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Random variable theory applied to the phase transition of KI-AgI system

10.24050/reia.v22i43.1819
Published 2025-01-01
Articles

Abstract

Silver iodide, AgI, doped with the MI family (M = K, Na, NH4, Li, among others), has been widely studied for its high ionic conductivity above 420 K. In the KI-AgI system, the silver ion (Ag+, 0.126 nm ionic radius) is substituted by the potassium ion (K+, 1.33 nm ionic radius), which modifies the crystal structure of the AgI solid electrolyte and, therefore, the electrostatic configuration of the system. The phase transition of this superionic system has been studied using a phenomenological model based on the free energy density as a function of the defect concentration at equilibrium and on the random variable theory represented by p or the probability that the carrier participates in the conduction. With a parallel plate geometry and the configuration of two Ag|sample|Ag| electrodes, electrical measurements were carried out using the impedance spectroscopy technique. The X–ray diffraction technique was used to demonstrate that the system is a mixture of two phases, one being g/β–AgI and the other being KI. In the proposed model, we have four fitting parameters: G, or the frequency ratio between interstitial ions and phonons; x, or the fraction between the energy to promote an ion to an interstitial position and the attractive interaction energy of the Frenkel pair. The abrupt jump in ionic conductivity at 420 K and the corresponding behavior in the temperature range from 350 K to 460 K were fitted for three cooling runs.

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References

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