Diffusive behavior in LiMPO4 with M = Fe, Co, Ni probed by muon-spin relaxation

In order to study the diffusive nature of lithium transition-metal phospho-olivines, we measured muon-spin relaxation (mu+SR) spectra for the polycrystalline LiMPO4 samples with M = Mn, Fe, Co, or Ni in the temperature range between 50 and 500 K. The mu+SR spectra under zero applied field are strongly affected by the magnetic moments of the 3d electrons in the M2+ ions so that, for LiMnPO4, it was difficult to detect the relaxation change caused by the diffusion due to the large Mn2+(S = 5/2) moments. However, diffusive behavior was clearly observed via the relaxation due to nuclear dipolar fields above similar to 150 K for LiFePO4, LiCoPO4, and LiNiPO4 as S decreased from 2 to 1. From the temperature dependence of the nuclear field fluctuation rate, self-diffusion coefficients of Li+ ions (D-Li) at 300 K and its activation energy (E-a) were estimated, respectively, as similar to 3.6(2) x 10(-10) cm(2)/s and E-a = 0.10(2) eV for LiFePO4, similar to 1.6(1) x 10(-10) cm(2)/s and E-a = 0.10(1) eV for LiCoPO4, and similar to 2.7(4) x 10(-10) cm(2)/s and E-a = 0.17(2) eV for LiNiPO4, assuming that the diffusing Li+ ions jump between the regular site and interstitial sites.