Magnetic structure evolution of LiFe_1−xCo_xPO₄ olivines upon delithiation

We present a comprehensive study of the nuclear and magnetic structures of the mixed olivine solid solution series LiFe_1−xCo_xPO₄ as well as their delithiated counterparts, for metal concentrations x = 0.2 - 0.6. Through neutron powder diffraction studies, we find that the LiFe_1−xCo_xPO₄ series orders in the magnetic space group Pnma^′, consistent with their Li(Fe/Co)PO₄ parent structures. After chemical delithiation using peracetic acid, however, the removal of over 30% of the lithium from the lattice results in a transition from an A_y-type to an A_x-type antiferromagnetic structure. The magnetic space group consistent with A_x antiferromagnetism was found to be Pn^′m^′a^′. Interestingly, magnetic susceptibility measurements of both series reveal that delithiation increases the magnetic transition temperature, T_N, from 50 K to 115 K. We find through analysis of the diffraction data that the unit cell parameters change linearly as a function of x; consequently, the exchange pathways between the Fe/Co magnetic cations are affected by the delithiation. We discuss how exchange pathways, metal oxidation states, and d-orbital occupation all contribute to the observed magnetic symmetries and trends in T_N in both series.