Unveiling the hidden effects of local potential energy surface on ion transport in superionic conductors

The potential energy surface (PES) fundamentally governs the motion of mobile ions in superionic conductors (SICs). In particular, flat local PES around mobile ion sites, indicative of low vibration frequencies and soft lattice dynamics, are strongly associated with reduced migration barriers and fast ion transport. However, a fast and quantitative method for evaluating local PES flatness remains lacking. Here, we combine the bond valence site energy method with isosurface analysis and harmonic vibration analysis to quantify local PES flatness through two complementary descriptors: the isosurface-enclosed volume (Viso; geometric perspective) and the site-specific vibration frequency (νsite; dynamical perspective), and unveiling their hidden effects on ion transport. Analysis of representative SICs reveals that large Viso (0.019–0.084 Å3) and low νsite (2.304–5.069 THz) at specific mobile ion sites are characteristic of fast ion transport, whereas non-SICs consistently exhibit smaller Viso (0.002–0.009 Å3) and higher νsite (8.465–12.872 THz). The proposed thresholds of Viso = 0.01 Å3 and νsite = 7 THz show a clear separation between known SICs and non-SICs in the current dataset and are expected to serve as practical criteria for future high-throughput screening of new SICs.