Magnetic phase transition and relaxation effects in lithium iron phosphate

We report the observation of a para-antiferromagnetic transition at ~50K in lithium iron phosphate, LiFePO₄ through DC magnetic susceptibility and Mössbauer spectroscopy. The ferrous ion Fe²⁺ (3d⁶, ⁵D) in LiFePO₄ exhibits relaxation effects with a relaxation frequency ~1.076×10⁷s^-1 at 300K. The temperature dependence of the frequency suggests the origin of the relaxation is of the spin-lattice type. The quadrupole splitting at low temperatures indicates that the excited orbital states mix strongly to the orbital doublet ground state via spin-orbit coupling. Modified molecular field model analysis yields a saturation value for hyperfine field B_hf~125kOe. The anomaly in susceptibility and Mössbauer parameters below 27K may be ascribed to a contribution of orbital angular momentum. The high value of the asymmetry parameter η (~0.8) of the electric field gradient obtained in the antiferromagnetic regime indicates a strongly distorted octahedral oxygen neighborhood for the ferrous sites.