Structure of conduction channels in superionic conductors: Crystallochemical modeling

The experimental and theoretical studies of the structure of conduction channels of superionic conductors are analyzed. A crystallochemical method of modeling the conduction channels is proposed in which they are treated as sets of polyhedron Vorono-Dirichlet sections outside of impenetrable spheres of the rigid sublattice. The allowance for both weak flexibility of the spheres and their mean-square displacements relative to the sublattice points yields a channel structure that “wraps” around the Vorono-Dirichlet polyhedron edges. Modeling the interionic potential for α-AgI with the help of equipotential surfaces suggests that the mobile-ion motion slips along the conduction channel walls. For α-AgI, the set of the equipotential surfaces specifies “the minimum energy trajectory” of transition into a superionic state, while the crystallochemical “wrapping” structure corresponds to a saddle point of a multidimensional potential surface. Symmetry selection rules are used for predicting mechanical trajectories as allowed oscillation modes for the tetrahedral and octahedral fragments of α-CuI.