Anharmonic lattice dynamics and superionic transition in AgCrSe₂.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid-State Chemistry

Background:

• Superionic conductors are crucial for thermoelectric energy conversion due to their intrinsically low lattice thermal conductivity.
• Understanding the atomic dynamics behind superionicity and ultralow thermal conductivity is essential but remains challenging.

Purpose of the Study:

• To comprehensively investigate the lattice dynamics and superionic diffusion mechanisms in superionic conductors.
• To elucidate the relationship between atomic dynamics, phonon behavior, and thermal conductivity.

Main Methods:

• Utilized energy- and momentum-resolved neutron and X-ray scattering techniques.
• Performed first-principles calculations to complement experimental findings.
• Analyzed lattice dynamics, phonon behavior, and ionic diffusion processes.

Main Results:

• Observed coexistence of long-wavelength transverse acoustic (TA) phonons with ultrafast silver ion diffusion in the superionic phase.
• Identified the breakdown of short-wavelength nondispersive TA phonons linked to Ag ion diffusion.
• Determined that strong phonon scattering from anharmonicity and Ag disorder causes intrinsically low thermal conductivity.
• Revealed that fast ionic diffusion originates from flat energy landscape basins and collective hopping behavior.

Conclusions:

• Settled unresolved questions regarding lattice dynamics and thermal conduction mechanisms in superionic conductors.
• Provided fundamental insights into the complex atomic dynamics governing superionic conductors.
• Highlighted the interplay between phonon behavior and ionic diffusion in determining thermal transport properties.