Scientists from the Institute of Metal Research (IMR) of the Chinese Academy of Sciences have directly observed liquid-like, glass-like and crystal-like phonon behaviors coexisting in a crystalline solid—the layered superionic conductor AgCrSe₂.

Their study, published in J. Am. Chem. Soc. on November 5, challenges the conventional boundaries among the dynamical behaviors of crystals, glasses, and liquids and provides new experimental evidence for a unified theory of heat transport.

Phonons are collective atomic vibrations that govern many fundamental material properties, including thermal conductivity. In the conventional picture, perfect crystals have well-defined phonon quasiparticles because their atomic lattices maintain long-range periodic order. In glasses and liquids, structural disorder strongly damps phonons, and transverse modes may even be suppressed.

AgCrSe₂ is a layered superionic conductor with a quasi-two-dimensional structure and a superionic transition near 475 K. Rigid edge-sharing [CrSe₆] octahedra form a stable periodic framework, while Ag⁺ ions occupy two equivalent interlayer sites. Below the transition, Ag⁺ ions preferentially occupy one site. Above it, the [CrSe₆] framework remains long-range ordered, but Ag⁺ ions hop randomly between the two equivalent sites, creating a nested structure of a rigid lattice framework and a fluid-like disordered sublattice.

Using high-resolution inelastic neutron scattering, first-principles calculations and molecular dynamics simulations, the researchers obtained the complete phonon dispersion of AgCrSe₂ single crystals. They found that transverse phonon behavior varies markedly with direction.

For transverse phonons propagating in-plane with in-plane atomic vibrations, the phonon dispersion shows a nesting feature that enhances phonon scattering. This enhanced scattering, together with the dynamic disorder of Ag⁺ ions, suppresses the quasiparticle character of these phonons, resulting in liquid-like behavior.

For transverse phonons propagating out-of-plane with in-plane atomic vibrations, scattering from the dynamic disorder of Ag⁺ ions is selective. These phonons gradually lose their quasiparticle character as the wavevector increases, exhibiting glass-like behavior. In contrast, transverse phonons propagating in-plane with out-of-plane atomic vibrations remain well-defined propagating modes even in the superionic state, displaying conventional crystal-like behavior.

Thermal conductivity measurements on single crystals reveal ultralow thermal conductivity and pronounced anisotropy in heat transport, consistent with the observed microscopic phonon dynamics.

These findings challenge conventional distinctions among states of matter at the level of lattice dynamics and offer new insights into controlling anisotropic phonon transport in solids.

Crystal structure of AgCrSe₂ and schematic illustrations of the characteristic transverse-phonon behavior in crystals, glasses, and liquids. (Image by IMR)