This Chapter Presents a Detailed Discussion of the Mössbauer Spectra of Iron Phosphate Containing Compounds and Minerals - Compounds that Are Increasingly Important in Understanding the Use of Lithium-Ion Phosphate in Battery Materials. an Extensive Discussion of the Thermodynamic Stability of These Compounds and Minerals is Followed by Several In-Depth Crystallographic Studies of the Various Structural Polymorphs of FePO 4. Then, the Paramagnetic and Magnetically Ordered Structures and Mössbauer Spectra of Α -FePO 4, Γ -FePO 4, Ζ -FePO 4, Several Metastable Polymorphs of FePO 4, and the Crystallographic Structures of Two Polymorphs of FePO 4 · 2H 2 O Are Discussed. the Mössbauer Spectral Properties of Various Oxides of Iron Phosphate Compounds and Minerals, Including their Temperature and Pressure Spectral Dependence of a -FePO 4, a -Fe 2 (PO 4)O, Fe 3 (PO 4)O 3, Fe 9 (PO 4)O 8, a -Fe 2 (PO 4), and Fe 9 (PO 4)O Are Covered in Detail. Finally, the Mössbauer Spectral Discussion Covers Some of the Difficulties Encountered in the Analysis of Magnetic Spectra; Particularly, the Confusion between Quadrupole Interaction, Quadrupole Shift, and Quadrupole Splitting in the Analysis of Magnetic Spectra is Illustrated in the Case of Η -Fe 0.9 Mn 0.1 PO 4.