Superionic conductors possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I superionic conductors (for example, AgI, Ag2Se and so on) are defined by a first-order transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a two-dimensional type I superionic conductor, α-KAg3Se2, by scattering techniques and complementary simulations. Quasi-elastic neutron scattering and ab initio molecular dynamics simulations confirm that the superionic Ag+ ions are confined to subnanometre sheets, with the simulated local structure validated by experimental X-ray powder pair-distribution-function analysis. Finally, we demonstrate that the phase transition temperature can be controlled by chemical substitution of the alkali metal ions that compose the immobile charge-balancing layers. Our work thus extends the known classes of superionic conductors and will facilitate the design of new materials with tailored ionic conductivities and phase transitions.
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Data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request. Source data are provided with this paper.
Boyce, J. B. & Huberman, B. A. Superionic conductors: transitions, structures, dynamics. Phys. Rep. 51, 189–265 (1979).
Faraday, M. V. I. I. Experimental researches in electricity. Philos. Trans. R. Soc. 128, 83–123 (1838).
Hull, S. Superionics: crystal structures and conduction processes. Rep. Prog. Phys. 67, 1233–1314 (2004).
Goodenough, J. B. Review lecture - fast ionic conduction in solid. Proc. R. Soc. A 393, 215–234 (1984).
Bruce, P. G. Solid State Electrochemistry (Cambridge Univ. Press, 1997).
Voneshen, D., Walker, H., Refson, K. & Goff, J. Hopping time scales and the phonon-liquid electron-crystal picture in thermoelectric copper selenide. Phys. Rev. Lett. 118, 145901 (2017).
Ding, J. et al. Anharmonic lattice dynamics and superionic transition in AgCrSe2. Proc. Natl Acad. Sci. USA 117, 3930–3937 (2020).
Bailey, T. P. & Uher, C. Potential for superionic conductors in thermoelectric applications. Curr. Opin. Green. Sustain. Chem. 4, 58–63 (2017).
Keen, D. A. Disordering phenomena in superionic conductors. J. Condens. Matter Phys. 14, R819 (2002).
Funke, K. AgI-type solid electrolytes. Prog. Solid. State Chem. 11, 345–402 (1976).
Derrington, C. & O’Keeffe, M. Anion conductivity and disorder in lead fluoride. Nat. Phys. Sci. 246, 44–46 (1973).
Boukamp, B. & Wiegers, G. Ionic and electronic processes in AgCrSe2. Solid State Ion. 9, 1193–1196 (1983).
Yao, Y.-F. Y. & Kummer, J. Ion exchange properties of and rates of ionic diffusion in beta-alumina. J. Inorg. Nucl. Chem. 29, 2453–2466, IN1, 2467–2475 (1967).
Engelsman, F., Wiegers, G., Jellinek, F. & Van Laar, B. Crystal structures and magnetic structures of some metal (I) chromium (III) sulfides and selenides. J. Solid State Chem. 6, 574–582 (1973).
Newsam, J. & Cheetham, A. Stoichiometric silver beta alumina studied at 25, 300 and 500 degrees C by powder neutron diffraction. J. Phys. Condens. Matter 2, 2335 (1990).
Tubandt, C. & Lorenz, E. Molekularzustand und elektrisches leitvermögen kristallisierter salze. Z. Phys. Chem. 87, 513–542 (1914).
Miyatani, S.-y Ionic conductivity in silver chalcogenides. J. Phys. Soc. Jpn 50, 3415–3418 (1981).
Majumdar, A. & Roy, R. Experimental study of the polymorphism of AgI. J. Phys. Chem. 63, 1858–1860 (1959).
Banus, M. D. Pressure dependence of the alpha-beta transition temperature in silver selenide. Science 147, 732–733 (1965).
Hu, T., Wittenberg, J. & Lindenberg, A. Room-temperature stabilization of nanoscale superionic Ag2Se. Nanotechnology 25, 415705 (2014).
Makiura, R. et al. Size-controlled stabilization of the superionic phase to room temperature in polymer-coated AgI nanoparticles. Nat. Mater. 8, 476–480 (2009).
Rettie, A. J. E. et al. Ag2Se to KAg3Se2: suppressing order–disorder transitions via reduced dimensionality. J. Am. Chem. Soc. 140, 9193–9202 (2018).
Mamontov, E. Fast oxygen diffusion in bismuth oxide probed by quasielastic neutron scattering. Solid State Ion. 296, 158–162 (2016).
Bée, M. Localized and long-range diffusion in condensed matter: state of the art of QENS studies and future prospects. Chem. Phys. 292, 121–141 (2003).
Hamilton, M., Barnes, A., Howells, W. & Fischer, H. Ag+ dynamics in the superionic and liquid phases of Ag2Se and Ag2Te by coherent quasi-elastic neutron scattering. J. Phys. Condens. Matter 13, 2425 (2001).
Chudley, C. & Elliott, R. Neutron scattering from a liquid on a jump diffusion model. Proc. Phys. Soc. 77, 353 (1961).
Embs, J. P., Juranyi, F. & Hempelmann, R. Introduction to quasielastic neutron scattering. Z. Phys. Chem. 224, 5–32 (2010).
Hempelmann, R. Quasielastic Neutron Scattering and Solid State Diffusion (Clarendon Press, 2000).
Wind, J., Mole, R. A., Yu, D. & Ling, C. D. Liquid-like ionic diffusion in solid bismuth oxide revealed by coherent quasielastic neutron scattering. Chem. Mater. 29, 7408–7415 (2017).
Niedziela, J. L. et al. Selective breakdown of phonon quasiparticles across superionic transition in CuCrSe2. Nat. Phys. 15, 73–78 (2019).
Bensch, W. & Dürichen, P. Crystal structure of potassium diselenotriargentate, KAg3Se2. Z. Kristallogr. N. Cryst. Struct. 212, 97–98 (1997).
Kvist, A. & Josefson, A.-M. The electrical conductivity of solid and molten silver iodide. Z. Naturforsch. A 23, 625–626 (1968).
Allen, R. L. & Moore, W. J. Diffusion of silver in silver sulfide. J. Phys. Chem. 63, 223–226 (1959).
Okazaki, H. Deviation from the Einstein relation in average crystals self-diffusion of Ag+ ions in α-Ag2S and α-Ag2Se. J. Phys. Soc. Jpn 23, 355–360 (1967).
Rom, I. & Sitte, W. Composition dependent ionic and electronic conductivities and chemical diffusion coefficient of silver selenide at 160 C. Solid State Ion. 101, 381–386 (1997).
Barnes, A., Lague, S., Salmon, P. & Fischer, H. A determination of the structure of liquid Ag2Se using neutron diffraction and isotopic substitution. J. Phys. Condens. Matter 9, 6159–6173 (1997).
Lee, S. & Xu, H. Using complementary methods of synchrotron radiation powder diffraction and pair distribution function to refine crystal structures with high quality parameters—a review. Minerals 10, 124 (2020).
Sharp, K. W. & Koehler, W. H. Synthesis and characterization of sodium polyselenides in liquid ammonia solution. Inorg. Chem. 16, 2258–2265 (1977).
Mamontov, E. & Herwig, K. W. A time-of-flight backscattering spectrometer at the Spallation Neutron Source, BASIS. Rev. Sci. Instrum. 82, 085109 (2011).
Arnold, O. et al. Mantid – data analysis and visualization package for neutron scattering and μSR experiments. Nucl. Instrum. Methods Phys. Res. A 764, 156–166 (2014).
Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 54, 11169 (1996).
Kresse, G. & Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, 15–50 (1996).
Kresse, G. & Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B 47, 558 (1993).
Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996).
Perdew, J. P. & Zunger, A. Self-interaction correction to density-functional approximations for many-electron systems. Phys. Rev. B 23, 5048 (1981).
He, X., Zhu, Y., Epstein, A. & Mo, Y. Statistical variances of diffusional properties from ab initio molecular dynamics simulations. npj Comput. Mater. 4, 18 (2018).
Toby, B. H. & Von Dreele, R. B. GSAS-II: the genesis of a modern open-source all purpose crystallography software package. J. Appl. Crystallogr. 46, 544–549 (2013).
Juhás, P., Davis, T., Farrow, C. L. & Billinge, S. J. L. PDFgetX3: a rapid and highly automatable program for processing powder diffraction data into total scattering pair distribution functions. J. Appl. Crystallogr. 46, 560–566 (2013).
Farrow, C. L. et al. PDFfit2 and PDFgui: computer programs for studying nanostructure in crystals. J. Phys. Condens. Matter 19, 335219 (2007).
We are indebted to W. Xu for assistance in the acquisition and analysis of X-ray total scattering data and to E. Mamontov for valuable discussions concerning the QENS data analysis. M.J.J. acknowledges HORIBA-Motor Industry Research Association (MIRA), University College London (UCL) and the Engineering and Physical Sciences Research Council (EPSRC) (EP/R513143/1) for a Collaborative Awards in Science and Engineering (CASE) studentship. This work was performed primarily at the Materials Science Division at Argonne National Laboratory, supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. We gratefully acknowledge the computing resources provided on Bebop, the high-performance computing clusters operated by the Laboratory Computing Resource Center at Argonne National Laboratory. First-principles modelling at Duke University (J.D., O.D.) was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under award no. DE-SC0019299. Work at Oak Ridge National Laboratory’s Spallation Neutron Source is supported by the US Department of Energy, Office of Basic Energy Sciences. The Oak Ridge National Laboratory is managed by UT–Battelle for the US Department of Energy under contract no. DEAC05-00OR22725. This work made use of the Integrated Molecular Structure Education and Research Center (IMSERC) facility at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-2025633) and Northwestern University. A.J.E.R. and M.J.J. gratefully acknowledge the Faraday Institution Lithium-Sulfur Technology Accelerator (LiSTAR) programme (FIRG014, EP/S003053/1) for funding.
The authors declare no competing interests.
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Rettie, A.J.E., Ding, J., Zhou, X. et al. A two-dimensional type I superionic conductor. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-01053-9