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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The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.
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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.
The authors declare that they have no competing interests.
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Niedziela, J.L., Bansal, D., May, A.F. et al. Selective breakdown of phonon quasiparticles across superionic transition in CuCrSe2. Nature Phys 15, 73–78 (2019) doi:10.1038/s41567-018-0298-2
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