Topological data analysis based on persistent homology has been applied to the molecular dynamics simulation for the fast ion-conducting phase (α-phase) of AgI to show its effectiveness on the ion migration mechanism analysis. Time-averaged persistence diagrams of α-AgI, which quantitatively record the shape and size of the ring structures in the given atomic configurations, clearly showed the emergence of the four-membered rings formed by two Ag and two I ions at high temperatures. They were identified as common structures during the Ag ion migration. The averaged potential energy change due to the deformation of the four-membered ring during Ag migration agrees well with the activation energy calculated from the conductivity Arrhenius plot. The concerted motion of two Ag ions via the four-membered ring was also successfully extracted from molecular dynamics simulations by our approach, providing new insight into the specific mechanism of the concerted motion.
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The migration of multiple ions triggered by that of one ion. Here, the term, concerted motion, is defined in a broad sense. In this article, if other ions move in conjunction with the migration of one ion, it is considered to be a concerted motion, regardless of the direction of transport, etc.
Here, we presume that each Ag ion migrates independently along the edge of the Voronoi polyhedron for I bcc lattice without interacting with each other.
In principle, the arrangement of Ag ions is random, but the following two constraints are imposed: (1) the number of Ag ions in a unit cell is limited to two to maintain the stoichiometry within a unit cell, and (2) each Ag ion in the unit cell must not be placed at least on the same surface of Voronoi polyhedron.