Negative thermal expansion in 2H CuScO2 originating from the cooperation of transverse thermal vibrations of Cu and O atoms

Abstract

Negative thermal expansion (NTE) originating from the transverse thermal vibrations of metal atoms is seldom reported, which is why the transparent conducting oxide 2H CuScO₂ is such a unique case. Using the density functional theory (DFT) and the quasi-harmonic approximation (QHA), the thermal properties of 2H CuScO₂ were investigated. The coefficient of thermal expansion (CTE) and the Grüneisen parameters of different vibrational modes were calculated, and we found that, up to a temperature of 200 K, 2H CuScO₂ displays a strong NTE behavior along the c-axis (i.e. along the O–Cu–O linkage), with an average CTE of approximately −2 × 10⁻⁶ K⁻¹. Our calculations are consistent with the experimental values. Furthermore, we reveal that low energy modes (0–150 cm⁻¹) originating from the cooperation of transverse vibrations of Cu and O atoms, which result in larger negative Grüneisen parameters and vibrational frequency softening phenomenon under pressure, are the main reasons for the NTE of such materials with a 2H structure. Our findings not only provide a better understanding of the NTE mechanism, but also present a report on detailed abnormal thermal properties in 2H CuScO₂ that have applications in electronic, electrochemical and optoelectronic devices.