Mn 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 LiMg_xNi_0.5−xMn_0.5]O₂, 0?x?0.5. Both layered LiMg_0.5Mn_0.5O₂ and monoclinic LiLi_1/3Mn_2/3]O₂ 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 LiMg_0.5−xNi_xMn_0.5O₂ and LiLi_(1−2x)/3Ni_xMn_(2−x)/3]O₂, 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, LiMg_0.25Ni_0.25Mn_0.5O₂ and LiLi_0.10Ni_0.35Mn_0.55]O₂, 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 LiMg_0.5−xNi_xMn_0.5O₂ (0?x?0.5) and LiLi_(1−2x)/3Ni_xMn_(2−x)/3]O₂ (x>0.2). It was shown that a fraction of Mn⁴⁺ are in an environment resembling the ordered “α,β”-type arrangement in Li_1−δ1Ni_δ1Li_{(1−2x)/3+δ1}Ni_2x/3−δ1)_α(Mn_(2−x)/3Ni_x/3)_β]O₂ (where and δ₁=0.06 were calculated), while the rest of Mn⁴⁺ are in the Ni,Mn-environment corresponding to the Li_1−δ2Ni_δ2Ni_1−yMn_y]O₂ () composition with a statistical Ni,Mn distribution. For LiLi_(1−2x)/3Ni_xMn_(2−x)/3]O₂ with x?0.2, IR spectroscopy indicated that the ordered α,β-type arrangement is retained upon Ni introduction into monoclinic LiLi_1/3Mn_2/3]O₂.