Ultrafast electron diffuse scattering in superionic conductor AgCrSe­₂

AgCrSe₂ is a layered material that exhibits superionic conductivity at high temperature (> 460 K). It has been recently attracting much attention for the study of transverse acoustic phonon and the emergence of ultra-low thermal conductivity in the “liquid state”, which is advantageous for thermoelectric functions [1]. In AgCrSe₂ crystals, there are two sites where Ag ions intercalated between CrSe₂ layers can reside. At low temperatures, all Ag ions occupy one of them to form a 2-dimensional triangular network, while in the superionic conducting state at high temperature, the occupancy becomes completely random between these two sites, i.e. order-disorder phase transition. In this study, we investigate the phononic dynamics in AgCrSe₂ by ultrafast electron diffraction method utilizing an ultrafast transmission electron microscope [2,3]. We discuss the phonon excitation and disorder effect by comparing the diffuse scattering in the picosecond to nanosecond range with the numerical simulations.

[1] B. Li, et al., Nat. Mater. 17, 226 (2018).

[2] A. Nakamura, Y. Chiashi et al., Nano Lett. 20, 4932–4938 (2020).

[3] A. Nakamura et al., Faraday Discuss., 237, 27-39 (2022).