Liquid, Glass and Crystal Phonons Coexist in One Solid

Below the transition, the silver ions settle into just one of those sites. Above it, the rigid chromium-selenium frame stays ordered, but the silver ions start hopping randomly between the two positions. The result is a solid containing a fluid-like, disordered sublattice embedded in a crystalline skeleton.

What did the experiments actually show?

The team combined high-resolution inelastic neutron scattering, first-principles calculations and molecular dynamics simulations to obtain the full phonon dispersion of AgCrSe₂ single crystals — essentially a complete map of how vibrations move through the material. The surprise was how strongly the behavior depended on direction:

• Liquid-like: transverse phonons traveling in-plane with in-plane atomic vibrations show a "nesting" feature in their dispersion that boosts scattering. Combined with the jumping silver ions, this destroys their particle-like character.
• Glass-like: transverse phonons traveling out-of-plane with in-plane vibrations experience selective scattering from the disordered ions. They gradually lose their quasiparticle character as their wavevector grows.
• Crystal-like: transverse phonons traveling in-plane with out-of-plane vibrations stay well-defined even in the superionic state, behaving like textbook crystal vibrations.

In plain terms, the same solid transports vibrations like a liquid along one direction, like a glass along another, and like an ordinary crystal along a third.

Why does ultralow heat transport matter?

Thermal conductivity measurements on the single crystals confirmed the microscopic picture: AgCrSe₂ shows ultralow thermal conductivity with pronounced anisotropy — heat moves at very different rates along different axes. That macroscopic behavior matches the direction-dependent phonon dynamics the neutron experiments revealed.

That's a useful combination for technology. Materials that barely conduct heat, and conduct it differently in different directions, are prized for thermoelectric devices, which convert waste heat into electricity. A design principle based on nested rigid-plus-fluid lattices could offer a new route to engineering such materials.

How solid are the conclusions?

The findings rest on a combination of direct measurement and simulation, and they concern one material, so caution is warranted before generalizing. Still, the study provides fresh experimental evidence for a unified theory of heat transport — one framework describing how crystals, glasses and liquids sit on a continuum rather than in separate boxes.

The work also challenges textbook distinctions among states of matter at the level of lattice dynamics. A crystal that vibrates like a liquid in one direction and like a perfect solid in another suggests the categories are less rigid than they appear — a point the IMR team argues the AgCrSe₂ data now demonstrate directly.

Whether the same triple-behavior trick appears in other superionic conductors remains an open question, and the researchers' framework points to new ways of controlling anisotropic phonon transport in solids.