Impact of in-plane disorders on the thermal conductivity of AgCrSe$_2$

TL;DR

This study investigates how Au substitution in AgCrSe$_2$ enhances anharmonic vibrations of Ag ions, significantly reducing lattice thermal conductivity and improving thermoelectric potential through increased phonon scattering.

Contribution

It reveals that increased in-plane vibrational anharmonicity due to Au substitution is a key mechanism lowering thermal conductivity in AgCrSe$_2$.

Findings

Au substitution reduces thermal conductivity

Enhanced Ag ion vibrations increase anharmonicity

Shortened phonon lifetime due to increased scattering

Abstract

Superionic conductors have recently attracted renewed attention for their use as thermoelectric materials due to their extremely low lattice thermal conductivity. Of central interest is why the superionic conductors exhibit such low thermal conductivity, and competing mechanisms have been proposed thus far. In this study, we investigate the effects of Cu and Au substitution for Ag site on the crystal structure and thermal properties of AgCrSe$_2$, which exhibits superionic conduction of Ag ions. We show that Au substitution significantly reduces the lattice thermal conductivity of AgCrSe$_2$. Powder structure analysis using synchrotron x-ray diffraction reveals that Au substitution increases the anisotropic atomic displacement parameter of Ag ions along the $a$ and $b$ axes. This result indicates that the amplitude of in-plane vibrations is enhanced, which is attributed to increased anharmonicity in the potential energy around Ag ions. The enhanced vibrational amplitude also suggests a reduction in the force constants between Ag ions. Consequently, the enhanced anharmonicity not only shortens the phonon lifetime ($\tau$) by increasing phonon-phonon scattering, but also increases the number of low-energy phonons, which further contributes to the reduction of $\tau$. This anharmonicity mechanism is applicable to other superionic conductors exhibiting ultra-low thermal conductivity, promoting their widespread use as thermoelectric materials.