Magnetism in olivine-type LiCo_1-xFe_xPO₄ cathode materials: Bridging theory and experiment

In the current paper, we present a non-aqueous sol-gel synthesis of olivine type LiCo_1-xFe_xPO₄ compounds (x = 0.00, 0.25, 0.50, 0.75, 1.00). The magnetic properties of the olivines are measured experimentally and calculated using first-principles theory. Specifically, the electronic and magnetic properties are studied in detail with standard density functional theory (DFT), as well as by including spin-orbit coupling (SOC), which couples the spin to the crystal structure. We find that the Co²⁺ ions exhibit strong orbital moment in the pure LiCoPO₄ system, which is partially quenched upon substitution of Co²⁺ by Fe²⁺. Interestingly, we also observe a non-negligible orbital moment on the Fe²⁺ ion. We underscore that the inclusion of SOC in the calculations is essential to obtain qualitative agreement with the observed effective magnetic moments. Additionally, Wannier functions were used to understand the experimentally observed rising trend in the Néel temperature, which is directly related to the magnetic exchange interaction paths in the materials. We suggest that out of layer M-O-P-O-M magnetic interactions (J) are present in the studied materials. The current findings shed light on important differences observed in the electrochemistry of the cathode material LiCoPO₄ compared to the already mature olivine material LiFePO₄.