Extreme phonon anharmonicity underpins superionic diffusion and ultralow thermal conductivity in argyrodite Ag₈SnSe₆

QY Ren and MK Gupta and M Jin and JX Ding and JT Wu and ZW Chen and SQ Lin and O Fabelo and JA Rodríguez-Velamazán and M Kofu and K Nakajima and M Wolf and FF Zhu and JL Wang and ZX Cheng and GH Wang and X Tong and YZ Pei and O Delaire and J Ma, NATURE MATERIALS, 22, 999-+ (2023).

DOI: 10.1038/s41563-023-01560-x

Ultralow thermal conductivity and fast ionic diffusion endow superionic materials with excellent performance both as thermoelectric converters and as solid-state electrolytes. Yet the correlation and interdependence between these two features remain unclear owing to a limited understanding of their complex atomic dynamics. Here we investigate ionic diffusion and lattice dynamics in argyrodite Ag8SnSe6 using synchrotron X-ray and neutron scattering techniques along with machine- learned molecular dynamics. We identify a critical interplay of the vibrational dynamics of mobile Ag and a host framework that controls the overdamping of low-energy Ag-dominated phonons into a quasi-elastic response, enabling superionicity. Concomitantly, the persistence of long-wavelength transverse acoustic phonons across the superionic transition challenges a proposed 'liquid-like thermal conduction' picture. Rather, a striking thermal broadening of low-energy phonons, starting even below 50 K, reveals extreme phonon anharmonicity and weak bonding as underlying features of the potential energy surface responsible for the ultralow thermal conductivity (<0.5 W m(-1) K-1) and fast diffusion. Our results provide fundamental insights into the complex atomic dynamics in superionic materials for energy conversion and storage. Superionic materials are of interest for solid-state batteries or thermoelectrics, yet a clear understanding of the atomistic mechanisms is lacking. Here it is shown that transverse acoustic phonons persist above the superionic transition in argyrodite Ag8SnSe6, and that the free-Se sublattice controls fast Ag cation diffusion.

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