Atomic dynamics in M Cr X 2 ( M = Ag , Cu ; X = S , Se ) across magnetic and superionic transitions

This work explores the atomic dynamics in layered chalcogenides (Ag,Cu)Cr(S,Se)${}_{2}$ across magnetic and superionic transitions. Combining neutron and x-ray scattering with first-principles simulations, the authors find low-energy vibration modes (phonons) with very strong anharmonicity, reflecting the weak bonding of the intercalated Cu or Ag ions vibrating in shallow potential energy wells. Upon warming, the Cu or Ag ions easily hop between two sublattices and the associated phonon modes break down into a quasielastic response. Because the Cr-Se crystalline framework remains robust, however, the long-wavelength transverse and longitudinal acoustic phonons do persist in the superionic phase. This work also examines the coupling between lattice dynamics and antiferromagnetic ordering in AgCrS${}_{2}$, and finds that magnetic quasielastic scattering in the paramagnetic phase is well separated from nuclear quasielastic associated with fast ionic diffusion. This work offers insights to guide the design of thermoelectric and electrolyte materials.