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ABSTRACT Superionic materials represent a regime intermediate between the crystalline and liquid states of matter. Despite the considerable interest in potential applications for solid-state

batteries or thermoelectric devices, it remains unclear whether the fast ionic diffusion observed in superionic materials reflects liquid-like dynamics or whether the hops of mobile ions

are inherently coupled to more conventional lattice phonons. Here we reveal a crossover from crystalline vibrations to relaxational dynamics of ionic diffusion in the superionic compound

Li6PS5Cl, a candidate solid-state electrolyte. By combining inelastic and quasi-elastic neutron-scattering measurements with first-principles-based machine-learned molecular dynamics

simulations, we found that the vibrational density of states in the superionic state strongly deviates from the quadratic behaviour expected from the Debye law of lattice dynamics. The

superionic dynamics emerges from overdamped phonon quasiparticles to give rise to a linear density of states characteristic of instantaneous normal modes in the liquid state. Further, we

showed that the coupling of lattice phonons with a dynamic breathing of the Li+ diffusion bottleneck enables an order-of-magnitude increase in diffusivity. Thus, our results shed insights

into superionics for future energy storage and conversion technologies. Access through your institution Buy or subscribe This is a preview of subscription content, access via your

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subscriptions * Read our FAQs * Contact customer support SIMILAR CONTENT BEING VIEWED BY OTHERS SOLIDS THAT ARE ALSO LIQUIDS: ELASTIC TENSORS OF SUPERIONIC MATERIALS Article Open access 19

January 2023 ELECTRONIC PADDLE-WHEELS IN A SOLID-STATE ELECTROLYTE Article Open access 02 January 2024 DIFFUSION MECHANISMS OF FAST LITHIUM-ION CONDUCTORS Article 12 September 2024 DATA

AVAILABILITY All data that support the conclusion of this work are available from the corresponding author upon reasonable request. The numerical data for the figures are available from the

Harvard Dataverse Repository at https://doi.org/10.7910/DVN/RCBK4U. Source data are provided with this paper. CODE AVAILABILITY The codes that support the findings of the study are available

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ACKNOWLEDGEMENTS The collection of neutron-scattering data, MD simulations and analysis by J.D., the simulations by H.-M.L. and manuscript writing by J.D. and O.D. were supported by a US

National Science Foundation DMREF project (Award DMR-2119273). The initial analysis and simulations by M.K.G. were supported by the DOE (Award DE-SC0019978). Sample synthesis by C.R. and

W.G.Z. was supported by the German Research Foundation (Grant No. ZE 1010/4-1). The use of Oak Ridge National Laboratory’s Spallation Neutron Source was sponsored by the Scientific User

Facilities Division, Office of Basic Energy Sciences, US DOE. Theoretical calculations were performed using the National Energy Research Scientific Computing Center, a US DOE Office of

Science User Facility supported by the Office of Science of the US DOE (Contract No. DE-AC02-05CH11231). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Department of Mechanical Engineering

and Materials Science, Duke University, Durham, NC, USA Jingxuan Ding, Mayanak K. Gupta & Olivier Delaire * John A. Paulson School of Engineering and Applied Sciences, Harvard

University, Cambridge, MA, USA Jingxuan Ding * Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai, India Mayanak K. Gupta * Institute of Inorganic and Analytical Chemistry,

University of Münster, Münster, Germany Carolin Rosenbach & Wolfgang G. Zeier * Department of Chemistry, Duke University, Durham, NC, USA Hung-Min Lin & Olivier Delaire * Neutron

Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA Naresh C. Osti & Douglas L. Abernathy * Institute of Energy Materials and Devices (IMD), IMD-4: Helmholtz-Institut

Münster, Forschungszentrum Jülich, Münster, Germany Wolfgang G. Zeier * Department of Physics, Duke University, Durham, NC, USA Olivier Delaire Authors * Jingxuan Ding View author

publications You can also search for this author inPubMed Google Scholar * Mayanak K. Gupta View author publications You can also search for this author inPubMed Google Scholar * Carolin

Rosenbach View author publications You can also search for this author inPubMed Google Scholar * Hung-Min Lin View author publications You can also search for this author inPubMed Google

Scholar * Naresh C. Osti View author publications You can also search for this author inPubMed Google Scholar * Douglas L. Abernathy View author publications You can also search for this

author inPubMed Google Scholar * Wolfgang G. Zeier View author publications You can also search for this author inPubMed Google Scholar * Olivier Delaire View author publications You can

also search for this author inPubMed Google Scholar CONTRIBUTIONS O.D. and J.D. designed the research. J.D., M.K.G., N.C.O. and D.L.A. performed the neutron-scattering measurements. C.R. and

W.G.Z. synthesized and characterized the samples. J.D., M.K.G. and H.-M.L. performed the simulations and analysed the data. J.D. and O.D. wrote the paper. CORRESPONDING AUTHOR

Correspondence to Olivier Delaire. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing interests. PEER REVIEW PEER REVIEW INFORMATION _Nature Physics_ thanks Helen

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CITE THIS ARTICLE Ding, J., Gupta, M.K., Rosenbach, C. _et al._ Liquid-like dynamics in a solid-state lithium electrolyte. _Nat. Phys._ 21, 118–125 (2025).

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