Probing magnetic order in LiMPO4 (M = Ni, Co, Fe) and lithium diffusion in LixFePO4

Muon spin relaxation measurements are reported on three members of the Li xMPO 4 series. The magnetic properties of stoichiometric samples with M= Ni, Co, Fe were investigated at low temperature. In LiNiPO 4 we observe different forms of the muon decay asymmetry in the commensurate and incommensurate antiferromagnetic phases, accompanied by a change in the temperature dependence of the muon oscillation frequency. In LiCoPO 4 the form of the muon decay asymmetry indicates that the correlation between layers decreases as the Néel temperature is approached from below. LiFePO 4 shows more conventional behavior, typical for a three-dimensional antiferromagnet. Measurements on Li xFePO 4 with x=0.8,0.9, and 1 show evidence for lithium diffusion below ∼250 K and muon diffusion dominating the form of the relaxation at higher temperature. The thermally activated form of the observed hopping rate suggests an activation barrier for lithium diffusion of ∼100 meV and a diffusion constant of D Li∼10 -10to10 -9 cm2 s -1 at room temperature. © 2011 American Physical Society