Low lattice thermal conductivity with two-channel thermal transport in the superatomic crystal PH4AlBr4

Designing novel crystalline materials composed of light and nontoxic elements, but with ultralow thermal conductivity insensitive to temperature, has been a long-standing challenge. One effective strategy is to utilize superatoms as building blocks to introduce hierarchical bonding and vibration for suppressing phonon velocity and enhancing high-order scattering. However, far fewer theoretical efforts have been made to understand the lattice dynamics and thermal transport in superatom-based materials. Herein, different from most of the existing works reported hitherto on atom-superatom hybrid systems, we carry out a comprehensive study on the phonon interaction and thermal transport in superatomic crystal PH4AlBr4 solely consisting of superatoms (superalkali and superhalogen), by employing homogeneous nonequilibrium molecular dynamics with active -learning potential, combined with density functional theory and unified theory of phonon thermal transport. We find that the supersalt PH4AlBr4 crystal exhibits amorphouslike ultralow lattice thermal conductivity of 0.329-0.286 W m-1 K-1 from 200 to 600 K. In particular, due to the strong quartic anharmonicity, the contribution of four-phonon scattering reaches 38% of total scattering rates at 300 K, while the phonon coherence's contribution is comparable with that of population, resulting from significant phonon localization. These theoretical findings demonstrate that superatom-assembled materials exhibit features distinguished from atom-based crystals and provide a unique platform for exploring ultralow thermal conductivity with enhanced two-channel thermal transport.

Automated summary

Designing novel crystalline materials with ultralow thermal conductivity that is insensitive to temperature has been a long-standing challenge. This study focuses on the phonon interaction and thermal transport in a superatomic crystal called PH4AlBr4, composed solely of superatoms. The results show that the supersalt PH4AlBr4 crystal exhibits amorphous-like ultralow lattice thermal conductivity, and the contribution of four-phonon scattering and phonon coherence play significant roles in the thermal transport. These findings demonstrate the unique features of superatom-assembled materials and provide insights for exploring ultralow thermal conductivity.