Antisite disorder in the battery material LiFePO₄

We report detailed magnetometry and high-frequency electron spin resonance (HF-ESR) measurements which allow detailed investigation on Li-Fe antisite disorder in single-crystalline LiFePO₄, i.e., exchange of Fe²⁺ and Li⁺ ions. The data imply that magnetic moments of Fe²⁺ ions at Li positions do not participate in long-range antiferromagnetic order in LiFePO₄ but form quasifree moments. Anisotropy axes of the magnetic moments at antisite defects are attached to the main crystallographic directions. The local character of these moments is confirmed by associated linear resonance branches detected by HF-ESR studies. Magnetic anisotropy shows up in significant zero-field splittings of Δ = 220(3) GHz, Δ` ∼ 50 GHz, and a highly anisotropic g factor, i.e., g_a = 1.4, g_b = 2.0, and g_c = 6.3. We demonstrate a general method to precisely determine Fe-antisite disorder in LiFePO₄ from magnetic studies which implies a density of paramagnetic Fe²⁺ ions at Li positions of 0.53%.