Theoretical study of negative thermal expansion mechanism of ZnF2

Abstract ZnF 2 is reported to exhibit negative thermal expansion (NTE) at lower temperatures very recently. In this article, we present the electronic and NTE properties of ZnF 2 using a first-principles calculation. Our results show that ZnF 2 is an insulator with a direct band gap and a strong hybridization occurs between Zn-3p, 4s and F-2p states. The related calculations on NTE properties are obtained within the quasi-harmonic approximation. The resulting relationship between volume and temperature confirms the NTE properties. Besides, we discuss the NTE mechanism in accordance to phonon vibrational modes. The phonon vibrational modes contributing to the NTE are singled out by Gruneisen parameters and all these modes are low-frequency optical phonons. The lowest frequency rigid unit mode (RUM) of ZnF 6 causes a rotary coupling between two adjacent octahedrons and makes the Zn–Zn distance shorter, which is most responsible for the NTE properties of ZnF 2 .