Resolving and Quantifying Nanoscaled Phases in Amorphous FeF

Article pubs.acs.org/JPCC

Resolving and Quantifying Nanoscaled Phases in Amorphous FeF3 by Pair Distribution Function and Mössbauer Spectroscopy Damien Dambournet,*,†,‡ Mathieu Duttine,†,‡,§ Karena W. Chapman,∥ Alain Wattiaux,§ Olaf Borkiewicz,∥ Peter J. Chupas,∥ Alain Demourgues,§ and Henri Groult†,‡ †

Sorbonne Universités, UPMC Université Paris 06, UMR 8234, PHENIX, F-75005, Paris, France CNRS, UMR 8234, PHENIX, F-75005, Paris, France § CNRS, Université Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France ∥ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: Probing the atomic structure of materials displaying a lack of long-range order has been a continuous challenge for the material science’s community. X-ray amorphous FeF3 has been shown to be a promising electrode material in Li and Na ion batteries. Providing structural information on this class of compounds is therefore of interest as it can help rationalize the material’s properties and further enabled its optimization. Herein, we used the pair distribution function and Mössbauer spectroscopy to provide unique insights into the atomic structure of amorphous FeF3. The results showed that amorphous FeF3 contained two phases built from corner-sharing of FeF6 octahedra. According to X-ray diffraction data, the PDF was successfully modeled based on two structural models related to the distorted ReO3 and the hexagonal-tungsten-bronze networks of FeF3. The lack of long-range order shown by conventional XRD data and PDF analysis was shown to arise mostly from disorder. This study provides detailed atomic structure with corresponding spectroscopic signature of amorphous phases. Quantitative analysis of both techniques indicated similar trends. This showed that our approach can be employed to determine the structure of other complex materials.

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hexagonal-tungsten-bronze network (Figure 1).2 Since the 1980s, these compounds have been widely investigated for their magnetic properties in relationship with their structural features.3 More recently, FeF3 has been considered as potential positive electrode material in lithium and sodium ion batteries owing to attractive redox properties.4 When used as an intercalation material, the theoretical capacity of FeF3 based on the exchange of 1 Li+/1e− per Fe can reach 237 mAh/g, a capacity higher than the 175 mAh/g of the commercially available LiFePO4. Even higher capacity can be accessed when FeF3 is used as conversion compound since it implied the reduction of trivalent to metallic iron thus achieving 737 mAh/g.5,6 Recently, X-ray amorphous FeF3 has shown very attractive electrochemical properties as compared to crystallized compounds.7,8 Although of particular interest, the rationalization of the physicochemical properties based on the structural features of amorphous compounds appeared to be much more complex as compared to bulk and crystallized materials.9 Amorphous compounds of FeF3 prepared by vapor deposition or fluorination methods have been extensively investigated by diffraction (X-ray and

INTRODUCTION

Atomic structures of trivalent iron fluorides are typically built from the assembly of FeF6 corner-sharing octahedra. These structures can be described by the connectivity of n-membered rings. The thermodynamically stable phase, denoted α-FeF3 (SG: R3̅c), displayed a 4-membered ring derived from the ReO3-type structure (Figure 1).1 The inclusion of interstitial water enabled the stabilization of 3-, 4-, and 6-membered rings as found in β-FeF3·(H2O)0.33 (SG: Cmcm) exhibiting the

Received: April 26, 2014 Revised: June 5, 2014 Published: June 6, 2014

Figure 1. Representation of the crystal structure of α-FeF3 and β-FeF3· (H2O)0.33 (along the c-axis). © 2014 American Chemical Society

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dx.doi.org/10.1021/jp504083g | J. Phys. Chem. C 2014, 118, 14039−14043

The Journal of Physical Chemistry C

Article

neutrons) data and Mössbauer spectroscopy.10 Leblanc et al. also proposed a description of amorphous FeF3·xHF based on a quasi-crystalline model consisting of a double layer of Fe3F11 enabling the fitting of the reduced atomic distribution function.11 The development of methods capable of providing detailed atomic structure with a coherence length that is below the conventional diffraction sensibility is a growing field as it concerns nanoscale materials that are often of technological interest. Those nanoscale compounds displaying atomic orderings that extend on nanometer-length scales include nanoparticles, bulk crystals with short-range order, and disordered crystals.12 The pair distribution function (PDF) analysis is a well-established technique capable of studying the local-intermediate structure of nanoscale, amorphous as well as crystalline materials. The Fourier transformation of the total structural function S(Q), obtained from a specialized scattering measurement, yields the PDF, G(r), a histogram of all the atom−atom distances within a sample, independent of crystallinity.13 The PDF data can be analyzed by directly comparing the correlations with reference samples,14 by refining the data using a Rietveld based method performed in the real space,15 by model-independent analysis of real-space features in the data,16 or by using constrained reverse-MonteCarlo.13 Fit of the PDF data can be based on crystalline models in a real-space analogue of the Rietveld method. When modeling PDF data using several phases, quantitative compositional information is provided by the scale factors. A recent study demonstrated the sensitivity of the PDF method in quantifying an amorphous component.17 Nevertheless, in some cases, PDF data of complex materials such as amorphous and/ or nanoscale compounds can be fitted using different models.18 Therefore, solving the atomic structure of complex materials required the use of a complementary technique to ensure an unbiased description of the solid.18,19 Local methods such as spectroscopy can be used as a complementary tool. For instance, Mössbauer spectroscopy can provide extensive and valuable information such as the oxidation state, the coordination mode, and the nature of ligands.20 Reconstruction of the Mössbauer spectrum can also enable semi-quantitative assessments. Finally, the determination of the magnetic temperature ordering allows to distinguish the coexistence of phases. Herein, we report on the complementary uses of Mössbauer spectroscopy and PDF to probe the structural features of amorphous FeF3. On one hand, Mö ssbauer spectroscopy allows one to probe the local environment of iron and the distinction of several phases in amorphous FeF3 through the characterization of their magnetic ordering temperature. On the other hand, their atomic structures were solved by fitting the PDF data. This study provides unique insight into the structure of an amorphous phase with detailed spectroscopic and structural data.

Figure 2. Typical X-ray diffraction powder pattern (λCu) of amorphous FeF3. Inset: Transmission electron micrograph.

The size of the particles is smaller than 8 nm. The high reactivity of metal fluoride toward the electron beam of the microscope prevents the observation of the crystalline state of the particles. High intensity synchrotron based X-ray (λ = 0.2128 Å) (Figure S1 in the Supporting Information) scattering measurements enabled detection of weak Bragg reflections belonging to the HTB-type structure (Q ∼ 10 nm−1), while the most intense line at Q ∼ 17 nm−1 is suggestive of the rhombohedral phase (α-FeF3). Nevertheless, anisotropic size effect impedes clear phase identification. No other phase such as the pyrochlore form of FeF3 was detected. Overall, synchrotron XRD allowed a better resolution of the Bragg peaks, but an accurate atomic structure of the sample cannot be obtained due to the broad Xray lines characteristic of an amorphous material and/or nanosized features. 57 Fe Mössbauer spectroscopy performed at room temperature on amorphous FeF3 is shown in Figure 3. The spectrum consists of a broad quadrupole doublet whose asymmetry is not due to textural effects. It was reconstructed using two distinct

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RESULTS AND DISCUSSION Amorphous FeF3 was prepared by thermal decomposition of βFeF3·3H2O (see Supporting Information). X-ray diffraction pattern of the as obtained FeF3 presents broad lines at around 25 and 50° (2θ, λCu) reflecting an absence of long-range order (Figure 2). Transmission electron microscopy (Figure 2, inset) showed that the thermal treatment induced a disintegration of the parent particle of FeF3·3H2O leading to the isolation of nanoparticles. This can be due to released gaseous species (e.g., H2O) inducing porosity during the decomposition process.21

Figure 3. Room temperature 57Fe Mössbauer spectrum of amorphous FeF3. (Inset: Quadrupole splitting distributions.) 14040



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PDF analysis was used to probe and identify the atomic structure of both phases. The PDF of amorphous FeF3 from high energy synchrotron data approached zero beyond 15 Å, confirming the absence of long-range order (Figure S2 in the Supporting Information). Based on the transmission electron microscopy (cf. inset Figure 2), no long-range order is expected beyond 50−80 Å due to size effects but the structural ordering evident in the PDF data is on a much shorter length scale. This suggests that the origin for the lack of long-range order is mostly due to disorder. Nevertheless, size effects that might be due to the existence of nanodomains within the nanoparticles cannot be entirely ruled out. Attempt to fit the experimental PDF using a single phase was unsuccessful. The residual to the single fit showed features at low r values consistent with the presence of a second phase in good agreement with synchrotron X-ray diffraction and Mössbauer data. A two-phase refinement was performed using different structures, and the best fit to the PDF data was achieved using structural models derived from the HTB type and ReO3 type of FeF3. To account for the absence of long-range order in amorphous FeF3, the refinement included optimized spherical particle-size parameters of 10 and 13 Å for the ReO3 and HTB phases, respectively. The final refinement shown in Figure 5 yields to a good fit to the experimental data

components. The isomer shift and (mean) quadrupole splitting values are characteristic of trivalent iron 6-fold coordinated to F− ions (Table 1). Each component was considered as the sum Table 1. Room Temperature 57Fe Mössbauer Parameters for Amorphous FeF3 and Those of the Precursor β-FeF3(H2O)3 and HTB-FeF3(H2O)0.33 taken from Leblanc et al.2a samples amorphous FeF3 β-FeF3(H2O)3 FeF3(H2O)0.33

δ (mm/s) (1) 0.40(1) (2) 0.46(1) 0.420(1) 0.435

ΔEQ (mm/s) b

0.67 0.51b 0.63b 0.637

Γ (mm/s)

area (%)

0.30(−) 0.30(−) 0.28(−) 0.51

59 41 100 100

δ = isomer shift relative to α-iron standard, 57Co(Rh) source; ΔEQ = quadrupole splitting; Γ = Lorentzian line width. bAverage value of quadrupole splitting distribution. a

of quadrupole doublets with Lorentzian shape (line width 0.3 mm/s) and distribution of quadrupole splitting values (see inset of Figure 3). To discern whether these two signals are due to different local environment or due to distinct phases, 57Fe Mössbauer spectra were measured at various temperatures (Figure 4). Two

Figure 5. PDF refinement of amorphous FeF3 (Rw = 13%) using a two-phase refinement (Rw = 13%, 1−10 Å). Contributions from the ReO3 and HTB structures to the fit have been included (black curves).

(Rw = 13%, 1−10 Å). PDF refinement estimated a percentage of 42(3)% and 58(3)% for the phases derived from the ReO3 and HTB types, respectively. Similar values were obtained from Mössbauer data highlighting the complementarity between both techniques.23 Assignments of the Mössbauer signatures are now being made possible. PDF analysis confirmed the occurrence of an HTB type phase which is characterized by the signal (1) detected by Mössbauer spectroscopy. The signal (2) can now be attributed to the ReO3-type phase (α-FeF3). A detailed atomic structure from both phases was extracted from the PDF refinement and compare with literature data (Table 2). The ReO3 component of amorphous FeF3 exhibited slight change as compared to bulk phase, with a 1% bond length contraction. The HTB component showed more complex structural deviations from the bulk structure. The orthorhombic HTB structure, which is built from two iron sites (Fe1 and Fe2) and four fluoride sites, showed larger cell volume and a broad

Figure 4. 57Fe Mössbauer spectra of amorphous FeF3 recorded at low temperatures.

sets of magnetic ordering temperatures were distinguished at about 30 K for the signal (1) and between 10 and 15 K for the signal (2), indicating the presence of two phases in amorphous FeF3. At room temperature and 4.2 K, Mössbauer parameters of the signal (1) are close to those found by Calage et al.3 for crystalline hydrolyzed HTB compound (Table S1 in the Supporting Information). This indicated that the signal (1) is related to the HTB-type structure whose presence was detected by high-energy XRD data. On the other hand, the signal (2) was more difficult to identify and the associated quadrupole doublet (at RT) and magnetic sextet (at 4.2 K) show hyperfine parameters close to those reported by Eibschütz et al.22 for another amorphous FeF3 compound (Table S1 in the Supporting Information). 14041



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After decomposition, the powder was placed in a glovebox for further characterization. Powder X-ray Diffraction Analysis. Powder diffraction patterns were recorded on PANalytical X’Pert (Cu Kα1 radiation) diffractometers in a Bragg−Brentano geometry (θ− 2θ). Transmission Electron Microscopy. Transmission electron microscopy analysis was performed using a JEOL 2010 UHR microscope operating at 200 kV equipped with a TCD camera. Mö ssbauer Spectroscopy. Mö ssbauer measurements were performed at room temperature using a constant acceleration Halder type spectrometer with a room temperature 57 Co source in transmission geometry. The velocity was calibrated using pure α-Fe as the standard material. The refinement of the Mössbauer spectra was performed using WinNormos software.32 Pair Distribution Function (PDF) Analysis. Amorphous FeF3 was loaded in a Kapton capillary which was sealed in a glovebox to prevent any contact with moisture. X-ray scattering data were measured at the 11-ID-B beamline at the Advanced Photon Source (Argonne National Laboratory). The high energy X-rays (λ = 0.2128 Å) were used in combination with a large amorphous-silicon based area detector to collect data to high values of momentum transfer Q ∼ 22 Å−1.33,34 Diffraction images were integrated within fit2D to obtained the onedimensional diffraction data.35 PDFs, G(r), were extracted from the data within PDFgetX2,36 after correcting for background and Compton scattering. Refinement of the PDF data was performed using the PDFgui program.14 Refined parameters were the instrument parameters, the scale factor, the lattice parameters, the atomic displacement parameters, and the atomic positions. The coherence length, accounting for the amorphous nature of the phases, was refined using the sp parameter in PDFgui.

Table 2. Structural Parameters Obtained from PDF Refinement of Amorphous FeF3 (denoted Am) for ReO3 and HTB Componentsa Am a (Å) d (Fe−F) (Å) a (Å) b (Å) c (Å) V (Å3) d (Fe−F) (Å) 4 × Fe1−F2 2 × Fe1−F3 2 × Fe2−F1 2 × Fe2−F2 2 × Fe2−F4 ⟨Fe−F⟩ a

ReO3 Type (SG: R3̅c) 5.27(1) 1.905 HTB Type (SG: Cmcm) 7.55(3) 12.73(7) 7.48(3) 718.9 2.109(30) 1.874(9) 1.995(20) 1.931(29) 1.948(26) 1.982

Cryst 5.362 1.923 7.423 12.730 7.526 711.17 1.948 1.945 1.945 1.945 1.949 1.946

Comparison with crystallized (denoted Cryst) homologues.

distribution of interatomic distances. This can be due to a finite size effect and/or due to the presence of OH groups substituting F− anions. Thermal treatment of the precursor FeF3·3H2O under inert atmosphere at 600 °C led to a phase mixture containing α-FeF3 and Fe2O3 (Figure S3 in the Supporting Information) revealing that hydrolysis reaction occurred during the decomposition of the precursor. The presence of OH groups substituting fluoride anions is also suggested by a broad distribution of Fe−F interatomic distances (Table 2). Bond lengths at around 2 Å are characteristic of Fe3+−O/OH, while more electronegative anion leads to shorter interatomic distances at 1.9−1.95 Å.24 Moreover, the low temperature 57Fe Mössbauer isomer shift of the HTB component, e.g., δ = 0.49 mm/s at 4.2 K, is similar to the one measured for hydrolyzed HTB (Table S2 in the Supporting Information, values obtained at 4.2 K),3 confirming the presence of OH groups substituting fluoride anions. Their presence might also be responsible for the observed phase mixture since OH groups have been shown to stabilize 3- and 6-membered rings as found in HTB type structure.25,26

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ASSOCIATED CONTENT

S Supporting Information *

Additional XRD, PDF, and Mössbauer data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

CONCLUSION In summary, the combination of PDF and Mössbauer analysis has been used to provide unique insights into the nature, the atomic structure, and spectroscopic signatures of amorphous FeF3. This study can help rationalize the electrochemical activity of amorphous electrode by identifying the atomic structural arrangement. In the present study, the ReO3‑ and HTB-type structures are known to be electrochemically active.6,27−30 Although the nanoscale is known to enhance the electrochemical activity, the role of structural disorder on the redox properties needs to be further investigated as it has been shown to provide a new route for the development of advanced cathode materials.31

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007−2013) under REA Grant Agreement No. [321879] (FLUOSYNES). Work done at Argonne and use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, were supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

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MATERIALS AND METHODS Synthesis of Amorphous FeF3. Amorphous FeF3 was prepared by thermal decomposition of FeF3·3H2O. The latter was prepared by a microwave-assisted synthesis method. The decomposition was performed under air for 2 h at 130−150 °C.

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Resolving and Quantifying Nanoscaled Phases in Amorphous FeF

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