Figure 3a shows the Fe K-edge XANES full spectra of YFM
xO (0 ≤
x ≤ 0.1) with Fe foil, FeO, Fe
2O
3, and Fe
3O
4 as standard compounds. The XANES spectra of pure and doped samples show an analogous pattern to each other, which substantiates the fact that Mn ions have occupied the Fe sites of YFO. The Fourier transformation of the EXAFS is also given. EXAFS features could provide useful information on both the short-range and the long-range orders (i.e., in the first shell and higher shell than the second).
Figure 3b shows the variation of the observed k
3-weighted EXAFS oscillation of YFM
xO (0 ≤
x ≤ 0.1) powders with standard compounds. The spectra of pure and Mn-doped samples are almost identical. The Fourier transforms of the k
3-weighted EXAFS spectra of YFM
xO (0 ≤
x ≤ 0.1) and reference samples are also shown in
Figure 3c. The first and the second neighbor distributions in distance are easier to separate from the other shells in the Fourier transform. For the YFM
xO (0 ≤
x ≤ 0.1) samples, there are some characteristic peaks in spectra: (1) There is a strong amplitude peak of about 1.54 Å, which corresponds to the Fe-O coordinate peak due to the first oxygen coordination sphere of Fe ions. (2) The second strong peak is about 3.29 Å, which is known as the Fe-Fe peak caused by the second rate of nearby metal ions. (3) The small intensity of other peaks is not yet clear. They are probably due to the multiple scattering processes in the first coordination shell. The magnified XANES spectra of YFM
xO (0 ≤
x ≤ 0.1) and reference samples are presented in
Figure 3d. The main peak in the spectrum consists of two parts: Pre-edge peak and post-edge peak. The pre-edge peak is usually related to quadrupole transition from 1
s core state to 3
d empty state, and contributions of dipolar transition originating from mixing of p with d orbitals, which is expected to be very weak for an Fe cation in an octahedral environment [
53,
54]. The invisible pre-edge peaks are observed in the spectra of both pure YFO and Mn-doped samples. The two post-edge peaks are attributed to the transfer of 2
p electrons in the oxygen 2
p band to the Fe 3
d orbital by a shakedown process [
55]. As seen in
Figure 3e, for all the powder samples, the absorption edge energies were close to that of the reference sample Fe
2O
3. The valence state of Fe in Fe
2O
3 is 3+, which means the Fe atoms in all YFM
xO samples have oxidation states of 3+. The enlargement of main peaks in the XANES spectrum of YFM
xO (0 ≤
x ≤ 0.1) samples is shown in
Figure 3f. It is well known that the pre-edge peak is a fingerprint of the octahedral coordination of Fe and the shifts of the pre-edge peak position towards higher energy with an increasing oxidation state [
56]. In our samples, there is no pre-edge peak position shift, which further proves Fe is in a 3+ valance state in all the samples. Our spectra show that the intensity of the pre-edge peaks changes as a function of Mn content. From the enlarged XANES spectra in
Figure 3f, it can be seen that, compared with the pure YFO, the intensity of the pre-edge peak is decreased for the Mn-doped sample with 0.025 <
x < 0.075 and the intensity slightly increases for the
x = 0.1 sample, which is due to the decrease in the symmetry of the Fe environment. A similar phenomenon has been observed in the other ABO
3 perovskite system [
57,
58]. The decreasing intensity in the pre-edge peak was caused by the 1
s–4
p dipole-allowed transition while the increasing intensity indicates the enhancement of the 1
s–3
d electric dipole-forbidden transition. The decrease in pre-edge intensity caused by Mn substitution also indicates the increase in local structure distortion around the Fe ions [
59]. The intensity of these two post-edge peaks first slightly increases and then decreases when
x = 0.1 (see
Figure 3f). This indicates that the 3
d–4
p transition and charge transfer from the O 2
p–Fe 3
d are enhanced with both low and high do** contents of Mn due to the loss of inversion octahedral symmetry of the oxygens around the Fe atoms [
60]. These evolutions indicate that the local geometry and structure of Fe have changed.
In addition to the XANES data above, further analysis is carried out using the EXAFS oscillations of YFM
xO (0 ≤
x ≤ 0.1) samples, and their theoretical fits are shown in
Figure 3g. It shows that the experimental data and fitted data are well matched in the spectra, which means the spectra are of good quality. The error noise is observed above ca. 9 Å
−1. Oscillations are still visible above ca. 12 Å
−1, being less intense at the higher K. This phenomenon may be related to the less symmetric environment around Fe cations.
Figure 3h shows the comparisons of the radial distribution functions between the experiment and fit for YFM
xO (0 ≤
x ≤ 0.1) samples. As presented in
Figure 3h, all samples show a strong peak at about 1.57 Å ascribed to the Fe-O scattering path in the first shell. There is a clear peak located at about 2.56 Å in the spectra. It could be recognized as a partial Fe atom, which coordinated with a Mn atom due to the electronic interaction. Moreover, the second dominant peak at about 3.30 Å refers to the Fe-Fe/Mn scattering path. Compared to the pure YFO, there is almost no shift of the peak position but the intensities of the Fe-O and Fe-Fe/Mn peaks are decreased as the Mn content increases. The reduction of the Fe-O peak intensity further indicates structural distortion, which is in line with the EXAFS analysis [
61]. The intensities of the Fe-Fe/Mn peaks also have the same trend, which may be related to the lower photoelectron scattering amplitude of Fe due to the Mn addition. These changes in the spectrum may be due to the Mn atoms becoming closer at high concentration levels, leading to inhomogeneous distribution in the system, which further changes the local environment of Fe atoms. Moreover, the wavelet transformation of Fe K-edge EXAFS plots is also provided in
Figure 3i. For both pure and Mn-doped samples, the signal from wavelet maxima near 5.8 Å
−1 can be associated with F-O bonds. Maximum intensities at 6 Å
−1 and 10.5 Å
−1 are attributed to the Fe-Fe/Mn bonds, further confirming the existence of Fe-Fe/Mn bonds in the system. The maximum at 10.5 Å
−1 is attributed to the Fe-Fe bond. These results are in good agreement with the EXAFS analysis.