Extreme phonon anharmonicity underpins superionic diffusion and ultralow thermal conductivity in argyrodite AgSnSe.

Ultralow thermal conductivity and fast ionic diffusion endow superionic materials with excellent performance both as thermoelectric converters and as solid-state electrolytes. Yet the correlation and interdependence between these two features remain unclear owing to a limited understanding of their complex atomic dynamics. Here we investigate ionic diffusion and lattice dynamics in argyrodite AgSnSe using synchrotron X-ray and neutron scattering techniques along with machine-learned molecular dynamics.

A two-dimensional type I superionic conductor.

Superionic conductors possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I superionic conductors (for example, AgI, AgSe and so on) are defined by a first-order transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a two-dimensional type I superionic conductor, α-KAgSe, by scattering techniques and complementary simulations.

Soft anharmonic phonons and ultralow thermal conductivity in Mg(Sb, Bi) thermoelectrics.

The candidate thermoelectric compounds MgSb and MgBi show excellent performance near ambient temperature, enabled by an anomalously low lattice thermal conductivity (κ) comparable to those of much heavier PbTe or BiTe Contrary to common mass-trend expectations, replacing Mg with heavier Ca or Yb yields a threefold increase in κ in CaMgSb and YbMgBi Here, we report a comprehensive analysis of phonons in the series Mg ( = Mg, Ca, and Yb; = Bi and Sb) based on inelastic neutron/x-ray scattering and first-principles simulations and show that the anomalously low κ of Mg has inherent phononic origins. We uncover a large phonon softening and flattening of low-energy transverse acoustic phonons in Mg compared to the ternary analogs and traced to a specific Mg- bond, which markedly enlarges the scattering phase-space, enabling the threefold tuning in κ These results provide key insights for manipulating phonon scattering without the traditional reliance on heavy elements.

High Thermoelectric Performance of AgSbPbSe Prepared by Fast Nonequilibrium Synthesis.

AgSbSe is a typical member of cubic I-V-VI semiconductors, which are known for their extremely low lattice thermal conductivity (κ). However, the low electrical conductivity of AgSbSe, below ∼10 S cm at room temperature, has hindered its thermoelectric performance. In this work, single-phase AgSbSe bulk samples with much higher electrical conductivity were synthesized via self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) for the first time.

Anharmonic lattice dynamics and superionic transition in AgCrSe.

Intrinsically low lattice thermal conductivity ([Formula: see text]) in superionic conductors is of great interest for energy conversion applications in thermoelectrics. Yet, the complex atomic dynamics leading to superionicity and ultralow thermal conductivity remain poorly understood. Here, we report a comprehensive study of the lattice dynamics and superionic diffusion in [Formula: see text] from energy- and momentum-resolved neutron and X-ray scattering techniques, combined with first-principles calculations.