Solid-solution behavior in the LiFePO4-FePO4 system was investigated using X-ray powder diffraction. A substantial dependence upon crystal size and morphology was observed. Partially delithiated samples of 4 x 2 x 0.2 mu m hydrothermally synthesized olivine-type LixFePO4 crystals containing only the two end-member phases were heated to form single-phase solid solutions. In samples with x > 0.6, a line phase with composition LiyFePO4 (y = 0.60 +/- 0.04) appeared as an intermediate during the transformation of the two-phase mixtures to single-phase LixFePO4. This phase then precipitated during cooling along with LiFePO4 and persisted at ambient temperature. Transmission electron microscopy indicated that upon cooling, phase separation occurred within the crystals along the b direction (space group Pnma), with LiFePO4 forming at the (010) surfaces. More complex behavior was found for samples with x < 0.6 and for crystals of smaller dimensions. (C) 2007 The Electrochemical Society.