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

Abstract

AbstractIn transition metal (M) compounds, the partial substitution of the host transition metal (Mh) to guest one (Mg) is effective to improve the functionality. To microscopically comprehend the substitution effect, degree of distribution of Mg is crucial. Here, we propose that a systematic EXAFS analysis against the Mg concentration can reveal the spatial distribution of Mg. We chose NaCo1−xFexO2 as a prototypical M compound and investigated the local intermetal distance around the guest Fe [dFe–M(x)] against Fe concentration (x). dFe–M(x) steeply increased with x, reflecting the larger ionic radius of high-spin Fe3+. The x-dependence of dFe–M(x) was analyzed by an empirical equation, $${d}_{\mathrm{F}\mathrm{e}-M}(x)=sxd_{\mathrm{F}\mathrm{e}-\mathrm{F}\mathrm{e}}+(1-sx)d_{\mathrm{F}\mathrm{e}-\mathrm{C}\mathrm{o}}$$dFe-M(x)=sxdFe-Fe+(1-sx)dFe-Co, where dFe–Fe and dFe–Co are the Fe–Fe and Co–Fe distances, respectively. The parameter s represents degree of distribution of Fe; s = 1, > 1, < 1 are for random, attractive, and repulsive distribution, respectively. The obtained s value (= 4.8) indicates aggregation tendency of guest Fe.