Extremely low phonon thermal conductivity of CdHgPb originating from quasi-rattling atoms

A comprehensive understanding of heat transfer necessitates the evaluation of both phonon and electronic contributions to thermal conductivity. Although the electronic thermal conductivity constitutes the dominant portion of the total thermal conductivity in conductors, the magnitude of phonon thermal conductivity is generally non-negligible, typically ranging from 1% to 40%. Through high-throughput screening of over 50,000 crystal structures using AI technology, we identified the metallic compound CdHgPb as having an exceptionally low phonon thermal conductivity, approximately 0.034 W mK-1. This value accounts for only 0.3% of the total thermal conductivity and can be considered negligible. As a result, it ranks as one of the materials with the lowest known phonon thermal conductivity, comparable even to that of air (approximately 0.025 W/mK under ambient conditions). To elucidate the origins of this anomaly, our analysis reveals that the anomalously low phonon thermal conductivity in CdHgPb is governed by the phonon lifetime, which is in turn determined by anharmonic phonon vibrations. Furthermore, the acoustic modes that contribute most significantly to the phonon thermal conductivity are associated primarily with the Hg atom. We conducted a detailed study of the Mean Square Displacement (MSD) and Crystal Orbital Hamilton Population (COHP) in the CdHgPb system to gain deeper insights. The results indicate that the Hg atom in CdHgPb acts as intrinsic quasi-rattlers, exhibiting behaviors akin to loosely bonded atoms, which play a pivotal role in reducing the phonon thermal conductivity to such an unusual extent. Our findings provide valuable insights into the mechanisms of heat conduction in metals, offering a broader perspective that could have significant implications for practical applications where low phonon thermal conductivity is desirable.