Comments on misuse of iron-57 transmission Mössbauer spectroscopy for surface characterization in Nature Communications 16, 5989 (2025)

arising from W. Lian et al. Nature Communications 10.1038/s41467-025-60919-7 (2025).

In disagreement with the claim in ref. ¹, the presence of surface Fe^IV = O on FeS₂, synthetic pyrite, upon reaction with chlorite is not supported by the room temperature Mössbauer spectra shown. These spectra have been reanalyzed herein with a simpler model involving fewer parameters—a reanalysis that agrees with the philosophy of Occam’s razor, see Wikipedia. These results are consistent with the well-known insensitivity of ⁵⁷Fe transmission Mössbauer spectroscopy to surface iron detection and study. Additional problems in ref. ¹ are delineated in the supplementary information.

We have used the experimental Mössbauer spectral data for supplementary Fig. 38 published in ref. ¹ and provided by the Editor. All three spectra have been fitted over the entire velocity range of ±12 mm/s but the results are shown in Fig. 1 over the relevant velocity range of ±4 mm/s.

Fig. 1. Room temperature Mössbauer spectra.

FeS₂, A, top, R-FeS₂-ClO₂, B, middle, and R-FeS₂, C, bottom. The black pluses represent the experimental data, the black, blue, and red solid lines represent the total fit, the Fe^II and Fe^IV components, respectively. The red plus symbols, at the top with their linear fit, are the residuals of the fits shown just below. In the left panel, the spectra have been fitted as proposed in ref. ¹, whereas in the right panel, the spectra have been fitted with a single quadrupole doublet. The corresponding fit parameters are given herein in Table 1.

Mössbauer spectrum A in supplementary Fig. 38 in ref. ¹ of the original FeS₂, synthetic pyrite, is shown at the top of Fig. 1. Our first fitting attempt of this spectrum used the parameters reported in supplementary Table 5 in ref. ¹ or in Table 1 in Lian et al.’s reply to this comment, the latter table contains at least some uncertainties. Only two parameters, the baseline and the total spectral absorption area have been fitted herein and a χ² of 0.964 has been obtained, indicating a satisfactory fit. Subsequently, the isomer shift and quadrupole splitting, and finally the linewidth have been fitted. The result of this fit is shown in the top of Fig. 1 and the five adjusted parameters with their uncertainties are given in Table 1 as shown below. This fit and these parameters agree with those presented for FeS₂, A, in Table 1 in Lian et al.’s reply to this comment.

Table 1.

The room temperature Mössbauer spectral parameters^a

Absorber, Spectrum label	δb, mm/s	ΔEQ, mm/s	Γc, mm/s	Textured	Area, %	Assignment	Total area, (%ε)x(mm/s)	χ2
FeS2, A	0.166 (6)	0.63 (1)	0.45 (1)	2	100	Fe(II)	2.72 (5)	0.969
R-FeS2-ClO2, Be	0.182 (2)	0.650 (3)	0.384 (4)	2.10 (4)	100	Fe(II)	29.5 (2)	1.107
R-FeS2, Ce	0.205 (1)	0.657 (2)	0.366 (3)	2.09 (3)	100	Fe(II)	26.9 (1)	1.182
R- FeS2-ClO2, Bf	0.186 (3)	0.660 (5)	0.382 (5)	2	93 (3)	Fe(II)	29.4 (2)	1.124
	0.11 (3)	0.52 (6)	0.29 (6)	2	7 (3)	Fe(IV)
R-FeS2, Cf	0.206 (2)	0.67 (1)	0.371 (5)	2	95 (5)	Fe(II)	26.9 (2)	1.159
	0.179 (9)	0.48 (3)	0.17 (7)	2	5 (5)	Fe(IV)

^aThe statistical uncertainties are given in parentheses.

^bThe isomer shift reference is unknown but is probably Rh at room temperature. Add +0.12 mm/s to refer to α-iron at 300 K^3,4.

^cFull width at half-maximum.

^dThe absence of texture is given by a parameter equal to 2.

^eSingle doublet fit as obtained and preferred herein.

^fTwo doublet fit as obtained herein and starting from the parameters reported in ref. ¹ and in Lian et al.’s reply to this comment.

Unfortunately, a comparison of the obtained isomer shift and quadrupole splitting for FeS₂ with the literature values is made difficult, because the isomer shift reference material is not reported in ref. ¹. Isomer shifts relative to α-iron at room temperature of 0.31(2) mm/s at 294 K and 0.40(2) mm/s at 78 K were reported² for a natural sample of pyrite. Hence, it appears that the room temperature isomer shift of 0.166(6) mm/s given in Table 1 is not referred to α-iron at room temperature but most likely to room temperature rhodium³. Quadrupole splittings, ΔE_Q, of 0.61(2) mm/s at 294 K and 0.62(2) mm/s at 78 K were reported² for a natural sample of pyrite and agree well with the value at room temperature of 0.63(1) mm/s given in Table 1 herein.

Next, our first fitting attempt of spectrum B in supplementary Fig. 38 in ref. ¹ for R-FeS₂-ClO₂, where R means reacted, with two symmetric doublets, one major doublet assigned to Fe^II in FeS₂ and a minor doublet, with the parameters quoted in supplementary Table 5 in ref. ¹ or Table 1 in the reply to this comment, gave a preliminary fit with a χ² of 1.121. Subsequent fits with adjustments of both isomer shifts and both quadrupole splittings, followed by fitting both linewidths, and finally, fitting the percent area of the minor doublet, gave a good fit with a χ² of 1.124, a fit which is shown in the middle-left panel of Fig. 1 and whose nine parameters with their respective uncertainties are given in Table 1 herein.

A fitting procedure similar to that used for R-FeS₂-ClO₂, was followed for R-FeS₂ spectrum C in supplementary Fig. 38 in ref. ¹. The resulting fit is shown in the bottom left panel of Fig. 1 and its parameters with their respective uncertainties are given in Table 1 herein.

The comparison of the spectral parameters for R-FeS₂-ClO₂ and R-FeS₂, given in Table 1 herein with those given in Table 1 in Lian et al.’s reply to this comment, shows good agreement for the isomer shift, quadrupole splitting, and linewidth of the major doublet. However, the adjustment of both linewidths and the percent area of the minor doublet affected its isomer shift and quadrupole splitting, slightly beyond the uncertainties. The percent area of the minor doublet found herein agrees within uncertainties with those given in Table 1 in Lian et al.’s reply to this comment. In brief, the spectral parameters given in the lower part of Table 1 herein show that, when all nine parameters of the model are refined, the isomer shift and quadrupole splitting of the minor doublet tend to approach the isomer shift and quadrupole splitting of the major doublet. Further, the linewidth of the minor doublet is smaller than that of the major doublet, without any physical reason for this difference, and even becomes smaller than the Heisenberg linewidth for iron-57 of 0.195 mm/s⁴. These two observations indicate that the presence of the minor doublet is highly dubious and questionable.

The room temperature isomer shifts of 0.05(7) and 0.08(14) mm/s relative to an unreported reference material, quoted in ref. ¹ and the reply to this comment, are below the limit of 0.20 mm/s, relative to α-iron at room temperature⁵, needed to unquestionably conclude that the doublet is indicative of high-spin Fe^IV, see Fig. S1 in the supplementary information. Further, the discussion of the difference between 0.05 and 0.08 mm/s that appears on page 3 right column in ref. ¹ is simply wrong and the so-called “regularity of isomer shift value change in related literature” supplementary text 2 in Lian et al.’s reply to this comment contains no physical basis for this discussion. If Lian et al. believe that 0.05(7) mm/s is significantly smaller than 0.08(14) mm/s, a belief unsupported by the uncertainties, the only physically sound analysis of this difference could go as follows. The smaller isomer shift results from a larger s-electron density at the iron nucleus⁶, a larger density that could result from the 3d-electron probability density extending further from the iron nucleus and, we guess, in the mind of Lian et al. towards the electronegative coordinated Cl. Of course, we do not even believe the Fe^IV component is present in the spectra and hence, this discussion is purely philosophical.

Because of the small relative area and the large uncertainties on the parameters of the minor doublet in both Mössbauer spectra of R-FeS₂-ClO₂ and R-FeS₂, and because only a small asymmetry is observed in the two doublets shown in the middle and bottom panels in Fig. 1, the adequacy of fits with only one asymmetric doublet, as a result of texture in the absorber, was investigated. Successful fits of the room temperature Mössbauer spectra of R-FeS₂-ClO₂ and R-FeS₂ with only six parameters and respective χ² values of 1.107 and 1.182 are shown in the middle and bottom right panels of Fig. 1 and the corresponding parameters are given in the top portion of Table 1. Note that the Fe^IV doublet is simply not needed and that both χ² values are not significantly larger than those obtained with a two symmetric doublet fit and the residuals do not show any significant misfit.

The isomer shifts and quadrupole splittings are slightly different from those observed for the original FeS₂ synthetic pyrite sample, most likely as a result of the preparative treatment of R-FeS₂-ClO₂ and R-FeS₂. There seems to be an increasing trend, perhaps accidental, in isomer shift and quadrupole splitting from FeS₂ to R-FeS₂-ClO₂ and then to R-FeS₂. The linewidth, see Table 1, is smaller than is observed for the original FeS₂ sample fitted herein, and may indicate a better crystallinity of the reacted samples. In the fitting program used herein, the texture parameter is defined as the relative area, 2, of the Δm_I = 0 transitions in an ⁵⁷Fe Mössbauer sextet of a random powder absorber. Hence, a symmetric doublet has a texture parameter of 2 and a departure from 2 in the fitted texture parameter indicates a non-random powder absorber. The texture parameters of 2.10(4) and 2.09(3) given in Table 1 are barely significantly different from 2 and thus both absorbers are very close to random powders.

In the original Mössbauer spectral data provided by the Editor, the number of counts in the baseline of the three spectra is given. It is worth noting that the number of counts in the baseline for Mössbauer spectrum A is about 713,000, in spectrum B is about 42,000, and in spectrum C is about 80,000. These numbers do not appear to fit the experimental conditions described in the supplementary text 1 in Lian et al.’s reply to this comment. These authors indicate: “we put pure FeS₂ into the detection area for a short [never specified] detecting time” and this short detection time yielded 713,000 counts in the baseline. They continue: “The detection time for PA≡Fe^IV = O [on R-FeS₂-ClO₂] … PS≡Fe^IV = O on R-FeS₂ was elongated” but this longer counting time gave small number of counts, 42,000 and 80,000, in the baseline and a huge 25 percent absorption in the spectra, see Fig. 1 herein, middle and bottom panels. These small number of counts in the baseline and the huge percent absorption may only result from using thick to very thick absorbers. This very thickness of the absorbers makes totally impossible the observation of eventual changes at the surface and in the first ten atomic layers below the surface that Lian et al. claimed¹ to observe. The contribution of 10 atomic layers to the overall absorption area may be estimated to be 0.02 %, well below the typical detectable minimum 1% area in Mössbauer spectroscopy.

Another new, somewhat puzzling, information in Lian et al.’s reply to this comment, is: “the Mössbauer spectra of R-FeS₂ and R-FeS₂-ClO₂ were in-situ characterized by adding corresponding solutions onto the FeS₂”. This addition of solutions suggest that the FeS₂ absorber was horizontal and hence that the drive of the Mössbauer spectrometer was operating in vertical mode. This is possible but unusual and surely should be mentioned if it is the case. Further, in view of the change in thickness of the absorbers for the acquisition of the three spectra, the form and the thickness in mg of Fe per cm² of the absorbers must be reported. Without this information, this research cannot be reproduced as it should be.

In conclusion, from the fits with one and two doublets of the room temperature Mössbauer spectra of R-FeS₂-ClO₂ and R-FeS₂, the presence of a minor doublet assigned to surface Fe^IV = O cannot be confirmed. The one doublet fits are perfectly adequate and of the same quality as the two doublet fits. The hyperfine parameters obtained for the Fe^II doublet show only small changes from those of pyrite. Because transmission Mössbauer spectroscopy is not a surface technique, the conclusion drawn above applies to the bulk of the samples. Only conversion electron Mössbauer spectroscopy⁷ could provide some information about the presence of Fe^IV on the surface of the grains in the samples.

Finally, the presence of ≡Fe^IV = O in R-FeS₂ and R-FeS₂-ClO₂ is supported by peaks at 920.7 and 929.2 cm^–1, respectively, in their in-situ Raman spectra shown in Fig. 1a and in supplementary Fig. 39 in¹. These 920.7 and 929.2 cm^–1 bands are far above the typical 750–860 cm⁻¹ values for the Raman signature of ≡Fe^IV = O, involving a strong, characteristic stretching band. The 920.7 and 929.2 cm^–1 values are actually higher than the highest reported value of 885 cm^–1 for the stretching band of ≡Fe^IV = O in zeolites⁸, a high value that was explained by the absence of a trans-axial ligand. The 920.7 and 929.2 cm^–1 values suggest a short Fe^IV = O bond, but unfortunately, the EXAFS analysis shown in Fig. 1f, g in ref. ¹ does not include the Fe^IV = O bond. The authors of¹ must provide an explanation for these large values of 920.7 and 929.2 cm^–1 and compare with their theoretical calculations.

The seven-split EPR signal shown in ref. ¹ in supplementary Fig. 6 is obtained in solution and in conditions so different from the powder absorber used in Mössbauer and Raman spectroscopy that any attempt of comparison is worthless.

In addition to the scientific problems outlined above¹, contains additional editorial and scientific problems that are detailed in the supplementary material associated with this comment and in a posted comment on PubPeer.com.

Observations and conclusions: The room temperature ⁵⁷Fe transmission Mössbauer spectra of R-FeS₂-ClO₂ and R-FeS₂ may be perfectly well fit with a single quadrupole doublet assigned to low-spin Fe^II, see the right portion of Fig. 1, in spite of claims by the authors that a second doublet attributed to high-spin Fe^IV is required to fit the spectra, see Fig. 1d in ref. ¹ and the left portion of Fig. 1 herein. Further, the two spectra do not provide any indication that either Fe^II or Fe^IV is present on the surface of the FeS₂ treated pyrite. This latter observation is completely consistent with the well-known fact that ⁵⁷Fe transmission Mössbauer spectra are not suitable for the study of surface iron sites. Thus, the present authors are forced to conclude that paper¹ has provided no Mössbauer spectral knowledge of any “planar asymmetric surface Fe^IV = O synthesis with pyrite and chlorite for efficient oxygen atom transfer reactions.”

Author contributions

The two authors have equally contributed to this work.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewer(s) for their contribution to the peer review of this work.

Code availability

The fitting code for Fig. 1 is available from the authors upon request.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-75476-w.