Ionics 4 (1998) 33 Studies of Polarization/Self-Depolarization and Electret-type Effect in AgI R.C. Agrawal*, R.K. Gupta and Mohan L. Verma Solid State Ionics Research Laboratory, School of Studies in Physics, Pt. Ravishankar Shukla University, Raipur - 492 010, M.P., India Abstract. A novel d.c. polarization/self-depolarization study and electret-type effect in AgI are reported. AgI pellets of varying thicknesses, placed between two blocking (graphite) electrodes, were subjected to an external d.c. potential. A state of complete polarization was attained within -10 min, irrespective of the sample thickness. At this state, the potential difference, developed across the sample pellet as a result of polarization/accumulation of mobile Ag § ions at the bulk/ negative electrode interface, was measured experimentally. The potential difference, obtained imme- diately after the removal of the external d.c. source, has been referred to as 'instant peak potential (Vp)'. As soon as the external voltage source is switched off, a process of self-depolarization is ini- tiated due to the chemical/self diffusion of polarized mobile Ag § ions throughout the bulk. 'Vp' gives a direct information regarding the extent of mobile ion concentration (n). 'Vp' measurements were carried out as a function of temperature and 'Log Vp vs 1/T' variation was compared with the 'Log n vs 1/T' Arrhenius plot, reported earlier in an entirely independent study. The two variations are almost analogous. This, in turn, supported as an earlier assertion that the abrupt conductivity increase in ~-AgI, after 13--->(x-phase transition at - 147 ~ is predominantly due to the excessive increase in 'n'. Furthermore, it has also been revealed that the Ag § ions play another unique role which led to the existence of 'persistent polarization' states in AgI. These states are identical to the 'electret-type effects', observed in a number of dielectric materials. The polarization state persisted for very long time in 'thermally stimulated polarized' sample. A detailed investigation of the per- sistence/retention of polarization in the thermally-stimulated-polarized sample is reported. 1. Introduction Silver iodide is one of the most extensively studied solid systems probably due to the fact that it exhibits a peculiar transition characteristics at ~ 147 ~ where it undergoes a structural transition from a low conducting hexagonal 13- phase to a very high conducting cubic (x-phase [1-3]. The (x-phase of AgI has been potentially employed to syn- thesize a large number of room temperature fast Ag § ion conducting systems by arresting this phase either in the solid solutions with other salts [4] or in the glassy net- work [5-7] or in the dispersexl composite electrolyte systems [8-10]. These solid state ionic materials, termed as 'Super Ionic Solids' or 'Solid Electrolytes' or 'Fast Ion Conductors', exhibit exceptionally high ionic conduc- tivity (~10 1 - 10 -4 S.cm -1) at room temperature, com- parable to the conductivity of liquid/aqueous electrolytes. (x-AgI is often termed as the archetypal of fast ion con- ductors with conductivity reaching ~ 2.6 S.cm 1 in the solid phase close to the melting point (Tm - 552 ~ Various experimental techniques have been used in the past to study the structural and transport properties as well as the role played by the mobile Ag + ions in AgI [4]. The electrical conductivity of ionic/superionic system is mainly governed by two basic transport parameters: the ionic mobility (~t) and the mobile ion concentration (n), as obvious from the well known equation for conductivity *Corresponding author