Comparison of the electronic structures of four crystalline phases of FePO₄

LiFePO₄ in the olivine structure is a promising cathode material for Li ion batteries. During normal battery operation, an olivine form of FePO₄ is produced. In addition to the olivine form, FePO₄ is known to form in a quartzlike structure, a high pressure CrVO₄-like structure, and a monoclinic structure. We report the results of a detailed density functional study of the electronic structures and total energies of these four crystalline structures of FePO₄. Partial density of states analysis of the four materials finds them all to be characterized by strong hybridization between the Fe and O contributions throughout their upper valence bands, consistent with recent x-ray spectroscopy studies of olivine FePO₄. Results obtained using the local density approximation for the exchange-correlation functional find the olivine structure to be more stable than the quartzlike structure by 0.1 eV, which is in good agreement with recent calorimetry experiments.