Phase Stability, Strong Four-Phonon Scattering, and Low Lattice Thermal Conductivity in Superatom-Based Superionic Conductor Na₃OBH₄

Superatom-based superionic conductors are of current interest due to their promising applications in solid-state electrolytes for rechargeable batteries. However, much less attention has been paid to their thermal properties, which are vital for safety and performance. Motivated by the recent synthesis of superatom-based superionic conductor Na3OBH4 consisting of superhalogen cluster BH4, we systematically investigate its lattice dynamics and thermal conductivity using the density functional theory combined with a self-consistent phonon approach. We reveal the bonding hierarchy features by studying the electron localization function and potential energy surface and further unveil the rattling effect of the BH4 superatom, which introduces strong quartic anharmonicity and induces soft phonon modes in low temperatures by assisting Na displacements, thus calling for the necessity of quartic renormalization and four-phonon scattering in calculating the lattice thermal conductivity. We find that the contribution of four-phonon processes to the lattice thermal conductivity increases from 13 to 32% when the temperature rises from 200 to 400 K. At room temperature (300 K), the phonon scattering phase space is enlarged by 133% due to the four-phonon interactions, and the lattice thermal conductivity is evaluated to be 5.34 W/mK, reduced by 24% as compared with a value of 6.99 W/mK involving three-phonon scattering only. These findings provide a better understanding of the lattice stability and thermal transport properties of superionic conductor Na3OBH4, shedding light on the role of strong quartic anharmonicity played in superatom-based materials.