J. Chem. Sci., Vol. 118, No. 1, January 2006, pp. 135–154. © Indian Academy of Sciences. Ionic conduction in the solid state P PADMA KUMAR and S YASHONATH Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012 e-mail:
[email protected] Abstract. Solid state ionic conductors are important from an industrial viewpoint. A variety of such conductors have been found. In order to understand the reasons for high ionic conductivity in these sol- ids, there have been a number of experimental, theoretical and computational studies in the literature. We provide here a survey of these investigations with focus on what is known and elaborate on issues that still remain unresolved. Conductivity depends on a number of factors such as presence of interstitial sites, ion size, temperature, crystal structure etc. We discuss the recent results from atomistic computer simula- tions on the dependence of conductivity in NASICONs as a function of composition, temperature, phase change and cation among others. A new potential for modelling of NASICON structure that has been proposed is also discussed. Keywords. Ionic conduction; solid state; atomistic computer simulations; NASICON structure. 1. Introduction 2. A brief history of superionic conductors There exist many solids with high ionic conductivity The study of ionic conduction in solid state origi- (>10–4 W–1 cm–1) and they are of immense use in di- nated way back in 1838 when Faraday discovered verse technological applications. Some of these solids that PbF2 and Ag2S are good conductors of electri- which are also good electronic conductors are often city.2,3 These solids are, the first ever discovered referred to as ‘mixed conductors’, while the term solid electrolyte and intercalation electrode respecti- ‘superionic conductor’ or ‘fast ion conductor’ is re- vely.