Negative thermal expansion in NbF3 and NbOF2: A comparative theoretical study*

Abstract

Thermal expansion control is always an obstructive factor and challenging in high precision engineering field. Here, the negative thermal expansion of NbF₃ and NbOF₂ was predicted by first-principles calculation with density functional theory and the quasi-harmonic approximation (QHA). We studied the total charge density, thermal vibration, and lattice dynamic to investigate the thermal expansion mechanism. We found that the presence of O induced the w bond in NbOF₂, thus weakening the transverse vibration of F and O in NbOF₂, compared with the case of NbF₃. In this study, we proposed a way to tailor negative thermal expansion of metal fluorides by introducing the oxygen atoms. The present work not only predicts two NTE compounds, but also provides an insight on thermal expansion control by designing chemical bond type.