Metastable Solid-Solution Phases in the LiFePO[sub 4]∕FePO[sub 4] System

Solid-solution behavior in the system was investigated using X-ray powder diffraction. A substantial dependence upon crystal size and morphology was observed. Partially delithiated samples of hydrothermally synthesized olivine-type crystals containing only the two end-member phases were heated to form single-phase solid solutions. In samples with , a line phase with composition appeared as an intermediate during the transformation of the two-phase mixtures to single-phase . This phase then precipitated during cooling along with and persisted at ambient temperature. Transmission electron microscopy indicated that upon cooling, phase separation occurred within the crystals along the direction (space group Pnma ), with forming at the (010) surfaces. More complex behavior was found for samples with and for crystals of smaller dimensions.