Infrared-active optical phonons in LiFePO₄ single crystals

Infrared-active optical phonons were studied in olivine LiFePO₄ oriented single crystals by means of both rotating analyzer and rotating compensator spectroscopic ellipsometry in the spectral range between 50 and 1400 cm⁻¹. The eigenfrequencies, oscillator strengths, and broadenings of the phonon modes were determined from fits of the anisotropic harmonic oscillator model to the data. Optical phonons in a heterosite FePO₄ crystal were measured from the delithiated ab-surface of the LiFePO₄ crystal and compared with the phonon modes of the latter. Good agreement was found between experimental data and the results of solid-state hybrid density functional theory calculations for the phonon modes in both LiFePO₄ and FePO₄.