Phase transition process and mechanism of LaOF in air: From experiment to theory

Abstract

Lanthanum oxyfluoride (LaOF) is widely used in multiple fields, such as optoelectronics, photocatalysis, all solid-state fluoride ion batteries, etc., and the requirements for its phase structure vary in different fields. However, there is currently no clear understanding of the phase transition process of LaOF in an air atmosphere. Therefore, in this article, LaOF with different phase structures was obtained by calcining LaCO3F in air. TG-DSC analysis, XRD, Raman, SEM, TEM, and first-principles calculations based on DFT were used to illustrate the phase transition mechanism and phase stability of LaOF in air. The experimental results indicate that when calcined in air, a rhombohedral phase (γ-LaOF) is obtained at 600 °C, a tetragonal phase (β-LaOF) is obtained at 750 °C, but at 925 °C, β-LaOF undergoes a phase transition again, transforming into γ-LaOF. In addition, the products begin to decompose into La2O3 when the calcination temperature reaches 1300 °C. Raman-assisted XRD data demonstrated the phase transition law of LaOF, namely γ-LaOF → β-LaOF → γ-LaOF → γ-LaOF + β-LaOF → γ-LaOF + La2O3, and γ-LaOF is more stable. The HSE functional calculation, based on first principles, was used to analyze the phase structures of three-dimensional LaOF. The results confirmed that the stability of γ-LaOF is higher than that of β-LaOF. Furthermore, the phonon spectrum calculation results indicate that the stability order is γ-LaOF > β-LaOF > α-LaOF. Additionally, first-principles molecular dynamics calculations were employed to explain the phase transition law of LaOF at different temperatures by examining the excess energy of γ-LaOF and β-LaOF.