Electrical conduction in some important copper based superionic solids

Abstract

Measurement of the thermoelectric power (S) and electrical conductivity (σ) of six superionic solids namely CuI, CuPb₃Br₇, Cu₂HgI₄, Cu₃CdI₅, Cu₃RbCl₄, Cu₇(C₆H₁₂NH₃)Br₈ and Cu₁₆Rb₄I₇Cl₁₃ are reported from 300 K to nearly the melting point of each material. The logσ T andS againstT ⁻¹ plots are linear in some temperature ranges with different slopes. For each material they show two distinct regions: one corresponding to superionic (SP) and the other to normal phase (NP). In the superionic phase, Cu⁺ ions are the main entity of charge carrier and an extended lattice gas model explains the transport mechanism fairly well. On the higher temperature side of SP, the other cation in the material starts contributing significantly toσ. In the normal phase the conduction is mainly due to Frenkel defects (Cu⁺ ions at interstitial sites). The enthalpy for migration and heat of transport of these defects has also been evaluated for CuI, CuPb₃Br₇, Cu₂HgI₄ and Cu₃CdI₅. The formation energy of defects has also been calculated for CuI and Cu₃CdI₅. Normal phase has not been obtained in Cu₃RbCl₄, Cu₇(C₆H₁₂NCH₃)Br₈ and Cu₁₆Rb₄I₇Cl₁₃ as their phase transition temperatures lie below room temperature.