Negative Thermal Expansion in Cuprite-Type M₂X (M = Cu, Ag, Au; X = O, S): A Quasi-Harmonic Lattice Dynamics Study

Understanding the mechanisms of negative thermal expansion (NTE) in solids is crucial for controlling this anomalous thermal response. In this work, we investigate NTE in the cuprite-type M2X family (M = Cu, Ag, Au; X = O, S) by first-principles quasi-harmonic lattice dynamics simulations. Au2S has a much more pronounced NTE effect than Cu2O and Ag2O in experimentally available structures, while the predicted Cu2S exhibits the most pronounced NTE response within this family. Their predicted NTE is dominated by low-frequency optical and transverse phonon modes, which drives the deformation of [XM4] tetrahedra, and arises from the competition between transverse and longitudinal vibrations in bridging atoms. Notably, we establish a characteristic ratio of transverse to longitudinal vibration permissibility that shows nearly linear correlation with NTE magnitude across this family. This work clarifies structure–property relationships for NTE in cuprite-type materials and reveals fundamental insights to guide the development of NTE materials.