Subsolidus phase relations of Li₂O–FeO–P₂O₅ system and the solid solubility of Li_1+x Fe_1−x PO₄ compounds under Ar/H₂ atmosphere

The phase relation of Li₂O–FeO–P₂O₅ ternary system under the 95 %Ar + 5 %H₂ atmosphere has been systematically investigated by X-ray diffraction (XRD), and there exist 8 binary compounds, 4 ternary compounds, 2 two-phase regions, and 17 three-phase regions. No other new lithium ferrous phosphates can be existed in the Li₂O–FeO–P₂O₅ ternary system under the 95 %Ar + 5 %H₂ atmosphere. In this system, the Li_1+x Fe_1−x PO₄ solid solution phase with the homogeneous range of −0.15 ≤ x ≤ 0.06 is determined. Their corresponding lattice parameters are obtained by the refinement results, and the results show that the lattice parameters (a, b, c, V) vary linearly with the increasing amount of excess Li-ion (x > 0) or with the increasing amount of excess Fe-ion (x < 0). The phase diagram determined in this paper can provide more information about the phase relation of Li₂O–FeO–P₂O₅ ternary system and serve as a guide for the future investigation of lithium iron phosphates in this system.