Abstract
Superionic materials that exhibit coexistence of rigid crystalline lattices and liquid-like fluctuating substructures have emerged as promising thermoelectric materials. The inadequate understanding of the phonon behavior in the superionic state, however, still prevents further revealing of the underlying correlation between the thermally induced liquid-like atomic dynamics and anomalous thermal transport properties. Herein, by adopting a hybrid scheme to directly characterize anharmonic phonon quasiparticles from ab-initio molecular dynamics, we manifest that low-energy transverse phonons dominated by Ag atoms totally collapse, whereas longitudinal optical phonons remain largely intact during the superionic transition. The ultralow thermal conductivity originates from the atomic level structural heterogeneity can be ultimately attributed to diffusive phonon dynamics. Our study also reveals that the extremely large selective phonon diffusive scattering can be counteracted by hydrostatic pressure induced deactivation of the liquid-like flow of Ag atoms. These results demonstrate the decisive role of ion superionicity in phonon scattering across superionic transition and may pave the way for new phonon engineering strategies in related superionic materials.
Highlights
The thermal conduction of condensed systems can be considered as microscopic atomic diffusion among equilibrium positions, where the averaged rearrangement time serve as an intrinsic criterion to distinguish solid from liquid[1,2]
The significant increase of ionic diffusivity across superionic transition has been first interpreted by calculating the root mean square displacement (RMSD) Δ(r), which often serves as an indicator of the solid-liquid phase transition[29]
We have explored the evolution of phonons in AgCrSe2 across the superionic transition through AIMD simulations
Summary
The thermal conduction of condensed systems can be considered as microscopic atomic diffusion among equilibrium positions, where the averaged rearrangement time serve as an intrinsic criterion to distinguish solid from liquid[1,2]. Compared with negligible diffusive time in liquid, which enables the fluidity, the hopping time between well-defined lattice sites is enormous for crystalline solid. The hierarchically multi-layered superionic crystals, are rare materials exhibiting long range liquidlike ionic diffusivity while simultaneously maintaining solid crystalline sublattices. They have attracted steady interest with promising applications in fuel cells[3], solid rechargeable batteries electrolytes[4], and high-efficient thermoelectric materials[5,6,7] by virtue of the unique solid-liquid duality. The efficiency is governed by a dimensionless figure of merit zT = α2T∕ρ(κL + κe), where α is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity, κL is the lattice thermal conductivity, and κe is the carrier thermal conductivity. Without an increase in ρ, substantial efforts have been invested to suppress the phonon propagation to reduce the κL to a glass-like value[8,9,10]
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