Mn4+ environment in layered Li[Mg0.5-xNixMn0.5]O2 oxides monitored by EPR spectroscopy

X-band and high-frequency EPR spectroscopy were used for studying the manganese environment in layered Li[MgxNi0.5-xMn0.5]O-2, 0 <= x <= 0.5. Both layered LiMg0.5Mn0.5O2 and monoclinic Li[Li1/3Mn2/3]O-2 oxides (containing Mn4+ ions only) were used as EPR standards. The EPR study was extended to the Ni-substituted analogues, where both Ni2+ and Mn4+ are paramagnetic. For LiMg0.5-xNixMn0.5O2 and Li[Li(1-2x)/3NixMn(2-x)/3]O-2, an EPR response from Mn4+ ions only was detected, while the Ni2+ ions remained EPR silent in the frequency range of 9.23-285 GHz. For the diamagnetically diluted oxides, LiMg0.25Ni0.25Mn0.5O2 and Li[Li0.10Ni0.35Mn0.55]O-2, two types of Mn4+ ions located in a mixed (Mn-Ni-Li)-environment and in a Ni-Mn environment, respectively, were registered by high-field experiments. In the X-band, comparative analysis of the EPR line width of Mn4+ ions permits to extract the composition of the first coordination sphere of Mn in layered LiMg0.5-xNixMn0.5O2 (0 <= x <= 0.5) and Li[Li(1-2x)/3 NixMn(2-x)/3]O-2 (x > 0.2). It was shown that a fraction of Mn4+ are in an environment resembling the ordered alpha,betatype arrangement in Li1-delta 1Ni delta 1[Li(1-2x)/3+delta 1Ni2x/3-delta 1)(alpha)(Mn(2-x)/3Nix/3)(beta)]O-2 (where x = 2/7 and delta(1) = 0.06 were calculated), while the rest of Mn4+ are in the Ni,Mn-environment corresponding to the Li1-delta 2Ni delta 2[Ni1-yMny]O-2 (y < 1/2) composition with a statistical Ni,Mn distribution. For Li[Li(1-2x)/3NixMn(2-x)/3]O-2 with x <= 0.2, IR spectroscopy indicated that the ordered alpha,beta-type arrangement is retained upon Ni introduction into monoclinic Li[Li1/3Mn2/3]O-2. (C) 2005 Elsevier Inc. All rights reserved.