Electrochemical, magnetic, and structural investigation of the Li _x(Mn_yFe_1-y)PO₄ olivine phases

A series of synthetic heterosite-purpurite, (M_yFe _1-y)PO₄ (y < 0.8), with negligible disorder and impurities, was obtained by chemical oxidation of the well-crystallized isotypic tryphillite-lithiophilite series, Li(Mn_yFe _1-y)PO₄ (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 Mn³⁺/Mn ²⁺ in the Li_x(Mn_yFe_1-y)PO ₄ electrode system. Introducing Mn ions into the 4c site did not cause significant change in the local geometry of M²⁺O₆ and PO₄ polyhedra, while the M³⁺O₆ octahedra became severely distorted with an increase in the number of Jahn-Teller active Mn³⁺ ions. The edge-sharing geometry of M³⁺O₆ and PO₄ polyhedra fixed the shared O3′-O3′ interatomic distance, causing selective strong elongation of the M³⁺-O3′ distance with small shrinkage of other M³⁺-O1, M³⁺-O2, and M³⁺-O3 bond lengths. The overall distortion of the MO ₆ octahedra with M = Mn³⁺ 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.