Article pubs.acs.org/cm
A Solution for Solution-Produced β‑FeSe: Elucidating and Overcoming Factors that Prevent Superconductivity Joshua T. Greenfield, Saeed Kamali, Kathleen Lee, and Kirill Kovnir* Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States S Supporting Information *
ABSTRACT: A new low-temperature solvothermal synthesis of superconducting β-FeSe has been developed using elemental iron and selenium as starting materials. We have shown that syntheses performed in aerobic conditions resulted in the formation of nonsuperconducting antiferromagnetic βFeSe, whereas syntheses performed in ultra-dry and oxygenfree conditions produced superconducting β-FeSe. Detailed characterization of both types of samples with magnetometry, resistivity, Mössbauer spectroscopy, synchrotron X-ray and neutron powder diffraction, and pair-distribution function analysis uncovered factors that trigger the loss of superconductivity in β-FeSe. Vacancies in the iron sublattice and the incorporation of disordered oxygen-containing species are typical for nonsuperconducting antiferromagnetic samples, whereas a pristine structure is required to preserve superconductivity. Exposure to ambient atmosphere resulted in the conversion of superconducting samples to antiferromagnetic ones. This synthetic method creates new possibilities for soft chemistry approaches to the synthesis and modification of iron-based superconductors.
■
INTRODUCTION
significant vapor pressure at high temperatures, further limiting control of the stoichiometry. A low-temperature solution route to β-FeSe would avoid the formation of high-temperature impurity phases and the loss of selenium to the vapor phase, and could further offer finer control of both the stoichiometry and the morphology of the products. It would also improve the scalability and energy efficiency of the reaction, which are highly desirable for industrial applications. A number of groups have reported solution-based syntheses that occur at temperatures lower than 623 K. One of the first reported methods involved refluxing Fe(CO)5 and selenium in trioctylphosphine oxide,7 while more recent reports call for FeCl2 or Fe(acac)3 as iron sources and milder solvents such as oleic acid, oleylamine, and short-chain polyols.8−11 Several of these methods have been shown to produce phase-pure samples of β-FeSe, with the products often appearing as nanosheets; however, no sample produced by a solution route has shown any evidence of superconductivity without postsynthetic high-temperature annealing.12 The lack of superconductivity indicates that there is a fundamental difference between solution-produced samples and those prepared by solid-state methods. One possible reason for this could be the sources of iron and selenium, which differ from the elemental precursors employed in solid-state reactions. The
The discovery of superconductivity at ∼8 K in the tetragonal form of iron(II) selenide (β-FeSe)1 spurred considerable interest in the compound, as it adopts the simplest crystal structure of any known iron-based superconductor.2−4 β-FeSe is composed of anti-PbO type layers of edge-sharing FeSe4 tetrahedra (Figure 1, inset). As these layers are found in all high-temperature iron-chalcogenide and iron-pnictide superconductors, β-FeSe can be thought of as a parent phase, making it an ideal platform for developing the superconducting structure-properties relationship for all iron-based superconductors. To this end a great deal of effort has gone into optimizing the conventional high-temperature solid-state synthesis, and a vast majority of the studies of the physical, magnetic, and electrical properties of β-FeSe have been performed on samples produced in this way. However, hightemperature routes have several intrinsic shortcomings when applied to metal chalcogenides in general, and specifically to the Fe−Se binary system. Although it is possible to perform direct reactions between iron and selenium powders, the desired tetragonal form is only stable at T < 730 K, more than 600 K below the liquidus for a stoichiometric mixture.5 There are numerous nonsuperconducting phases stable at high temperatures that inevitably form upon cooling a melt, even in quenched reactions, and though extended annealing at 573− 673 K can improve the final product, it cannot completely remove the impurity phases.6 Additionally, selenium exhibits a © 2014 American Chemical Society
Received: November 11, 2014 Revised: December 12, 2014 Published: December 17, 2014 588
DOI: 10.1021/cm5041612 Chem. Mater. 2015, 27, 588−596
Article
Chemistry of Materials
powder X-ray diffraction (PXRD) patterns were collected in the range 5° < 2θ < 120° with an Inel diffractometer employing Co Kα radiation (λ = 1.7902 Å). High-resolution synchrotron X-ray diffraction data were collected at beamline 11-BM (λ = 0.413 Å) at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). Rietveld refinement of the synchrotron data was performed with Jana200619 in the spacegroup P4/nmm (no. 129); Berar’s correction was applied.20 Sample morphology was characterized with transmission electron microscopy (TEM, Jeol 2500SE) operating at 200 kV, and elemental analysis was performed with energy-dispersive X-ray (EDX) microanalysis (Oxford INCA energy) on a Hitachi S4100T scanning electron microscope. Fourier transform infrared (FT-IR) spectroscopy was performed on a Bruker ALPHA-P spectrometer in ATR geometry to verify that all of the solvent had been removed from the samples. Temperature-dependent magnetic susceptibility (2−300 K, H = 1− 10 mT) and isothermal magnetization (2 K, H = 0−5 T) were measured on pelletized samples with a Quantum Design MPMS-XL SQUID magnetometer. Samples were affixed between two long pieces of Kapton tape to minimize diamagnetic contributions from the sample holder. Electrical resistivity was measured with a physical properties measurement system (Quantum Design PPMS) from 2 to 300 K on cold-pressed pellets by the standard four-probe AC method with a pincontact assembly (Wimbush Scientific). 57 Fe Mössbauer spectra were collected at multiple temperatures from 5 to 300 K using a conventional constant-acceleration spectrometer equipped with a 57Co/Rh source held at room temperature. Samples were mixed with boron nitride to reduce preferred orientation. Least-squares fitting of the spectra was performed with the Recoil software package,21 and all centroid shifts (δ) are given with respect to metallic α-iron at room temperature. Neutron pair distribution function (PDF) measurements were performed at the Nanoscale-Ordered Materials Diffractometer (NOMAD)22 at the Spallation Neutron Source at Oak Ridge National Laboratory. Approximately 50 mg of sample was finely ground and loaded into 2 mm diameter silica capillaries (Hampton Research). The scattering structure factor S(Q) was obtained by normalizing diffraction data against a solid vanadium rod and subtracting the background using the IDL routines developed for the NOMAD instrument. The PDF was obtained by the Fourier transform of S(Q) at Qmax = 31.5 Å−1, which was convoluted with the Lorch function to minimize high-Q noise. The samples were measured at 300 K under an Ar flow. Additional high-energy total X-ray scattering data (λ = 0.2114 Å) were collected at the PDF beamline 11-ID-B at APS, ANL. S(Q) was calculated from the PDFgetX2 software package23 after corrections for background scattering, X-ray transmission and Compton scattering. Ground samples were sealed inside silica capillaries and measured at ambient conditions. A large-area amorphous silicon-based detector (Perkin-Elmer) was used to collect high values of momentum transfer (Qmax = 23.7 Å−1). Structural models were refined against the neutron and X-ray PDF data within PDFgui.24
Figure 1. Conventional powder X-ray diffraction pattern of β-FeSe; λ = 1.7903 Å (Co Kα). Experimental pattern: black. Calculated peaks (JCPDS #85-0735): red. Inset: general view of the crystal structure. Fe: black. Se: yellow. Unit cell: black lines.
relative insolubility of these sources in conventional solvents precludes their use in most forms of solution-based reactions. However, it is well-known that many inorganic compounds can be dissolved under solvothermal conditions, and with the assistance of appropriate mineralizing compounds most materials can be brought into solution.13 In the past, solvothermal methods have been employed in the synthesis of a wide variety of metal chalcogenides, utilizing solvents such as water, methanol, and ethylenediamine.14−18 Here we report the solvothermal synthesis of β-FeSe from elemental precursors, with a detailed analysis of the factors that lead to the presence or absence of superconductivity in solutionproduced samples.
■
EXPERIMENTAL SECTION
Synthesis. Single-phase samples of β-FeSe were prepared using a solvothermal method. Iron powder (Alfa Aesar, 99.998%), selenium powder (Alfa Aesar, 99.999%), and ammonium chloride (Alfa Aesar, >99.5%) were used as received. Ethane-1,2-diol (Acros Organics, 99.95%) was used as received for samples produced under aerobic conditions. For samples prepared in air-free conditions, all operations were performed in a glovebox with an argon atmosphere. The solvent was sparged with argon for air-free samples, and it was vacuumdegassed at 373 K and dried over 4 Å molecular sieves (Acros Organics) for air- and water-free samples. In a typical synthesis, Fe (405.4 mg, 7.26 mmol), Se (409.8 mg, 5.19 mmol), and NH4Cl (534.9 mg, 10.0 mmol) were placed in a 125 mL PTFE-lined stainless steel acid digestion vessel (Parr Instrument Company), to which ethane1,2-diol (82.5 mL) was added to achieve a filling fraction of 66%. After the components were thoroughly mixed, the vessel was sealed tightly and heated for 1−10 days at 473 K. Following natural cooling to room temperature (inside a glovebox for air-free samples) the products were filtered, washed with fresh portions of ethane-1,2-diol, and dried under vacuum, yielding very fine dark-gray to black powders. Characterization. Samples were maintained under inert atmosphere during preparation, transport to instrumentation, and measurement unless otherwise specified. When samples were mounted, exposure to the atmosphere was minimized (≤1 min). Conventional
■
RESULTS AND DISCUSSION Synthesis. The solvothermal synthesis was optimized by varying the solvent, the Fe:Se molar ratio, the presence of a mineralizer, the reaction temperature and duration, and the exposure of the reaction and products to air and water (Table SI 1, Supporting Information). It was found that the desired phase could be formed from reactions performed in water or ethanol, but only in a mixture with several selenium-rich products and unreacted iron and selenium. Ethylenediamine is commonly used as a solvent for this type of system, but in our previous work, we have shown that it is too strongly coordinating to yield binary products.25 The oxygen-bearing analogue, ethane-1,2-diol, was selected as a solvent as it is widely available, easy to work with, and has been successfully 589
DOI: 10.1021/cm5041612 Chem. Mater. 2015, 27, 588−596
Article
Chemistry of Materials used in other solution-based syntheses of β-FeSe.9,26 Initial reactions of stoichiometric ratios of iron and selenium in ethane-1,2-diol, carried out under aerobic conditions, produced polycrystalline samples in which β-FeSe was the primary phase. Several admixtures were also present, including unreacted iron and selenium, as well as the selenium-rich phases FeSe2 and Fe3Se4, though in significantly smaller quantities than observed in reactions performed in water or ethanol. The coordinating ability of ethane-1,2-diol was insufficient for the reaction to proceed completely, as evidenced by the presence of unreacted starting materials. This can often be remedied by the addition of a mineralizing compound; when the salt NH4Cl was included in the reaction, unreacted iron was no longer observed in the products. However, following the reaction the solvent took on a deep red color and the seleniumrich phases were still present, indicating that some amount of iron remained in solution. The Fe:Se molar ratio was then varied from 1:1 to 2.5:1, and as the relative concentration of iron increased, the number of admixtures and overall amount of selenium-rich products decreased. Starting Fe:Se ratios greater than 1.25, 1.30, and 1.35 to 1 marked the disappearance of selenium, FeSe2, and Fe3Se4, respectively, from the products; ratios greater than 1.75: 1 led to the reappearance of elemental iron in the products. At a ratio of 1.4:1, the only observed product was β-FeSe (Table SI 1, Supporting Information), and this was taken to be the optimal ratio for further experiments. The temperature of the reaction was also varied from 433 to 513 K. Temperatures below 453 K yielded unreacted iron, βFeSe, and FeSe2, whereas temperatures greater than 493 K produced β-FeSe, Fe3Se4, and Fe3O4. The most common impurities formed in high-temperature solid-state reactions, Fe7Se8 and δ-FeSe (NiAs-type), were not observed under any reaction conditions. These results are in excellent agreement with Grivel et al.’s in situ observation of solid-state product formation, which showed evidence of FeSe2 forming above 398 K and Fe3Se4 forming above 494 K.27 However, they did not observe the formation of β-FeSe below 523 K, while in the present work it had already formed as the primary phase in reactions at 433 K. 473 K was selected as the optimal temperature to maximize the rate of the reaction and avoid the formation of impurity phases. The duration of the reaction was also varied from 1 to 10 days; at 473 K and with the optimized ratio of 1.4 Fe:1 Se, phase-pure samples of β-FeSe could be produced in as little as 24 h, but the crystallinity of the product was markedly improved if the reaction was allowed to continue for 5−10 days. A PXRD pattern for a typical reaction can be seen in Figure 1; all peaks were well described by the database pattern for β-FeSe (JCPDS #85-735), and there were no visible peaks from impurity phases. The width of the diffraction peaks indicated that the product was nanostructured, as has been reported for all other samples produced from solution.7−12,26,28 Characterization by TEM confirmed the nanoscale particle size, with the sample forming as nanosheets ranging from 100 to 500 nm across (Figure SI 1, Supporting Information), though the ultrasonication involved in preparing the sample may have reduced the particle size through exfoliation.29 Samples stored under ambient conditions were found to be highly insulating, whereas samples produced by solid-state methods were conductive;1,30 when placed in the scanning electron microscope for elemental analysis, solutionproduced samples would charge upon exposure to the electron beam and visibly degraded during imaging. EDX results
suggested iron-rich compositions, as well as the presence of a significant quantity of oxygen. Although the Fe:Se ratio was almost certainly affected by the charging and degradation of the sample, with selenium presumably being lost to the vapor phase, the presence of oxygen was unexpected, as no oxygencontaining impurities were visible by PXRD. Furthermore, FTIR measurements showed no evidence of residual solvent or adsorbed water, indicating that the oxygen was somehow incorporated into the sample. It is known that exposure to even small amounts of oxygen during solid-state syntheses can result in nonsuperconducting samples of β-FeSe,30 and it has been shown that samples exposed to oxygen or ambient conditions will lose their superconductivity in a short period of time.31 However, the physical reasons for this behavior remain unknown. To determine whether the lack of superconductivity in solutionproduced samples could arise from exposure to air or residual water, either in the headspace of the solvothermal vessel or dissolved in the solvent, three samples were prepared with increasing levels of air and water exclusion. All samples were prepared according to the optimized synthesis with 1.4 Fe:1 Se, and the reactions occurred at 473 K for 10 days. Sample I (airexposed) was treated in aerobic conditions; Sample II (air-free) was prepared and handled in the inert atmosphere of an argonfilled glovebox, and the solvent was sparged with argon to remove dissolved oxygen; Sample III (air/H2O-free) was treated under similar conditions to Sample II, but the solvent was vacuum-degassed at 100 °C and dried over 4 Å molecular sieves to remove residual water. All operations with Samples II and III were performed under inert conditions, and exposure to ambient atmosphere was limited to less than 1 min during transfer to various instruments for characterization. Magnetism and Resistivity. Laboratory PXRD patterns appeared similar for all three samples, but each displayed markedly different magnetic properties, as can be seen in Figure 2A. Bulk superconducting samples of β-FeSe produced by solidstate reactions do not exhibit any magnetic ordering at finite temperatures, and show a superconducting transition at roughly 8 K.32 In contrast, all previously reported magnetic characterizations of solution-produced samples show antiferromagnetic ordering near 50 K.7,10 Sample I exhibited the same broad antiferromagnetic peak centered around 40 K, in excellent agreement with the literature. Sample III did not show any antiferromagnetic ordering, but instead exhibited the characteristic superconducting transition at 8 K. It has been suggested that long-range antiferromagnetic order and superconductivity are competitive in this system,33 and the exclusion of air and water from the synthesis appeared to completely suppress longrange magnetic ordering, allowing for superconductivity to persist. Sample II seemed to have intermediate magnetic properties; it was unclear whether there was an antiferromagnetic ordering at 5 K, or the onset of superconductivity in a very small volume fraction of the sample superimposed on a significant paramagnetic tail. Samples II and III also contained some form of ferromagnetic impurity, as evidenced by the positive slope of the high-temperature susceptibility and the significant splitting between zero-field-cooled (ZFC) and field-cooled (FC) measurements (Figure SI 2, Supporting Information); it is most likely a very small admixture of unreacted iron, as has been reported in many solid-state samples,6 though in a quantity too small to detect by other methods of character590
DOI: 10.1021/cm5041612 Chem. Mater. 2015, 27, 588−596
Article
Chemistry of Materials
aerobic conditions were too resistive to measure, freshly made samples exhibited much lower resistivities. Sample I showed no evidence of superconductivity, as the resistivity only increased with decreasing temperature. Samples II and III were much more conductive, and both exhibited sudden drops in resistivity, confirming the superconducting transition at 8 K. The small magnitude and relative breadth of the transition was due to the poor contact across grain boundaries in the coldpressed pellets. 57 Fe Mö ssbauer Spectroscopy. To further assess the magnetic properties and local structure of solution-produced βFeSe, Mössbauer spectra were collected at temperatures from 5 to 300 K (Figure 3). Typical Mössbauer spectra reported for solid-state superconducting samples of β-FeSe consist of a single nonmagnetic doublet with a centroid shift (δ) of ∼0.55 mm/s and an electric quadrupole splitting (ΔEQ) of ∼0.29 mm/s, corresponding to the single crystallographic position of Fe2+ within the structure.28,32 This doublet remains unsplit down to 4.2 K, indicating that no magnetic ordering occurs in the iron sublattice of β-FeSe. At room temperature, all three solution-produced samples contained such a nonmagnetic doublet, in excellent agreement with the literature; however, Sample I exhibited two additional doublets, similar to what has been observed for “oxygenpoisoned” solid-state samples,30 while Samples II and III did not. Furthermore, of the three doublets in Sample I, only one remained nonmagnetic at low temperature; the second ordered by 150 K and the third split into two sextets below 80 K. As the only phase visible by PXRD was β-FeSe, the fact that the nonmagnetic doublet accounted for only 44% of the total intensity presented an interesting problem. One possibility was that the other three components were generated by disordered vacancies within the structure, while another was that they arose from nanostructured or amorphous impurity phases with diffraction peaks too broad to be resolved by PXRD. Samples II and III each consisted of a single nonmagnetic doublet at temperatures down to 50 K, indicating that exposure to air and water had a significant deleterious effect on the structure of Sample I. Although no impurity phases could be observed in the spectra for both the air-free and the air/H2Ofree samples, they still were not identical to spectra obtained from solid-state samples. Specifically, between 50 and 25 K, there was a distinct broadening of the doublet that persisted down to 5 K. These spectra could be fit with two components, but in two separate ways, either with slightly different centroid shifts and similar electric quadrupole splitting (Figure 3) or with similar centroid shifts and different quadrupole splitting (Figure SI 4, Supporting Information). In each case, both components freely refined to intensities of 50%, suggesting that both samples underwent a structural transition that resulted in two crystallographic iron positions, instead of the single position in β-FeSe. This splitting cannot be explained by a transition into the orthorhombic Cmma phase, which is known to occur below 90 K,39 as there remains only one iron position in the orthorhombic structure. Therefore, it is possible that there is a phase change to a lower symmetry at temperatures well below the known tetragonal-to-orthorhombic transition; a similar reduction in symmetry has been proposed based on lowtemperature TEM and electron diffraction studies.40 Synchrotron Powder X-ray Diffraction. To determine if minute structural differences were responsible for the differing magnetic and superconducting properties, and to more accurately assess the presence of impurity phases, high-
Figure 2. (A) Zero-field-cooled (ZFC) molar magnetic susceptibility for Samples I−III. Inset: χ−χ25K for Samples II and III in the region of 0−25 K. (B) AC resistivity for Samples I−III. Inset: expanded lowtemperature region. Sample I: red circles. Sample II: blue triangles. Sample III: black squares.
ization. A magnetic anomaly was observed around 120 K in all three samples, though it was most pronounced in Sample III; a similar anomaly has been observed in β-FeSe,34 FeSe1−xTex,35 and FeTe1−xSx,36 and has been attributed to spin- or chargeordering events in impurities such as Fe7Se8 and Fe3O4.32,37 However, the presence of this feature was inconsistent among samples prepared under similar conditions, all of which exhibited the superconducting transition at 8 K (Figure SI 3, Supporting Information). This suggested that the signal was not intrinsic to β-FeSe, and indeed stemmed from an impurity phase. Although the feature appeared very similar to the welldocumented Verwey transition of magnetite (Fe3O4),38 this assignment made little sense, given the synthetic conditions under which it was most often observed; if the signal arose from iron oxide, it should be most pronounced in samples produced under aerobic conditions, and not in reactions from which air and water were rigorously excluded. It is therefore unlikely that this signal was from the Verwey transition of a magnetite impurity, but the possibility cannot be ruled out, as none of the impurities that could give rise to this anomaly were detectable by any other method of characterization. Electrical resistivity measurements were used to confirm the superconducting nature of Samples II and III, as can be seen in Figure 2B. Although samples produced and stored under 591
DOI: 10.1021/cm5041612 Chem. Mater. 2015, 27, 588−596
Article
Chemistry of Materials
Figure 3. 57Fe Mössbauer spectra for Samples I−III collected from 5 to 300 K. Experimental data: black circles. Calculated spectrum: black line. Components: red, blue, orange, magenta, and cyan lines. Centroid shift (δ), quadrupole splitting/quadrupole shift (ΔEQ/ε), hyperfine field (Bhf), and intensity (I) are given for individual components of each spectrum.
resolution synchrotron PXRD was performed for all three samples (Figures 4 and SI 5 (Supporting Information) and Table 1). All three profiles were extremely similar, and all observed Bragg peaks were well described by the tetragonal βFeSe structure. The broad peak centered at 6° 2θ originated from the amorphous silica with which the samples were mixed to reduce the effects of preferred orientation and X-ray
Table 1. Refined Structural Parameters of Samples I−III from Room-Temperature Synchrotron PXRD Data Sample
I
II
III
Fe:Se a (Å) c (Å) V (Å3) zSe Rp/Rwp (%) GOF
0.910(5):1 3.7873(1) 5.5144(3) 79.098(6) 0.2628(3) 4.68/5.61 0.97
0.996(3):1 3.7735(1) 5.5223(1) 78.635(2) 0.2653(1) 5.11/6.12 0.94
1.001(2):1 3.7723(1) 5.5225(1) 78.588(2) 0.2656(1) 4.62/5.48 1.00
absorption. No other peaks were observed in any of the samples, indicating that any impurities were either amorphous or present in exceptionally small quantities. Rietveld refinement revealed that Sample I was significantly iron-deficient, with the iron occupancy refining to only 91%. This was accompanied by an unexpected shift in the lattice parameters; if 9% of the iron positions were vacant, there should be a considerable contraction of the a lattice parameter as the surrounding iron atoms in the basal plane relaxed into the vacancy (Figure 5C). Counter to this, the unit cell parameter a was significantly larger in Sample I than in Samples II and III, suggesting that something was present in or around the iron vacancies that occupied a sufficient volume to cause an expansion in the basal plane (Figure 5D). As the effect was most pronounced in the sample that had the greatest exposure to air and water, it is very likely to be an oxygen-containing species. Refinements performed using structural models with an additional oxygen atom located in a variety of positions yielded no significant improvement over the initial model, indicating that the species responsible for the expansion of the basal plane does not exhibit long-range ordering in the structure. Samples II and III were structurally very similar, and the refined lattice parameters matched closely with those of
Figure 4. Rietveld refinement of the room-temperature synchrotron PXRD pattern for superconducting Sample III. Experimental data: black crosses. Calculated profile: orange line; difference profile: red line. Vertical ticks indicate peak positions for β-FeSe (space group P4/ nmm, No. 129), and no additional peaks were observed. Inset: Comparison of the experimental profiles of Samples I−III in the region of 12−15° 2θ, showing slight shifts in the lattice parameters and peak widths. Sample I: red circles. Sample II: blue triangles. Sample III: black squares. 592
DOI: 10.1021/cm5041612 Chem. Mater. 2015, 27, 588−596
Article
Chemistry of Materials
Figure 5. Room-temperature neutron diffraction pair-distribution function (PDF) analysis of Samples I−III. (A) Short-range PDF for Samples I−III. Sample I: red circles. Sample II: blue triangles. Sample III: black squares. (B) Medium-range least-squares fitting for Sample III. Experimental data: black squares. Calculated PDF: orange line. Difference profile: red line. (C, D) Structural models of the basal plane of β-FeSe showing an iron vacancy and a possible position for an oxygen atom. Iron: black. Selenium: yellow. Oxygen: red.
superconducting samples made via solid-state methods.39 Both samples were stoichiometric within experimental uncertainty, but they were not completely identical. Sample II exhibited slightly broader diffraction peaks, and also showed small increases in a and V compared to Sample III, suggesting that the presence of trace quantities of water during the synthesis was enough to slightly alter the structure, effectively limiting the superconducting properties of the product. While synchrotron PXRD experiments revealed subtle differences in the structures of the three samples, further characterization of the local structure was necessary to achieve a better understanding of why Samples II and III were superconducting while Sample I was not. Neutron and X-ray PDF. The local structures of Samples I−III were assessed by room-temperature neutron and X-ray powder diffraction pair-distribution function (PDF) analysis; the results are summarized in Figures 5 and SI 6 and Table SI 2, Supporting Information. Both measurements gave very similar results, but several distinguishing features were much more apparent in the neutron PDFs. Similar to the results from synchrotron PXRD, Sample I was significantly different from Samples II and III, but in this case, all three samples had distinguishing features. Most notably, the PDF for Sample I contained strong peaks that were not predicted by the β-FeSe structure (Figure 5A). The most prominent peak was centered at 1.95 Å, with a smaller and broader peak appearing at 3.00 Å. The intensity of the shortest Fe−Fe pair was also diminished, and was shifted from 2.65 to 2.75 Å; this supported the idea that oxygen was incorporated into the structure around vacant iron positions, as the peak at 1.95 Å corresponds to the typical Fe-O bond length in a tetrahedral geometry.41 These peaks were also present in the X-ray PDF, but at a significantly
reduced intensity (Figure SI 7, Supporting Information), further supporting the theory that a light atom such as oxygen was responsible for the additional peaks. One possible position for this oxygen atom is shown in Figure 5D; refinements were attempted for models containing such an atom, and while these models had peaks at the correct distances (Figure SI 6, Supporting Information), no model could adequately describe the high intensity of the first peak without unreasonably high occupancy of the oxygen atoms. The PDF of Sample I also indicated that the short-range (
