文摘
A series of synthetic heterosite-purpurite, (MnyFe1-y)PO4 (y < 0.8), with negligible disorder andimpurities, was obtained by chemical oxidation of the well-crystallized isotypic tryphillite-lithiophiliteseries, Li(MnyFe1-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 functionof Mn content to increase understanding of the electrochemical activity loss of Mn3+/Mn2+ in theLix(MnyFe1-y)PO4 electrode system. Introducing Mn ions into the 4c site did not cause significant changein the local geometry of M2+O6 and PO4 polyhedra, while the M3+O6 octahedra became severely distortedwith an increase in the number of Jahn-Teller active Mn3+ ions. The edge-sharing geometry of M3+O6and PO4 polyhedra fixed the shared O3'-O3' interatomic distance, causing selective strong elongationof the M3+-O3' distance with small shrinkage of other M3+-O1, M3+-O2, and M3+-O3 bond lengths.The overall distortion of the MO6 octahedra with M = Mn3+ was much larger than the correspondingchange in the unit-cell orthorombicity and significantly increased asymmetry in the M-O-Msuperexchange interaction. All samples exhibited antiferromagnetism; however, the trivalent series hadmore than a sevenfold larger decrease in Neel temperature TN (from ca. 130 K at y = 0 to ca. 50 K aty = 0.8) compared to the divalent series (from ca. 52 K at y = 0 to ca. 35 K at y =1) as a function ofthe Mn content y.