Superionic crystals exhibit ionic mobilities comparable to liquids while maintaining a periodic crystalline lattice. The atomic dynamics leading to large ionic mobility have long been debated. A central question is whether phonon quasiparticles—which conduct heat in regular solids—survive in the superionic state, where a large fraction of the system exhibits liquid-like behaviour. Here we present the results of energy- and momentum-resolved scattering studies combined with first-principles calculations and show that in the superionic phase of CuCrSe2, long-wavelength acoustic phonons capable of heat conduction remain largely intact, whereas specific phonon quasiparticles dominated by the Cu ions break down as a result of anharmonicity and disorder. The weak bonding and large anharmonicity of the Cu sublattice are present already in the normal ordered state, resulting in low thermal conductivity even below the superionic transition. These results demonstrate that anharmonic phonon dynamics are at the origin of low thermal conductivity and superionicity in this class of materials.

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We thank O. Hellman and M. Stone for helpful discussions. We are grateful to J. Z. Tischler for algorithms enabling deconvolution of the energy resolution from the inelastic X-ray phonon scattering data. We would also like to acknowledge technical support from D. Dunning, T. Russell and S. Elorfi at the SNS. J.L.N., J.D. and T.L.-A. were supported as part of the S3TEC EFRC, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences under award no. DE-SC0001299. D.B. and O.D. were supported by the US DOE, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under the Early Career Award no. DE-SC0016166 (principal investigator O.D.). A.F.M. was supported by the US DOE, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The research at Oak Ridge National Laboratory’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US DOE. This research used resources of the Advanced Photon Source, a US DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. Ab initio molecular dynamics calculations were performed using resources of 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 under contract no. DE-AC02-05CH11231. Density functional theory simulations for this research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the US DOE under contract no. DE-AC05-00OR22725.

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Author notes

    • Jennifer L. Niedziela

    Present address: Nuclear Security and Isotope Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

  1. These authors contributed equally: J. L. Niedziela, Dipanshu Bansal.


  1. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

    • Jennifer L. Niedziela
    •  & Andrew F. May
  2. Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC, USA

    • Jennifer L. Niedziela
    • , Dipanshu Bansal
    • , Jingxuan Ding
    • , Tyson Lanigan-Atkins
    •  & Olivier Delaire
  3. Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

    • Georg Ehlers
    •  & Douglas L. Abernathy
  4. Advanced Photon Source, Argonne National Laboratory, Lemont, IL, USA

    • Ayman Said
  5. Department of Physics, Duke University, Durham, NC, USA

    • Olivier Delaire


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J.L.N., D.B. and O.D. performed and analysed the neutron scattering measurements with support from G.E. and D.L.A. J.L.N., D.B. and O.D. performed and analysed the X-ray measurements with support from A.S. A.F.M. synthesized the samples and performed transport measurements. T.L.-A. performed diffusivity and heat capacity measurements. D.B. and J.D. performed simulations. J.L.N., D.B. and O.D. wrote the manuscript and all authors commented on the manuscript. O.D. supervised the project.

Competing Interests

The authors declare that they have no competing interests.

Corresponding authors

Correspondence to Jennifer L. Niedziela or Dipanshu Bansal or Olivier Delaire.

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