In the present work, the oxidation and spin state of Fe and the local structure around Fe in the supercapacitor birnessite with different contents of the Fe dopant were investigated using Mossbauer spectroscopy. It was found that Fe ions were exclusively present as high spin Fe3+ in octahedral coordination with about 70% iron occupying the Mn3+ positions and about 30% iron occupying the Mn4+ positions in the [MnO6] octahedra for all Fe-doped birnessite samples. Based on these new findings, the trend of typical cell parameters, selected bond lengths of the Fe-doped birnessites and their corresponding quadrupole splittings in the Mossbauer spectra were well explained by considering both the weakened Jahn–Teller effect during the replacement of Mn3+ by Fe3+ and the expansion of octahedra during the replacement of Mn4+ by Fe3+. The present work offers some new insights into the understanding of the mechanism of the heterogeneous atomic doping on the crystal structure of birnessite, with importance for both mineralogy and material science.

中文翻译：

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57Fe Mössbauer 光谱研究 Fe 掺杂的超级电容器水钠锰矿，$$ ({\text{Na}},{\text{K}})_{x} {\text{Mn}}^{4 + }_{2 - x} {\text{Mn}}^{3 + }_{x} {\text{O}}_{4} \cdot 1.5{\text{H}}_{2} {\text{O}} $$

在目前的工作中，使用穆斯堡尔光谱研究了具有不同 Fe 掺杂剂含量的超级电容器水钠锰矿中 Fe 的氧化和自旋态以及 Fe 周围的局部结构。发现 Fe 离子仅以八面体配位中的高自旋 Fe3+ 形式存在，其中约 70% 的铁占据 Mn3+ 位置，约 30% 的铁占据 [MnO6] 八面体中所有 Fe 掺杂的水钠锰矿样品中的 Mn4+ 位置。基于这些新发现，通过考虑在 Fe3+ 取代 Mn3+ 过程中减弱的 Jahn-Teller 效应，可以很好地解释典型电池参数的趋势，Fe 掺杂的水钠锰矿的选定键长及其在 Mossbauer 光谱中相应的四极分裂Fe3+ 置换 Mn4+ 过程中八面体的膨胀。