Anharmonicity and lattice thermal conductivity of negative thermal expansion materials Zn(CN)₂ and Cd(CN)₂ by first-principles calculations

Certain negative thermal expansion (NTE) materials have been reported to exhibit anomalous lattice thermal conductivity (LTC), making them particularly interesting for theoretical studies. In Zn(CN)₂ and Cd(CN)₂, cyanide bonds have induced great NTE behavior and anharmonicity, making us believe they would exhibit anomalous LTC. To investigate this, we employed first-principles calculations based on density functional theory and solved the linearized Boltzmann transport equation within the relaxation time approximation to calculate the LTC. Our analysis reveals that both Zn(CN)₂ and Cd(CN)₂ have low LTC, with Cd(CN)₂ exhibiting a notably anomalous glass-like behavior. Some phonons with long lifetimes, short mean free paths (MFPs), and strong anharmonicity contribute most to the anomalous LTC observed in both materials. Analysis of the phonon spectral functions and the weighted joint density of states reveals that three-phonon interactions are significant in both materials, impacting their LTCs, particularly in Cd(CN)₂.