Infrared and Raman spec troscopy have been used to investigate the Li-ion battery cathode materials Li-x (MnyFe1-y)PO4 for 0 < x <= 1 and y = 0.0, 0.5 and 1.0. In the Raman spectrum of Li(Mn0.5Fe0.5)PO4, the frequency of the pO(4)(3)(-) symmetric stretching mode is very similar to that found in LiFePO4 and LiMnPO4, suggesting that the P-O bond lengths are similar across the entire Li(MnyFe1-y)PO4 solid-solution series. The more localized v(1) and V-3 bands (symmetric and antisymmetric PO43- stretching modes, respectively) in the infrared spectra exhibit two-mode behavior for all three Li(MnyFe1-y)PO4 compositions studied, whereas the PO43- antisymmetric bending modes (V-4) and the Li+ cage mode frequencies vary with Mn2+ concentration. The vibrational modes in Li-x(Mn0.5Fe0.5)PO4 are sensitive to lithium content (x) and are, thus, used to investigate changes in the (Mn0.5Fe0.5)PO4 framework under cycling. The lack of spectral changes during the Fe2+/Fe3+ redox couple (3.7 V versus Li+/Li) suggests that the pO(4)(3-) anions have similar local environments in Li((Mn0.5Fe0.5(II))-Fe-(II))PO4 and Li-0.5((Mn0.5Fe0.5(III))-Fe-(II))PO4. Two-phase behavior is confirmed during Mn2+/Mn3+ redox couple (4.2 V), and the infrared spectrum for ((Mn0.5Fe0.5(III))-Fe-(III))PO4 is similar to that of FePO4 since both phases contain 4 anions coordinated only to trivalent transition metal ions. However, the pO(4)(3-) anions are much more distorted in ((Mn0.5Fe0.5(III))-Fe-(III))PO4 compared to FePO4, probably as a result of deformation of the MnO6 octahedra induced by Jahn-Teller active Mn3+ ions in the ((Mn0.5Fe0.5)-Fe-(III)((III)))PO4 structure. Raman spectra suggest that the carbon layer coating the Li(Mn0.5Fe0.5)PO4 particles (to improve electronic contact between particles) is predominantly composed of sp(2)-hybridized carbon atoms. (c) 2007 Elsevier B.V. All rights reserved.