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

Abstract

in transition metal (M) compounds, the partial substitution of the host transition metal (M h) to guest one (M g) is effective to improve the functionality. To microscopically comprehend the substitution effect, degree of distribution of M g is crucial. Here, we propose that a systematic EXAFS analysis against the M g concentration can reveal the spatial distribution of M g. We chose NaCo 1−x fe x o 2 as a prototypical M compound and investigated the local intermetal distance around the guest Fe [d fe-M (x)] against Fe concentration (x). d fe-M (x) steeply increased with x, reflecting the larger ionic radius of high-spin Fe 3+. The x-dependence of d fe-M (x) was analyzed by an empirical equation, d Fe−M (x) = sxd Fe−Fe + (1 − sx)d Fe−Co , where d fe-fe and d fe-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.

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“…In addition, the intensity of the Fe–O scattering peak in the R -spacing is decreased upon discharging to 0 V (Figure S16). Compared with that of pristine Fe 3 O 4 @CNS (Figure b), the EXAFS spectrum at 0 V shows a new peak at about 3.21 Å (Figure c), which should be attributed to the Fe–Na scattering path . In addition, the contour plot has one intensity at 8.0 Å –1 rather than Fe–Fe interaction (6.5 Å –1 ) according to wavelet transform analysis, further indicating the presence of the Fe–Na scattering peak (Figure f).…”

“…Compared with that of pristine Fe 3 O 4 @CNS (Figure 5b), the EXAFS spectrum at 0 V shows a new peak at about 3.21 Å (Figure 5c), which should be attributed to the Fe−Na scattering path. 52 In addition, the contour plot has one intensity at 8.0 Å −1 rather than Fe−Fe interaction (6.5 Å −1 ) according to wavelet transform analysis, further indicating the presence of the Fe−Na scattering peak (Figure 5f). EXAFS fitting shows that the Fe−Na coordination number at the 0 V discharged state is 1.4 (Table S5).…”