Comparison of the electronic structures of four crystalline forms of FePO$_{4}$.

The purpose of this study is to investigate the stability of FePO$_{4}$, the fully charged form of the Li ion battery material, LiFePO$_{4}$. The electronic structures of four phases of FePO$_{4}$, having the CrVO$_{4}$,\footnote{J. P. Attfield and co-workers, {\em{J. Solid State Chem.}} {\bf{57}}, 357 (1985)} quartz,\footnote{H. N. Ng and C. Calvo, {\em{Can. J. Chem.}} {\bf{53}}, 2064 (1975)} monoclinic,\footnote{ Y. Song and co-workers, {\em{Inorg. Chem.}} {\bf{41}}, 5778 (2002)} and olivine\footnote{A. S. Andersson and co-workers, {\em{Solid State Ionics}} {\bf{130}}, 41 (2000)} structures are studied using density functional theory with LSDA and GGA exchange-correlation functionals. A ferromagnetic spin configuration is chosen for all of the FePO$_{4}$ calculations. The band structures, density of states, and total energies are compared among all four phases. The results show that the CrVO$_{4}$ phase is metallic while the other phases are insulating. The band gaps of the insulating phases are found to increase with decreasing density of the material.