Charge carrier interactions in ionic conductors: A classical molecular-dynamics and Monte Carlo study on AgI

The equilibrium concentration of ionic and electronic charge carriers in io nic crystals as a function of temperature, concentration of dopants, and ch emical environment is phenomenologically well understood as long as these p oint defects can be considered sufficiently dilute. However, there are case s, usually at temperatures close to the melting point, where the defects ap pear in higher concentrations. In these cases interactions come into play a nd cause anomalous increases in the conductivity or even phase transitions. Recently Hainovsky and Maier showed that for various Frenkel disordered ma terials this anomalous conductivity increase at high temperature can be des cribed by a cube root term in the chemical potential of the defects. This q uasi-Madelung approach does not only allow ionic conductivities and heat ca pacities to be computed, it also leads to a phenomenological understanding of the solid-liquid or superionic transition temperatures. In the present s tudy we analyze this approach on the atomistic level for AgI: The defect co ncentrations as well as defect energies, including excess energies, are com puted as a function of temperature by molecular-dynamics and Monte Carlo si mulations based on a classical semiempirical potential. The simulations sup port the cube-root model, yield approximately the same interaction constant s and show that the corrections in the chemical potential are of an energet ic nature. In agreement with structural expectations, the simulations revea l that two different kinds of interstitials are present: Octahedral interst itials, which essentially determine the ionic transport at higher temperatu re, and tetrahedral ones, which remain substantially associated with the va cancies. It is shown how these refinements have to be introduced into the c ube root. (C) 2000 American Institute of Physics. [S0021- 9606(00)70314-X].