Abstract
Pushing the intrinsic lattice thermal conductivity (LTC) in crystalline materials to lower bounds is crucial for fundamental materials research towards emerging technologies including thermoelectric energy conversion and thermal management in both hypersonic aircraft and next-generation turbine systems. However, in the ultralow LTC regime ( < 1 Wm-1K-1), the competition between propagative (particle-like) and coherent phonons—arising from off-diagonal components—poses a significant challenge in further reducing LTC. We perform quantitative analysis of 4700 materials using density functional theory (DFT), spanning all crystallographic groups, to elucidate the interplay between diagonal and off-diagonal phonon contributions. We identify a critical balance between these transport mechanisms, where intermediate phonon lifetimes ( ~ 1 ps) and slow group velocities ( ~ 1 km/s) collectively suppress both contributions, enabling ultralow LTC. Results from a large dataset of 31,058 structures by machine learning models strongly resemble the DFT trends of two-channel phonon transport. Leveraging these models, we screen 25,882 additional materials and confirm their properties with DFT, identifying 12 candidates with ultralow room-temperature LTC—including a record-low value of 0.132 Wm-1K-1. Our large-scale analysis reveals fundamental insights into dual-channel phonon transport, enabling rational design of ultralow LTC materials and accelerating the discovery of advanced phononic crystals with tailored thermal transport properties.
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Data availability
The DFT dataset containing 4700 structures used to train the ALIGNN models as well as the ALIGNN predicted phonon properties of 31,225 structures are shared in the supporting Excel files. The atomic structures with relevant structure information are provided in CIF format embedded in the Excel files. All data reported herein are available upon reasonable request to the corresponding author.
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Acknowledgements
This work was supported by the NSF (award number 2030128, 2110033, 2311202, 2320292) and supported in part by ADAPT in SC and SC EPSCoR (Grant No. 24-GC02). ADAPT in SC is funded by the NSF under award number OIA-2242812. R.R. acknowledges financial support by the Severo Ochoa Centres of Excellence Program under grant CEX2023-001263-S, and by the Agencia Estatal de Investigación under grant PID2024-162811NB-I00. Calculations were performed at Theia, an AI-focused HPC cluster at the University of South Carolina supported by NSF (award number 2320292), and at the Centro de Supercomputación de Galicia (CESGA) and at the Barcelona Supercomputing Center (BSC) within actions FI-2024-1-0012, FI-2024-2-0015, FI-2025-1-0010, and FI-2025-2-0015 of the Red Española de Supercomputación (RES).
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M.H. conveyed the idea and designed and supervised the study. G.J.S. provided fruitful discussion and insights into the problem. R.R. performed partial DFT calculations. A.R. performed the off-diagonal calculations. C.L. wrote the CSLD code for IFC fitting. J.O. and M.A. participated in establishing workflow and processing data. A.R. prepared the draft of the manuscript. R.R., G.J.S. and M.H. revised the manuscript. All authors contributed to discussions and interpretation of results in the manuscript.
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Rodriguez, A., Rurali, R., Lin, C. et al. Approaching lower bound of lattice thermal conductivity by simultaneously suppressing diagonal and off-diagonal phonon contributions. npj Comput Mater (2026). https://doi.org/10.1038/s41524-026-02018-9
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DOI: https://doi.org/10.1038/s41524-026-02018-9
