The rechargeable lithium-ion cell is an advanced energy-storage system. However, high cost, safety hazards, and chemical instability prohibit its use in large-scale applications. An alternative cathode material, LiFePO(4), solves these problems, but has a kinetic problem involving strong electron/hole localization. One reason for this is believed to be the limited carrier density in the fixed monovalent Fe(3+)PO(4)/LiFe(2+)PO(4) two-phase electrode reaction in LixFePO4. Here, we provide experimental evidence that LixFePO4, at room temperature, can be described as a mixture of the Fe(3+)/Fe(2+) mixed-valent intermediate LialphaFePO4 and Li1-betaFePO4 phases. Using powder neutron diffraction, the site occupancy numbers for lithium in each phase were refined to be alpha=0.05 and 1-beta=0.89. The corresponding solid solution ranges outside the miscibility gap (0

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A series of synthetic heterosite-purpurite, (Mn y Fe 1-y )PO 4 (y < 0.8), with negligible disorder and impurities, was obtained by chemical oxidation of the well-crystallized isotypic tryphillite-lithiophilite series, Li(Mn y Fe 1-y )PO 4 (ordered olivine structure, space group Pnma). Comparative magnetic and X-ray/ neutron powder diffraction investigations of the two solid-solution lines were performed as a function of Mn content to increase understanding of the electrochemical activity loss of Mn 3+ /Mn 2+ in the Li x (Mn y Fe 1-y )PO 4 electrode system. Introducing Mn ions into the 4c site did not cause significant change in the local geometry of M 2+ O 6 and PO 4 polyhedra, while the M 3+ O 6 octahedra became severely distorted with an increase in the number of Jahn-Teller active Mn 3+ ions. The edge-sharing geometry of M 3+ O 6 and PO 4 polyhedra fixed the shared O3′-O3′ interatomic distance, causing selective strong elongation of the M 3+ -O3′ distance with small shrinkage of other M 3+ -O1, M 3+ -O2, and M 3+ -O3 bond lengths. The overall distortion of the MO 6 octahedra with M ) Mn 3+ was much larger than the corresponding change in the unit-cell orthorombicity and significantly increased asymmetry in the M-O-M superexchange interaction. All samples exhibited antiferromagnetism; however, the trivalent series had more than a sevenfold larger decrease in Neel temperature T N (from ca. 130 K at y ) 0 to ca. 50 K at y ) 0.8) compared to the divalent series (from ca. 52 K at y ) 0 to ca. 35 K at y )1) as a function of the Mn content y.

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