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 LixMPO4 series. The magnetic properties of stoichiometric samples with M = Ni, Co, Fe were investigated at low temperature. In LiNiPO4 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 LiCoPO4 the form of the muon decay asymmetry indicates that the correlation between layers decreases as the Neel temperature is approached from below. LiFePO4 shows more conventional behavior, typical for a three-dimensional antiferromagnet. Measurements on LixFePO4 with x = 0.8,0.9, and 1 show evidence for lithium diffusion below similar to 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 similar to 100 meV and a diffusion constant of D-Li similar to 10(-10) to 10(-9) cm(2) s(-1) at room temperature.