On May 9, 2026, Assistant Professor Chen Wang from the Institute for Advanced Study, Shenzhen University, has published a research paper entitled “Disentangling Mass‑Energy Coupling to Reveal the Intrinsic Lattice Thermal Conductivity of Superionic Cu2-xS” in the Journal of Physical Chemistry Letters, a Nature Index journal. This study addresses the strong mass‑energy coupling and the resulting Soret/Dufour effects in superionic copper sulfides. The research team developed a computational approach that combines the Schur complement framework with cepstrum analysis, successfully extracting the intrinsic lattice thermal conductivity under zero net ion current condition and clarifying the phonon-ion correlation effects in the Cu2-xS system. Chen Wang is the first author, Associate Professor Yue Chen from the Department of Mechanical Engineering, The University of Hong Kong, is the corresponding author, and the Institute for Advanced Study, Shenzhen University, is the first affiliation.
Superionic systems have become a hot topic in phonon thermal transport research due to their extremely low lattice thermal conductivity. However, the strong mass‑energy coupling in copper sulfides prevents the conventional Green‑Kubo method from obtaining converged lattice thermal conductivity values. Using a machine‑learning moment tensor potential combined with nanosecond‑scale equilibrium molecular dynamics simulations, the team discovered unique dynamical behavior in non‑stoichiometric Cu1.8S: the percolating vacancy network suppresses random hopping through confined segmental motion while simultaneously promoting correlated directional diffusion, leading to a Cu ion diffusion coefficient even lower than that of stoichiometric Cu2S. This strongly correlated motion generates huge fluctuations in the convective energy current, which is the fundamental reason for the divergence of the conventional Green‑Kubo integral.
By combining the Schur complement scheme with cepstrum analysis, the study removed the extrinsic thermal transport contribution arising from ion current in the frequency domain, thus obtaining the pure phononic lattice thermal conductivity under zero net ion current condition. The results show that after the SCCB correction, the intrinsic lattice thermal conductivities of Cu1.8S and Cu2S become comparable, indicating that the anomalously low or even negative apparent lattice thermal conductivity observed experimentally for Cu1.8S is actually an artifact caused by strong mass‑energy coupling. Furthermore, longitudinal/transverse current correlation analysis reveals that the transverse acoustic phonon branches do not “melt” in either compound; instead, the high-energy optical phonons in Cu1.8S become even more stable due to the enhanced charge contrast of the vacancy lattice. This finding challenges the conventional “phonon‑liquid” picture and provides new insight for independently regulating ionic and thermal transport properties in superionic conductors.
This work was supported by the National Natural Science Foundation of China (Grant No. 12504019), the Pearl River Talent Program of Guangdong Province (Grant No. ZJQNRC20241219163147045), and the Research Grants Council of Hong Kong (Grant Nos. C7002-22Y, N_HKU702/24, C6020-22GF, C1002-24Y).
Link: https://doi.org/10.1021/acs.jpclett.6c00679
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