Aggregation tendency of guest Fe in NaCo1?xFexO2 (x?<?0.1) as investigated by systematic EXAFS analysis

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  • Aggregation tendency of guest Fe in NaCo1−xFexO2 (x < 0.1) as investigated by systematic EXAFS analysis

Abstract

<jats:title>Abstract</jats:title><jats:p>In transition metal (<jats:italic>M</jats:italic>) compounds, the partial substitution of the host transition metal (<jats:italic>M</jats:italic><jats:sub>h</jats:sub>) to guest one (<jats:italic>M</jats:italic><jats:sub>g</jats:sub>) is effective to improve the functionality. To microscopically comprehend the substitution effect, degree of distribution of <jats:italic>M</jats:italic><jats:sub>g</jats:sub> is crucial. Here, we propose that a systematic EXAFS analysis against the <jats:italic>M</jats:italic><jats:sub>g</jats:sub> concentration can reveal the spatial distribution of <jats:italic>M</jats:italic><jats:sub>g</jats:sub>. We chose NaCo<jats:sub>1−<jats:italic>x</jats:italic></jats:sub>Fe<jats:sub><jats:italic>x</jats:italic></jats:sub>O<jats:sub>2</jats:sub> as a prototypical <jats:italic>M</jats:italic> compound and investigated the local intermetal distance around the guest Fe [<jats:italic>d</jats:italic><jats:sub>Fe–<jats:italic>M</jats:italic></jats:sub>(<jats:italic>x</jats:italic>)] against Fe concentration (<jats:italic>x</jats:italic>). <jats:italic>d</jats:italic><jats:sub>Fe–<jats:italic>M</jats:italic></jats:sub>(<jats:italic>x</jats:italic>) steeply increased with <jats:italic>x</jats:italic>, reflecting the larger ionic radius of high-spin Fe<jats:sup>3+</jats:sup>. The <jats:italic>x</jats:italic>-dependence of <jats:italic>d</jats:italic><jats:sub>Fe–<jats:italic>M</jats:italic></jats:sub>(<jats:italic>x</jats:italic>) was analyzed by an empirical equation, <jats:inline-formula><jats:alternatives><jats:tex-math>$${d}_{\mathrm{F}\mathrm{e}-M}(x)=sxd_{\mathrm{F}\mathrm{e}-\mathrm{F}\mathrm{e}}+(1-sx)d_{\mathrm{F}\mathrm{e}-\mathrm{C}\mathrm{o}}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>d</mml:mi><mml:mrow><mml:mi>F</mml:mi><mml:mi>e</mml:mi><mml:mo>-</mml:mo><mml:mi>M</mml:mi></mml:mrow></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>x</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mo>=</mml:mo><mml:mi>s</mml:mi><mml:mi>x</mml:mi><mml:msub><mml:mi>d</mml:mi><mml:mrow><mml:mi>F</mml:mi><mml:mi>e</mml:mi><mml:mo>-</mml:mo><mml:mi>F</mml:mi><mml:mi>e</mml:mi></mml:mrow></mml:msub><mml:mo>+</mml:mo><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>-</mml:mo><mml:mi>s</mml:mi><mml:mi>x</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:msub><mml:mi>d</mml:mi><mml:mrow><mml:mi>F</mml:mi><mml:mi>e</mml:mi><mml:mo>-</mml:mo><mml:mi>C</mml:mi><mml:mi>o</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math></jats:alternatives></jats:inline-formula>, where <jats:italic>d</jats:italic><jats:sub>Fe–Fe</jats:sub> and <jats:italic>d</jats:italic><jats:sub>Fe–Co</jats:sub> are the Fe–Fe and Co–Fe distances, respectively. The parameter <jats:italic>s</jats:italic> represents degree of distribution of Fe; <jats:italic>s</jats:italic> = 1, > 1, < 1 are for random, attractive, and repulsive distribution, respectively. The obtained <jats:italic>s</jats:italic> value (= 4.8) indicates aggregation tendency of guest Fe.</jats:p>

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