Significance
Unveiling the unusual atomic dynamics in superionic conductors is critical for the design of energy conversion and storage materials, for example to rationalize their thermal transport properties in thermoelectric applications or their fast ionic conductivity in solid-state electrolytes. We combine neutron/X-ray scattering techniques with first-principles calculations to discover hybrid lattice dynamics in
A
g
C
r
S
e
2
, where the phonon wavelength controls the breakdown of transverse acoustic modes in the superionic phase. We attribute the ultralow thermal conductivity to strong phonon anharmonicity combined with disorder in the Ag sublattice. This understanding provides insights into the thermal conduction mechanism in superionic conductors and sheds light on the role of lattice dynamics in the emergence of superionic behavior.