Fig. 5.

Superionic diffusion of Ag ions and the diffusion barrier. ($A$) Quasielastic neutron scattering from CNCS with $E_i\hspace{0.17em}=$ 4 meV at 300, 460, 530, and 600 K, respectively, integrated over 1.4 $\le$ $\left|Q\right|$ $\le$ 1.6. The Lorentzian width increasing on heating represents faster diffusion. (B and C) In-plane probability distribution of Ag atoms from AIMD at ($B$) 300 K and ($C$) 600 K. At 300 K, Ag atoms vibrate near their equilibrium positions at $\alpha$ sites. The diffusion occurs occasionally at 300 K and becomes dominant at 600 K. Threefold rotation symmetry and periodicity are applied to the plot. ($D$) The probability density of Ag atoms on the plane perpendicular to the shared edge of neighboring $\alpha$ and $\beta$ sites. The upper red triangle is the projection of the $\alpha$-tetrahedron, and the lower one is the $\beta$-tetrahedron. The angle $\theta$ between the Ag atom positions and the center of the $\alpha$-tetrahedron is defined as the reaction coordinate. ($E$) The free-energy barrier along the reaction pathway. A low barrier of 0.11 eV is observed along an extended region between $\alpha$ and $\beta$ sites. ($F$) Pair distribution function projected on Ag–Ag bonds at 300 K (blue) and 600 K (orange). Dashed lines are the distances between $\alpha$–$\alpha$ and the $\alpha$–$\beta$ sites. Strong repulsion between Ag atoms is observed, as no Ag–Ag bond with length of 2.3 Å (the nearest $\alpha$–$\beta$ distance) appears in AIMD.