Abstract
It has been suggested that type II phase transitions in ionic conductors, in which the conductivity changes continuously, are of the order-disorder type. We present data on various properties of RbAg4I5 near the 208K phase transition which supports the view that there is partial ordering of the mobile silver ions, in a manner analogous to the cooperative Jahn-Teller transitions. The specific heat is shown to have Ising-like critical exponents and birefringence data show a linear coupling between the order parameter and the lattice strain as is characteristic of cooperative Jahn-teller systems. One of the predictions of such a model is the variation of the ionic resistivity with the degree of short range order in the system. A novel method of obtaining the temperature derivative of log R, is shown to give data strongly resembling the specific heat in the critical region which in turn demonstrates the proportionality between R and the short range order. X-ray data are presented which demonstrate that a structural component is present. Although a Landau theory analysis of this transition predicts a first order phase transition, no discontinuous behavior nor hysteresis was observed.
Published Version
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