI von einem Strom in der GroBenordnung von A begleitet wird. Seine GroBe und Richtung ksnn durch ein polarisierendes, elektrisches Feld beeinfluBt werden. IntroduetionIt is well known that ammonium chloride undergoes a number of structure modifications a t atmospheric pressure a t certain well defined transition temperatures [I].These modifications are characterized by the presence of ordering and disordering of ions [l, 21. It is now becoming increasingly clear that reliable understanding of the various electric properties of NH,C1 during its different phase transitions should be made in terms of order -disorder phenomenon. This report covers some of these properties and is considered as an extending part of a previously published work [I, 21.We report here what we believe to be very interesting observations of dielectric measurements and spontaneous thermal current in the immediate vicinity of a critical phase transition region. The sitbstance studied here initially chosen because the nature of the transition was apparently simple and well understood [3, 41. The lattice here has a cubic symmetry both above and below the transition point. The stable transformations of ammonium chloride are :