A first-principle study of the effect of OH− doping on the elastic constants and electronic structure of HTB-FeF3

Abstract

FeF₃ with a hexagonal-tungsten-bronze structure (HTB-FeF₃) belongs to one type of promising cathode material for Li-ion batteries. However, poor conductivity has hindered its use for further practice. Moreover, its fundamental information, such as its geometrical structure, elastic properties and electronic structure, can rarely be found. In this study, first-principles calculations were implemented to perform a systematic investigation about the effect of OH⁻ doping on orthorhombic HTB-FeF₃. First, the single crystal elastic constants of HTB-FeF_3−x(OH)_x (x = 0, 0.083, 0.167, 0.333, 0.667, 0.833) were successfully obtained from the energy-strain curve calculations. The bulk and Young's modulus, as well as the Poisson's ratio for ideal polycrystalline HTB-FeF_3−x(OH)_x were calculated. The directionally dependent Young's modulus of HTB-FeF_2.167(OH)_0.833 and HTB-FeF₃ was also further calculated to analyze their elastic anisotropy. These results show that the partial substitution of OH⁻ for F⁻ ions can improve the elastic property of HTB-FeF₃. Moreover, HTB-FeF_3−x(OH)_x can be classified as a ductile material. Furthermore, the ductility of polycrystalline HTB-FeF_3−x(OH)_x can be enhanced with increasing OH⁻ doping concentrations. In order to clarify the mechanism of OH⁻ doping on the orthorhombic HTB-FeF₃, the crystal structure and chemical bonding properties of the orthorhombic FeF_2.167(OH)_0.833 and HTB-FeF₃ crystals were further investigated. The calculated results indicate that strong O–H⋯F hydrogen bonds play a crucial role on the strengthening effect in HTB-FeF_2.167(OH)_0.833. Moreover, OH⁻ doping can improve the electronic conductivity of HTB-FeF₃.