Probing Anharmonic and Heterogeneous Carrier Dynamics Across Sublattice Melting in a Minimal Model Superionic Conductor

TL;DR

This work addresses the microscopic origin of sublattice melting and rapid ion transport in superionic conductors by introducing a minimal two-sublattice model with a rigid host lattice and a soft carrier sublattice coupled by long-range interactions. The simulations reveal three dynamical regimes—crystalline, sublattice-melt, and fully molten—and show that carrier transport in the intermediate regime arises from cooperative, highly anharmonic motion with pronounced dynamic heterogeneity, rather than independent hopping. Density tuning emerges as a robust control parameter, broadening or narrowing the sublattice-melting window and modulating carrier anharmonicity, thereby linking lattice softness to collective transport. These findings provide a microscopic framework for designing mechanically robust solid electrolytes capable of high conduction near ambient conditions, by leveraging sublattice melting and cooperative dynamics as design principles.

Abstract

Despite decades of research, the microscopic origin of sublattice melting and fast ion transport in superionic conductors remains elusive. Here, we introduce a chemically neutral minimal binary model consisting of a rigid host lattice stabilized by short-range steric repulsion and a soft carrier sublattice interacting via long-range Wigner-type forces. This contrast naturally produces distinct melting temperatures and an intermediate sublattice-melting phase in which carriers become fluidlike while the host remains crystalline. Molecular-dynamics simulations identify three dynamical regimes-crystalline, sublattice-melt, and fully molten-marked by sharp changes in diffusivity, structural correlations, and dynamic heterogeneity. Near sublattice melting, carrier motion is strongly anharmonic and spatially heterogeneous, beyond mean-field hopping descriptions. By tuning the density, we demonstrate that sublattice melting can be continuously controlled, establishing a direct link between lattice softness, anharmonicity, and collective ion transport. This work provides a unified microscopic foundation for designing mechanically robust, high-performance superionic conductors operable near ambient conditions.

Figures (15)

• Figure 1: Structural representations of the host-carrier system:(A) Schematic representation of the interaction scheme between interparticle pairs: the host-host interaction length is denoted as $\sigma_\mathrm{H} = \sigma_\mathrm{HH}$, the host-carrier interaction length as $\sigma_\mathrm{CH}$, the non-additive interaction length $\sigma_\mathrm{CC}$ between carrier-carrier is represented by the red dotted line, and the effective-carrier interaction length is chosen to be $\sigma_\mathrm{C}$ (see Methods for more details). (B) Low-temperature configuration obtained using a Wigner-type interaction, illustrating the ordered host lattice (blue) with carrier particles (red) occupying energetically favorable interstitial sites. (C--F) Representative snapshot configurations illustrating crystalline, sublattice-melt, and fully molten states across increasing temperature (top to bottom). For clarity, each snapshot includes an inset highlighting a magnified region of the configuration, emphasizing local structural arrangements and carrier environments.
• Figure 2: Selective sublattice melting and structural evolution: (A) Schematic representation of the diffusivity $D_{\alpha}$ as a function of inverse temperature $1/T$, illustrating three distinct regimes: (I) crystalline, (II) sublattice melting, and (III) full melting. The characteristic temperatures $T_m (\sim 0.35)$, $T_f (\sim 0.50)$, and $T_l (\sim 3.50)$ are indicated, corresponding respectively to the onset of sublattice melting, the freezing of carrier motion, and the liquid-like regime. (B) Representative snapshots illustrating structural evolution across these regimes. (C) Radial distribution functions $g_{\alpha\alpha}(r)$ ($\alpha \in \{{\rm C, H}\}$, where $C$: Carrier and $H$: Host) corresponding to each regime. Region II shows disordered carrier sublattice coexisting with ordered host lattice, evidencing selective carrier melting.
• Figure 3: Trajectory of carriers at different time intervals and temperature regimes: Panels (A--F) show carrier trajectories within region II-III of the diffusion plot (as shown in Fig. \ref{['fig:fig2']}), corresponding to the sublattice melting regime. Panels (A, B) present trajectories at the melting temperature of host within region III, beyond the sublattice melting regime, while panels (C, D) correspond to the higher-temperature side of the sublattice melting regime (region II). Panels (E, F) illustrate more heterogeneous behavior in the trajectories on the lower-temperature side of region II. Results are shown for packing fraction $\varphi = 0.85$. Particle identities are color-coded as indicated by the accompanying colorbar.
• Figure 4: Microscopic dynamics and quantify the extent of dynamical heterogeneity: (A-C) Carrier dynamics are characterized by the mean-squared displacement (MSD), the self-intermediate scattering function $F_{s,\mathrm{C}}(q,t)$, and the four-point susceptibility $\chi_{4,\mathrm{C}}(t)$ for the system of packing fraction $\varphi = 0.85$ at different temperatures (please see the top color bar). The characteristic temperatures $T_m (\sim 0.35)$, $T_f (\sim 0.50)$, and $T_l (\sim 3.50)$—marking the onset of sublattice melting, the freezing of carrier motion, and the liquid-like regime, respectively—are indicated by red circles, blue squares, and black triangles connected with dotted lines. (D-E) Inverse-temperature dependence of the Stokes-Einstein (SE) ratios $D_{\rm{C}}\tau_{\alpha, \mathrm{C}}$ and $D_{\rm{C}}\tau_{\chi_{4,\mathrm{C}}}$, highlighting pronounced SE violation at low temperatures (E). A guideline in (D) emphasizes the slope change near the sublattice-melting transition.
• Figure 5: Interplay between dynamic and static heterogeneity near the sublattice-melting point: (A) Spatial map of mean square displacement over $t \in [t_0,\, t_0 + \tau_\alpha]$ at $T = 0.45$, showing coexisting liquid-like and immobile crystalline domains. Displacement vectors in (A) further reveal string-like cooperative motion and hopping pathways within liquid-like regions, contrasted with localized, heterogeneous vibrations in crystallized domains. (B) Static configuration at $t_0$, coloured by nearest carrier-carrier distance, revealing anharmonic regions that align with the dynamically mobile zones in (A). Together, the two panels show that dynamical heterogeneity emerges from underlying static distortions of the carrier sublattice.