First-principles calculations of the atomic structure and electronic states of LixFeF3

We calculate the atomic and electronic structures of trirutile-type LixFeF3 (x = 0, 0.25, 0.5, 0.75, and 1) by first-principles calculations and evaluate the relative stability among the optimized structures by energy analysis. Li0.5FeF3 is more stable than the three-phase coexistence of FeF3, FeF2, and LiF, whereas the other compositions are unstable. The analyses of the local electron density, local atomic volume, and local atomic configurations show that the formal valence of Fe atoms decreases from trivalent (3+) to divalent (2+) after Li insertion. In addition, we calculate Fe K-edge x-ray absorption near-edge structure (XANES) spectra in LixFeF3 and compare them with observed spectra. The calculated XANES spectra agree well with the corresponding observed spectra in areas such as the spectral shape and relative position of the main peaks associated with Fe3+ and Fe2+. In particular, partial XANES spectra of Fe3+ in LixFeF3, for x = 0.25, 0.5, and 0.75, have a specific peak between the main peaks, associated with Fe3+ and Fe2+. The detailed study reveals that the energy level and intensity ratio of the Fe3+ main peaks depend on the adjacent cation site of Fe.