Investigation Into the Effect of Sulfate and Borate Incorporation on the Structure and Properties of Srfeo3-Δ

crystals

Article Investigation into the Effect of Sulfate and Borate Incorporation on the Structure and Properties of SrFeO3-δ

Abbey Jarvis and Peter Raymond Slater *

School of Chemistry, University of Birmingham, Birmingham B15 2TT, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-121-414-8906

Academic Editor: Stevin Pramana Received: 28 April 2017; Accepted: 3 June 2017; Published: 7 June 2017

Abstract: In this paper, we demonstrate the successful incorporation of sulfate and borate into SrFeO3-δ, and characterise the effect on the structure and conductivity, with a view to possible utilisation as a cathode material in Solid Oxide Fuel Cells. The incorporation of low levels of sulfate/borate is sufﬁcient to cause a change from a tetragonal to a cubic cell. Moreover, whereas heat treatment of undoped SrFeO3-δ under N2 leads to a transformation to brownmillerite Sr2Fe2O5 with oxygen vacancy ordering, the sulfate/borate-doped samples remain cubic under the same conditions. Thus, sulfate/borate doping appears to be successful in introducing oxide ion vacancy disorder in this system.

Keywords: solid oxide fuel cell; cathode; perovskite; sulfate; borate

1. Introduction Research into solid oxide fuel cells (SOFCs) as alternate energy materials has grown due to their high efficiency and consequent reduction in greenhouse gas emissions. Specifically, research into perovskite materials has attracted significant interest for potential SOFC materials including cathode, electrolyte and anode materials (see, for example, the review articles [1,2]). Traditionally, doping strategies for perovskite materials has involved the introduction of cations of a similar size e.g., Sr2+ for 3+ 2+ 3+ La , Mg for Ga to give the oxide ion conducting electrolyte La1−xSrxGa1−yMgyO3−x/2−y/2 [3]. More recently, we have applied oxyanion (silicate, phosphate, sulfate, borate) doping strategies to improve the properties of solid oxide fuel cell materials, initially demonstrating the successful incorporation into Ba2In2O5 [4]. Research on oxyanion incorporation into perovskites was initially reported for superconducting cuprate materials. This work showed that a wide range of oxyanions (carbonate, borate, nitrate, sulfate, and phosphate) could be incorporated into the perovskite structure [5–12]. In this doping strategy, the central ion of the oxyanion group is located on the perovskite B cation site, with the oxygens of this group occupying either three (borate, carbonate, nitrate) or four (sulfate, phosphate) of the available six anion positions around this site, albeit suitably displaced to achieve the correct coordination for the oxyanion group. As an extension to this work on cuprate superconductors, we have demonstrated successful oxyanion doping of perovskite materials for SOFC applications. This has included oxyanion (sulfate, silicate, phosphate) doping of Ba2(In/Sc)2O5 electrolyte materials [13–16]. Doping with these oxyanions introduces disorder on the oxygen sublattice, leading to improvements in the ionic conductivity. This work has also been extended to potential solid oxide fuel cell electrode materials, with successful oxyanion doping in the perovskite systems SrCoO3 [17–20], SrCo0.85Fe0.15O3 [21], SrMnO3 [18], CaMnO3 [22] and SrFeO3-δ [23], with the results suggesting improved performance/stability.

Crystals 2017, 7, 169; doi:10.3390/cryst7060169 www.mdpi.com/journal/crystals Crystals 2017, 7, 169 2 of 13

Following the prior work demonstrating the successful incorporation of silicate into SrFeO3-δ [23],Crystals we 2017 report, 7, 169 in this paper an investigation into the effect of sulfate and borate doping.2 of 13 The Crystalsundoped2017 ,system7, 169 SrFeO3-δ has attracted interest due to its high ionic and electronic conductivity,2 of 13 howeverFollowing under lowthe priorp(O2 )work the oxide demonstrating ion (and electronicthe successful conductivity) incorporation are significantlyof silicate into reduced. SrFeO3- δThis is[23], due we to reportoxygen in loss this leading paper anto investigationa transformati intoon tothe the effect oxygen of sulfate vacancy and ordered borate doping.brownmillerite The Following the prior work demonstrating the successful incorporation of silicate into SrFeO [23], 3-δ 3-δ structure,undoped Srsystem2Fe2O 5SrFeO (Figure has1). Weattracted have thereforeinterest due investigated to its high the ionic effect and of electronic borate/sulfate conductivity, doping on we report in this paper an investigation into the effect of sulfate and borate doping. The undoped thishowever transformation. under low p(O2) the oxide ion (and electronic conductivity) are significantly reduced. This systemis due SrFeO to oxygen3-δ has loss attracted leading interest to a transformati due to its highon to ionic the andoxygen electronic vacancy conductivity, ordered brownmillerite however under lowstructure, p(O2) the Sr2 oxideFe2O5 (Figure ion (and 1). electronic We have conductivity)therefore investigated are signiﬁcantly the effect reduced. of borate/sulfate This is due doping to oxygen on lossthis leading transformation. to a transformation to the oxygen vacancy ordered brownmillerite structure, Sr2Fe2O5 (Figure1). We have therefore investigated the effect of borate/sulfate doping on this transformation.

Figure 1. Structure of Sr2Fe2O5 (showing oxygen vacancy ordering) (left) and SrFeO3 (right). Figure 1. Structure of Sr2Fe2O5 (showing oxygen vacancy ordering) (left) and SrFeO3 (right). 2. ResultsFigure and Discussion 1. Structure of Sr2Fe2O5 (showing oxygen vacancy ordering) (left) and SrFeO3 (right). 2. Results and Discussion 2. Results and Discussion 2.1. SrFe1−xSxO3-δ 2.1. SrFe1−xSxO3-δ 2.1. SrFe1−xSxO3-δ 2.1.1. X-ray Diffraction Data 2.1.1. X-ray Diffraction Data 2.1.1.A X-ray range Diffraction of samples Data of SrFe1−xSxO3-δ with increasing sulfate content (0 ≤ x ≤ 0.1) were prepared. A range of samples of SrFe1−xSxO3-δ with increasing sulfate content (0 ≤ x ≤ 0.1) were prepared. X-ray Adiffraction range of samplesanalysis of showed SrFe1−x SthatxO3- δwithout with increasing sulfate doping sulfate contentSrFeO3- δ(0 forms ≤ x ≤ a0.1) tetragonal were prepared. perovskite X-ray diffraction analysis showed that without sulfate doping SrFeO3-δ forms a tetragonal perovskite in line with prior reports. This is illustrated in Figure 2 (expanded region 2θ = 45 to 60°) where peak inX-ray line with diffraction prior reports.analysis Thisshowed is illustrated that without in Figuresulfate2 doping (expanded SrFeO region3-δ forms 2 θ a= tetragonal 45◦ to 60◦ perovskite) where peak splitting can clearly be observed. Upon doping with sulfate, there is a transformationθ to a cubic cell splittingin line with can clearlyprior reports. be observed. This is illustrated Upon doping in Figure with 2 sulfate, (expanded there region is a transformation 2 = 45 to 60°) where to a cubic peak cell (0.025splitting ≤ x can≤ 0.075), clearly where be observed. no peak Up splittingon doping is now with ob sulfate,served there (Figure is a 2).transformation Above x = 0.075, to a cubic small cell SrSO 4 (0.025 ≤ x ≤ 0.075), where no peak splitting is now observed (Figure2). Above x = 0.075, small SrSO 4 impurities(0.025 ≤ x ≤appear, 0.075), wheresuggesting no peak that splitting the solubility is now limitobserved of sulfate (Figure in 2).SrFe Above1−xSxO x3- =δ is0.075, x ≈ 0.075. small SrSO4 impurities appear, suggesting that the solubility limit of sulfate in SrFe1−xSxO3-δ is x ≈ 0.075. impurities appear, suggesting that the solubility limit of sulfate in SrFe1−xSxO3-δ is x ≈ 0.075.

Figure 2. Cont.

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Figure 2. a b c FigureFigure 2.2. X-rayX-ray diffractiondiffraction patternspatternspatterns for forfor ( (a()a) ) SrFeO SrFeOSrFeO3-3-δ3-δ,(,δ , (b(b)) ) SrFe SrFeSrFe0.9750.9750.975SSS0.0250.0250.025OOO3-3-δ3-,δ ,δ (,(c(c) ) SrFe)SrFe SrFe0.950.950.95SS0.050.05SO0.05O3-3-δO,δ , 3-(d(δd), ) (d) SrFe S O ,(e) SrFe S O , and (f) SrFe S O . The data show a tetragonal cell for SrFeSrFe0.9250.925S0.925S0.0750.075OO0.0753-3-δ,δ ,( e(e)3- )SrFeδ SrFe0.90.9SS0.10.1OO0.93-3-δ,δ 0.1,and and3- ( fδ(f) )SrFe SrFe0.850.85SS0.150.15OO0.853-3-δ.δ .The The0.15 data data3-δ show show a a tetragonal tetragonal cell cell for for SrFeO SrFeO3-3-δ,δ , ◦ ◦ SrFeO(as(as highlightedhighlighted3-δ, (as highlighted byby thethe peakpeak by the splittisplitti peakng:ng: splitting: seesee expandedexpanded see expanded regionregion region FigureFigure Figure 22θθ ==2 45θ45 = toto 45 60°),60°),to 60 whilewhile), while thethe the sulfate-doped samples are cubic. SrSO impurities (highlighted by an asterisk) are observed for sulfate-dopedsulfate-doped samplessamples areare cubic.cubic. SrSOSrSO4 4 impurities4impurities (highlighted(highlighted byby anan asterisk)asterisk) areare observedobserved forfor samplessamples withwith with higherhigher higher sulfatesulfate sulfate contentscontents contents (x(x (x == = 0.1 0.1 and and 0.15). 0.15).

InIn order orderorder to toto provide provideprovide further furtherfurther support supportsupport for the forfor incorporation thethe incorporationincorporation of sulfate, ofof sulfate, equivalentsulfate, equivalentequivalent Fe-deﬁcient Fe-deficientFe-deficient samples 0.95 3-δ weresamplessamples prepared werewere withoutpreparedprepared addition withoutwithout of addition sulfate.addition The ofof sulfate. X-raysulfate. diffraction TheThe X-rayX-ray data diffractiondiffraction for SrFe 0.95 datadataO3- δforfor(Fe-deﬁcient, SrFeSrFe0.95OO3-δ 0.95 0.05 3-δ no(Fe-deficient,(Fe-deficient, sulfate) and no no SrFe sulfate) sulfate)0.95S0.05 and andO3- SrFe δSrFeare0.95 comparedSS0.05OO3-δ are are in compared Figurecompared3. These in in Figure Figure data 3. show3. These These the data data presence show show of the the impurities presence presence 0.95 3-δ forofof impurities SrFeimpurities0.95O3- δ forforalong SrFeSrFe with0.95OO3- peakδ alongalong splitting, withwith consistentpeakpeak splitting,splitting, with a consistentconsistent tetragonal withwith cell, asaa tetragonal fortetragonal the undoped cell,cell, asas SrFeO forfor the3-theδ 3-δ parentundopedundoped phase. SrFeO SrFeO In3- contrast,δ parentparent impurities phase. phase. InIn are contrast,contrast, not observed impurities impurities forthe are are sulfate-containing notnot observedobserved forfor phase thethe sulfate-containingsulfate-containing SrFe0.95S0.05O3-δ, andphasephase the SrFe SrFe cell0.950.95 isSS0.05 now0.05OO3-3-δ cubic.,δ ,and and the the Therefore, cell cell is is now now this cubic. cubic. comparison Therefore, Therefore, provides th thisis comparison comparison further evidence provides provides for further further the successful evidence evidence incorporationforfor the the successful successful of sulfate. incorporation incorporation of of sulfate. sulfate.

FigureFigure 3. 3. X-ray X-ray diffractiondiffraction diffractionpatterns patterns patterns for for for (a ( )a(a SrFe) )SrFe SrFe0.950.950.95OO3-O3-δ3-δ δ(Fe-deficient,(Fe-deficient, (Fe-deficient, no no no sulfate) sulfate) sulfate) and and and (b ( )b(b SrFe) )SrFe SrFe0.950.950.95SS0.05S0.050.05OOO3-3-3-δδ.δ. .

TheThe datadata data showshow show impuritiesimpurities impurities (highlighted(hig (highlightedhlighted byby by anan an asterisk)asterisk) asterisk) and an andd a a tetragonala tetragonal tetragonal cell cell cell for for for SrFe SrFe SrFe0.950.950.95OOO3-3-δ3-δ,δ , while while thethe the sulfate-containingsulfate-containing sample sample sample SrFe SrFe SrFe0.950.950.95SS0.05S0.050.05OO3-3-δδ δis is phase phasephase pure purepure and andand cubic. cubic.

CellCell parameters parameters for for all all these these phases phases were were determineddetermi determinedned using using the the Rietveld Rietveld method method (an (an example example ﬁtfit fit is shown in Figure 4). The variation of the cell parameters for SrFe1−1x−xSxxO3-3-δδ with increasing sulfate isis shownshown inin FigureFigure4 ).4). The The variation variation of of the the cell cell parameters parameters for for SrFe SrFe1−xSSxOO3-δ with increasing sulfatesulfate contentcontent isis given given inin Table Table1 1. .1. TheThe The datadata data showshow show aa a smallsmall small generalgeneral general increaseincrease increase withwith with increasingincreasing increasing sulfatesulfate sulfate content,content, content, andand this this will will be be discussed discussed inin in moremore more detaildetail detail inin in SectionSection Section 2.1.42.1.4. 2.1.4..

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Figure 4. Observed, calculated and difference X-ray diffraction proﬁle for SrFe S O δ. Figure 4. Observed, calculated and difference X-ray diffraction profile for SrFe0.9750.9750.025S0.025O3-3-δ.

InIn addition addition to to the the determination determination of of the the unit unit cell cell parameters, parameters, site site occupancies occupancies were were reﬁned refined for for thethe Fe/S Fe/S site. site. TheseThese occupanciesoccupancies areare givengiven inin Table Table1 ,1, and and show show that that the the reﬁned refined values values are are in in good good agreementagreement with with the the expected expected values. values.

TableTable 1. 1.Lattice Lattice parametersparameters and Fe/S Fe/S site site occupancies occupancies obtained obtained from from Rietveld Rietveld refinement refinement using using XRD XRDdata datafor forSrFe SrFe1−xSx1O−3-xδS. xOThe3-δ .structure The structure of SrFeO of3- SrFeOδ was3- δrefinedwas refined using usinga tetragonal a tetragonal space spacegroup group(P4/mmm). (P4/mmm). The structures The structures of the ofsulfate-doped the sulfate-doped samples samples were refined were refined using usinga cubic a cubicspacespace group group(Pm3 (Pmm). 3m).

SrFe1−xSxO3-δ SrFe1−xSxO3-δ S (x) 0 0.025 0.05 0.075 S (x) 0 0.025 0.05 0.075 a (Å) a (Å) 3.8648(1) 3.8648(1) 3.8723(1) 3.8776(1) 3.8776(1) 3.8766(1) 3.8766(1) c (Å) c (Å) 3.8487(1) 3.8487(1) ------V (Å3) V (Å3) 57.486(4) 57.486(4) 58.066(2) 58.303(4) 58.303(4) 58.260(4) 58.260(4) Rwp (%)Rwp (%) 1.84 1.84 1.671.67 2.01 2.01 1.97 1.97 Rexp (%) 0.92 0.92 0.90 0.90 Rexp (%) 0.92 0.92 0.90 0.90 Fe occupancy 1 0.98(1) 0.96(1) 0.94(1) S occupancyFe occupancy - 1 0.02(1)0.98(1) 0.96(1) 0.04(1) 0.94(1) 0.06(1) Fe/S UisoS occupancy 0.003(1) - 0.009(1) 0.02(1) 0.04(1) 0.011(1) 0.06(1) 0.008(1) Fe/S Uiso 0.003(1) 0.009(1) 0.011(1) 0.008(1)

2.1.2. Stability under N2 2.1.2. Stability under N2 The effect of heating the SrFe1−xSxO3-δ samples under N2 was then examined. Upon heating The effect◦ of heating the SrFe1−xSxO3-δ samples under N2 was then examined. Upon heating under N2 to 950 C, the X-ray diffraction data showed that SrFeO3-δ transforms to the oxygen vacancy under N2 to 950 °C, the X-ray diffraction data showed that SrFeO3-δ transforms to the oxygen vacancy ordered brownmillerite type Sr2Fe2O5 (Figures1 and5). This would be expected to be unfavorable forordered fuel cell brownmillerite applications duetype toSr the2Fe2O ordering5 (Figures of 1 oxygen and 5). vacancies, This would which be expected is expected to be to unfavorable lower the oxidefor fuel ion cell conductivity. applications In contrast,due to the for ordering the sulfate-doped of oxygen samples, vacancies, the which disordered is expected cubic perovskite to lower the is retainedoxide ion under conductivity. a similar In heat contrast, treatment. for the For sulfate-doped the x = 0.025 samples, sample, therethe disordered are some verycubic weakperovskite peaks is (seeretained expanded under XRD a similar ﬁgure) heat associated treatment. with For the the brownmillerite x = 0.025 sample, structure, there butare some for the very higher weak sulfate peaks contents(see expanded (x ≥ 0.05), XRD a single figure) phase associated cubic cell with is observed.the brownmillerite In line with structure, the reduction but for in the the higher Fe oxidation sulfate statecontents towards (x ≥ 3+,0.05), there a single is an phase increase cubic in cell the is cell observed. parameters In line associated with the withreduction the larger in the sizeFe oxidation of Fe3+ 3+ versusstate towards Fe4+ (Table 3+,2 ).there is an increase in the cell parameters associated with the larger size of Fe versus Fe4+ (Table 2).

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FigureFigure 5. 5.X-ray X-ray diffraction diffraction patterns patterns of of ( a()a) SrFeO SrFeO3-3δ-,(δ, b(b)) SrFeSrFe0.9750.975S0.025OO3-3-δ, δand, and (c) ( cSrFe) SrFe0.950.95S0.05SO0.053-δO after3-δ ◦ afterheating heating under under N2 to N 9502 to °C. 950 C.

TableTable 2. Lattice2. Lattice parameters parameters for for SrFe SrFe1−x1S−xxSOxO3-3-δδ afterafter heatingheating inin airair andand NN22.

SrFe1−xSxO3-δ SrFe1−xSxO3-δ 0.025 0.05 0.075 S (x) 0.025 0.05 0.075 S (x) Air Dry N2 Air Dry N2 Air Dry N2 Air Dry N2 Air Dry N2 Air Dry N2 a (Å)a (Å) 3.8723(1) 3.8723(1) 3.9231(1)3.9231(1) 3.8776(1)3.8776(1) 3.9256(1)3.9256(1) 3.8766(1) 3.9280(1) c (Å)c (Å) ------VV (Å (Å3) 3) 58.066(2) 58.066(2) 60.379(1)60.379(1) 58.303(4)58.303(4) 60.496(1)60.496(1) 58.260(4) 60.606(1) Rwp (%) 1.67 3.10 2.01 3.09 1.97 3.20 Rwp (%) 1.67 3.10 2.01 3.09 1.97 3.20 Rexp (%) 0.92 2.59 0.90 2.51 0.90 2.50 Rexp (%) 0.92 2.59 0.90 2.51 0.90 2.50

2.1.3.2.1.3. Thermogravimetric Thermogravimetric Analysis Analysis Thermogravimetric analysis (TGA) (heat treatment in N to reduce the Fe oxidation state to 3+) Thermogravimetric analysis (TGA) (heat treatment in N2 2 to reduce the Fe oxidation state to 3+) waswas then then utilized utilized to to determine determine the the oxygen oxygen contents contents of of the the samples samples as as prepared prepared in in air. air. This This analysis analysis resultedresulted in in an an interesting interesting observation observation associated associated with with Mass Mass spectrometry spectrometry analysis analysis of the of evolved the evolved gas. For the undoped sample SrFeO δ, the loss of mass is only associated with oxygen as indicated by the gas. For the undoped sample 3-SrFeO3-δ, the loss of mass is only associated with oxygen as indicated massby the spectrometry mass spectrometry data. However, data. However, for the for sulfate-doped the sulfate-doped samples, samples, a mass a loss mass associated loss associated with CO with2 was observed in addition to the expected mass loss due to O . CO2 was observed in addition to the expected mass loss due2 to O2. Thus,Thus, the the results results indicated indicated the the presence presence of some of some carbonate carbonate in the in sulfate-doped the sulfate-doped samples. samples. In order In ◦ to remove this carbonate, heat treatment in O (up to 900 C) was carried out for these SrFe − S O δ order to remove this carbonate, heat treatment2 in O2 (up to 900 °C) was carried out1 forx x these3- samples. After this oxygen treatment, TGA analysis indicated no presence of carbonate. This is SrFe1−xSxO3-δ samples. After this oxygen treatment, TGA analysis indicated no presence of carbonate. illustrated in Figure6, where a mass loss associated with CO is not observed after heat treatment in This is illustrated in Figure 6, where a mass loss associated2 with CO2 is not observed after heat O for SrFe S O δ. treatment2 0.95in O0.052 for 3-SrFe0.95S0.05O3-δ.

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100 2.0

99.5

99 1.5

98.5 /A -12 %

*10 98 1.0 Mass/ current

97.5 Ion

97 0.5

96.5

96 0.0 300 400 500 600 700 800 900 Temperature/ ○C

2 2 FigureFigure 6. PlotPlot of mass vs. temperature and ion current (for m/z = 44; CO 2)) vs. vs. temperature temperature (under (under N N2)) forfor SrFe 0.95SS0.050.05OO3-δ3- preparedδ prepared in inair air (black) (black) and and O O2 2(red),(red), showing showing aa mass mass loss loss associated associated with with CO 2 inin thethe air air synthesised synthesised sample, sample, which which is is eliminated after heat treatment in O2.. Solid Solid lines lines indicate indicate %mass %mass andand dashed dashed lines indicate ion current.

The TGA results indicating the presence of carbonate may at first glance suggest the existence The TGA results indicating the presence of carbonate may at first glance suggest the existence of of a small amount of SrCO3 impurity. However, the temperature at which the CO2 is lost is a small amount of SrCO3 impurity. However, the temperature at which the CO2 is lost is significantly significantly lower than would be expected for SrCO3. In order to illustrate this, TGA data for SrCO3 lower than would be expected for SrCO3. In order to illustrate this, TGA data for SrCO3 were also were also collected and compared to the data for the SrFe0.95S0.05O3-δ. This experiment shows a collected and compared to the data for the SrFe0.95S0.05O3-δ. This experiment shows a significant significant difference in the temperature at which the loss of CO2 occurs for SrFe0.95S0.05O3-δ and difference in the temperature at which the loss of CO2 occurs for SrFe0.95S0.05O3-δ and SrCO3. SrCO3. In particular, the starting temperature for CO2 loss for SrFe0.95S0.05O3-δ occurs at a significantly In particular, the starting temperature for CO2 loss for SrFe0.95S0.05O3-δ occurs at a significantly lower lower temperature (≈◦490 °C vs. ≈760◦ °C for SrCO3), which may suggest that this carbonate is present temperature (≈490 C vs. ≈760 C for SrCO3), which may suggest that this carbonate is present in in the perovskite structure, i.e., we have a mixed sulfate/carbonate-doped sample—SrFe1−x−ySxCyO3-δ. the perovskite structure, i.e., we have a mixed sulfate/carbonate-doped sample—SrFe1−x−ySxCyO3-δ. Further work is required to investigate this possibility, although as noted in the introduction, Further work is required to investigate this possibility, although as noted in the introduction, carbonate carbonate has been shown previously to be accommodated in perovskite materials. has been shown previously to be accommodated in perovskite materials. Given the dual (O2 and CO2) mass loss for the air-synthesized SrFe1−xSxO3-δ samples, it is not Given the dual (O2 and CO2) mass loss for the air-synthesized SrFe1−xSxO3-δ samples, it is possible to determine reliable oxygen contents for these samples. In addition, as detailed by Starkov not possible to determine reliable oxygen contents for these samples. In addition, as detailed by et al., the determination of oxygen contents in partially substituted ferrites is non-trivial without a Starkov et al., the determination of oxygen contents in partially substituted ferrites is non-trivial reliable fixed reference point [24]. Since it is possible that there may still be a small amount of Fe4+ in without a reliable fixed reference point [24]. Since it is possible that there may still be a small amount these4+ samples after the N2 treatment, there is not a conclusive fixed reference oxygen point, and so of Fe in these samples after the N2 treatment, there is not a conclusive fixed reference oxygen point, any calculated oxygen contents would only be rough approximations. and so any calculated oxygen contents would only be rough approximations.

2.1.4. Heat Treatment under O2 2.1.4. Heat Treatment under O2

The samples (described above) heated under O2 were also examined by X-ray diffraction The samples (described above) heated under O2 were also examined by X-ray diffraction (Figure7). (Figure 7). All the sulfate-doped SrFe1−xSxO3-δ samples were shown to retain their original cubic All the sulfate-doped SrFe1−xSxO3-δ samples were shown to retain their original cubic structure while structure while undoped SrFeO3-δ remained tetragonal. undoped SrFeO3-δ remained tetragonal.

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Figure 7. X-rayX-ray diffraction diffraction patterns patterns of of (a ()a SrFeO) SrFeO3-δ,3- (δb,() bSrFe) SrFe0.975S0.9750.025SO0.0253-δ, O(c3-) δSrFe,(c0.95) SrFeS0.05O0.953-δS, 0.05andO 3-(dδ),

andSrFe (0.925d)S SrFe0.075O0.9253-δ afterS0.075 heatingO3-δ after under heating O2. under O2.

Cell parameters and site occupancies were determined by Rietveld refinement refinement using the X-ray diffraction data. From From these these structure structure refinements refinements (Table (Table 33),), therethere isis aa decreasedecrease inin thethe unitunit cellcell parameters upon heating heating in in O O2.2 .This This can can be be more more clearly clearly seen seen in Figure in Figure 8 where8 where the the variation variation in cell in cellvolume volume versus versus sulfate sulfate content content is compared is compared for samples for samples heated heated in O in2 and O2 andthose those just justheated heated in air. in air.These These data data show show a reduction a reduction in cell in cell volume volume on onheating heating in inoxygen, oxygen, which which can can be be correlated correlated with with a agreater greater concentration concentration of of the the smaller smaller Fe Fe4+4+ asas a aresult result of of an an increase in oxygenoxygen content.content. The cell parameter data also show an interesting variation with sulfate content, with an approximately linear increase in cellcell volumevolume upup toto xx == 0.05.0.05. The The fact fact that there is no further increase for x == 0.075 may therefore suggestsuggest thatthat thethe sulfatesulfate solubilitysolubility limitlimit isis closercloser toto xx == 0.05. The increase in cell volume with increasing sulfate content is at firstfirst glanceglance unexpectedunexpected given that S6+ isis significantly significantly smaller smaller than than Fe Fe3+/4+3+/4+. One. One possible possible explanation explanation could could relate relate to changes to changes in the in theFe3+ Fe/Fe3+4+/Fe ratio4+ ratioon sulfate on sulfate incorporation. incorporation. However, However, this cell this volume cell volume increase increase is also isalso seen seen for forthe theN2 3+ 3+ Ntreated2 treated samples samples (Table (Table 2), 2where), where we we only only have have Fe Fe, and, and thus thus another another factor factor must must be significant. be significant. In Inthis this respect, respect, the the cell cell volume volume increase increase may may be be a associatedssociated with with the the extra extra oxygen oxygen associated associated with this dopant. Thus, if we take thethe casecase ofof thethe NN22 treatedtreated samples,samples, wewe areare effectivelyeffectively replacingreplacing Fe(III)OFe(III)O1.51.5 with SOSO33,, whichwhich means means the the introduction introduction of anof extra an extra 1.5 oxide 1.5 oxide ions per ions sulfate, per sulfate, which might which be might expected be toexpected contribute to contribute to an expanded to an expanded cell size. cell size.

Table 3.3. LatticeLattice parametersparameters andand Fe/S Fe/S sitesite occupanciesoccupancies for for SrFe SrFe1−1x−xSSxxO3-3-δ, , obtainedobtained from RietveldRietveld reﬁnementrefinement usingusing XRDXRD datadata forfor SrFeSrFe11−xxSSxOxO3-δ3- afterδ after heating heating in inO2 O. 2.

SrFe1−xSxO3-δ Heated in O2 SrFe1−xSxO3-δ Heated in O2 S (x) 0 0.025 0.05 0.075 S (x) 0 0.025 0.05 0.075 a (Å) 3.8651(1) 3.8641(1) 3.8692(1) 3.8691(1) a (Å) 3.8651(1) 3.8641(1) 3.8692(1) 3.8691(1) c (Å) c (Å) 3.8477(1) 3.8477(1) ------V (Å3)V (Å3) 57.349(3) 57.349(3) 57.694(2) 57.694(2) 57.924(2) 57.924(2) 57.922(3) 57.922(3) Rwp (%)Rwp (%) 4.164.16 3.29 3.29 3.79 3.793.93 3.93 Rexp (%)Rexp (%) 3.713.71 2.81 2.81 2.73 2.732.79 2.79 Fe occFe occ 1(-) 1(-) 0.97(2) 0.97(2) 0.94(2) 0.94(2) 0.93(2) 0.93(2) S occ - 0.03(2) 0.06(2) 0.07(2) S occ - 0.03(2) 0.06(2) 0.07(2)

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Figure 8. Plot of unit cell volume vs. x for SrFe1−xSxO3-δ heated in air (●) and heated in O2 (■). Figure 8. Plot of unit cell volume vs. x for SrFe1−xSxO3-δ heated in air ( ) and heated in O2 (). Figure 8. Plot of unit cell volume vs. x for SrFe1−xSxO3-δ heated in air (●) and heated in O2 (■). 2.1.5. Conductivity Data 2.1.5.2.1.5. Conductivity Conductivity Data Data Following the successful incorporation of sulfate, the conductivities of the SrFe1−xSxO3-δ samples wereFollowing Followingexamined the inthe successful air. successful In general, incorporation incorporation the SrFe1−x ofS xofO sulfate, 3-sulfate,δ samples the the conductivitieswere conductivities found to ofhave of the the similar SrFe SrFe11− −conductivities,xxSSxxOO3-3-δ samplesδ samples werewithwere examined the examined exception in air.in air. of In aIn general, notable general, the decrease the SrFe SrFe1− 1atx−xSS xlowerxO3-δδ samples samplestemperatures were were found foundfor the to to havehigher have similar similarsulfate conductivities, content conductivities, (x = with0.075)with the the exceptionsample exception (Figure of aof notable 9).a notableThis decrease observation decrease at lower atof lowera temperaturesdecrease temperatures in conductivity for thefor higherthe forhigher sulfatehigher sulfate contentdopant content levels (x = (x 0.075 is = ) samplecomparable0.075) (Figure sample to9). the(Figure This silicon-doped observation 9). This observation SrFeO of a decrease3-δ system of a in decrease where conductivity it inwas conductivity proposed for higher that dopantfor athigher higher levels dopant doping is comparable levels levels is tothe comparable silicate silicon-doped disrupts to the silicon-doped SrFeO the Fe-O3-δ system network SrFeO where resulting3-δ system it was in where proposeda decrease it was that inproposed conductivity. at higher that doping at [23].higher Another levels doping the factor, levels silicate disruptshowever,the silicate the could Fe-O disrupts networkrelate the to lowFe-O resulting levels network inof ainsulating decreaseresulting impurities inin conductivity.a decrease in this in [highconductivity.23]. Anothersulfate content factor,[23]. Another sample, however, factor,given could relatethathowever, tothe low cell could levelsparameter relate of insulating todata low suggest levels impurities ofthat insulating the su inlfate this impurities solubility high sulfate in limit this content highmay sulfatebe sample,closer content to givenx = sample,0.05. that given the cell parameterthat the cell data parameter suggest that data the suggest sulfate that solubility the sulfate limit solubility may be limit closer may to be x = closer 0.05. to x = 0.05.

Figure 9. Plot of log σ vs. 1000/T for SrFeO3-δ (●), SrFe0.975S0.025O3-δ (○), SrFe0.95S 0.05O3-δ (■) and σ FigureSrFeFigure0.925 9. SPlot9.0.075 PlotO of3-δ log (of□) inlog vs.air. σ 1000/Tvs. 1000/T for SrFeOfor SrFeO3-δ ( 3-δ), ( SrFe●), SrFe0.9750.975S0.025S0.025OO3-δ3-δ ( (○),), SrFeSrFe0.950.95S0.050.05OO3-δ3- δ(■() and) and

SrFeSrFe0.9250.925S0.075S0.075OO3-3-δ ((□) )in in air. air. # 2.2. SrFe1−xBxO3-δ 2.2.2.2. SrFe SrFe1−x1−BxBxOxO3-3-δδ 2.2.1. X-ray Diffraction Data 2.2.1.2.2.1. X-ray X-ray Diffraction Diffraction Data Data The possible incorporation of borate into SrFeO3-δ was then examined, with samples of SrFeThe1−ThexB possiblexO 3-possibleδ prepared incorporation incorporation for 0 ≤ xof ≤of borate0.15.borate The intointo results SrFeOSrFeO showed3-3-δ waswas thatthen then a examined, examined,higher borate with with content samples samples was of of SrFeachievableSrFe1−x1B−xBxOxO3- 3-comparedδδ prepared to for the 0sulfate-doped ≤≤ xx ≤ 0.15.0.15. samples,The The results results with showed single showed phasethat that aborate-doped ahigher higher borate borate samples content content for wasx was≤ achievable0.1achievable (Figure compared 10).compared In terms to theto theof sulfate-doped the sulfate-doped effect of samples,borate samples, incorporation with with single single phaseon phase the borate-dopedstructure, borate-doped similar samples samples results for for werex ≤x ≤0.1 0.1 (Figure 10). In terms of the effect of borate incorporation on the structure, similar results were

Crystals 2017, 7, 169 9 of 13 Crystals 2017, 7, 169 9 of 13 Crystals 2017, 7, 169 9 of 13

(Figureobserved 10 ).as In for terms the SrFe of the1−x effectSxO3-δ ofsamples. borate incorporationIn particular, onborate the structure, doping resulted similar resultsin a similar were cubic observed cell observed as for the SrFe1−xSxO3-δ samples. In particular, borate doping resulted in a similar cubic cell asas forfor thethe SrFesulfate-doped1−xSxO3-δ samples. In particular, borate doping resulted in a similar cubic cell as for the sulfate-dopedas for the sulfate-doped samples. samples.

FigureFigure 10. 10.X-ray X-ray diffraction diffraction patterns patterns for for (a) ( SrFeOa) SrFeO3-δ,(3-δb, ()b SrFe) SrFe0.950.95BB0.050.05OO3-3-δδ,,( (cc)) SrFe SrFe0.90.9BB0.10.1OO3-δ3-. δ. Figure 10. X-ray diffraction patterns for (a) SrFeO3-δ, (b) SrFe0.95B0.05O3-δ, (c) SrFe0.9B0.1O3-δ.

CellCell parametersparameters andand sitesite occupancies occupancies were were determined determined using using the the Rietveld Rietveld method method (an (an example example ﬁt Cell parameters and site occupancies were determined using the Rietveld method (an example isfit shown is shown in Figurein Figure 11). 11). fit is shown in Figure 11).

FigureFigure 11. 11.Observed, Observed, calculated calculated and and difference difference X-ray X-ray diffraction diffraction proﬁle profile for for SrFe SrFe0.950.95BB0.050.05O3-δ. . Figure 11. Observed, calculated and difference X-ray diffraction profile for SrFe0.95B0.05O3-δ. The refinements gave Fe/B occupancies in good agreement with those expected (Table 4). TheThe reﬁnementsrefinements gavegave Fe/BFe/B occupanciesoccupancies inin goodgood agreementagreement withwith thosethose expectedexpected (Table(Table4 ).4). Furthermore, in this case, a decrease in the unit cell was observed on borate doping in agreement Furthermore,Furthermore, in in this this case, case, a decreasea decrease in thein the unit unit cell wascell was observed observed on borate on borate doping doping in agreement in agreement with with the smaller size of B3+ compared to Fe3+/4+, and the fact that unlike the situation for sulfate thewith smaller the smaller size of Bsize3+ compared of B3+ compared to Fe3+/4+ to, Fe and3+/4+ the, and fact the that fact unlike that the unlike situation the situation for sulfate for doping sulfate doping there is no additional oxygen associated with the dopant (i.e., we are effectively replacing theredoping is no there additional is no additional oxygen associated oxygen associated with the dopant with the (i.e., dopant we are (i.e., effectively we are replacingeffectively Fe(III)O replacing1.5 Fe(III)O1.5 with BO1.5). withFe(III)O BO1.51.5). with BO1.5).

Table 4. Lattice parameters and Fe/B site occupancies obtained from Rietveld analysis using X-ray Table 4. Lattice parameters and Fe/B site occupancies obtained from Rietveld analysis using X-ray diffraction data for SrFe1−xBxO3-δ. The structure of SrFeO3-δ was refined using a tetragonal space group diffraction data for SrFe1−xBxO3-δ. The structure of SrFeO3-δ was refined using a tetragonal space group (P4/mmm). The structures of the doped samples were refined using a cubic space group (Pm3m). (P4/mmm). The structures of the doped samples were refined using a cubic space group (Pm3m). SrFe1−xBxO3-δ SrFe1−xBxO3-δ B (x) 0 0.05 0.1 B (x) 0 0.05 0.1 a (Å) 3.8648(1) 3.8593(1) 3.8561(1) a (Å) 3.8648(1) 3.8593(1) 3.8561(1) c (Å) 3.8487(1) - - c (Å) 3.8487(1) - -

Crystals 2017, 7, 169 10 of 13

Table 4. Lattice parameters and Fe/B site occupancies obtained from Rietveld analysis using X-ray

diffraction data for SrFe1−xBxO3-δ. The structure of SrFeO3-δ was reﬁned using a tetragonal space group (P4/mmm). The structures of the doped samples were reﬁned using a cubic space group (Pm3m).

SrFe1−xBxO3-δ Crystals 2017, 7, 169 B (x) 0 0.05 0.1 10 of 13 a (Å) 3.8648(1) 3.8593(1) 3.8561(1) c (Å) V (Å3) 3.8487(1) 57.486(4) 57.483(2) - 57.336(4) - V (Å3) Rwp (%) 57.486(4) 1.84 57.483(2)3.23 3.67 57.336(4) Rwp (%)Rexp (%) 1.84 0.92 2.30 3.23 2.52 3.67 Rexp Fe(%) occupancy 0.92 1 0.92(1) 2.30 0.89(1) 2.52 Fe occupancy 1 0.92(1) 0.89(1) B occupancyB occupancy - - 0.08(1) 0.08(1) 0.11(1) 0.11(1) Fe/B UisoFe/B Uiso 0.003(1)0.003(1) 0.015(1) 0.015(1) 0.022(1) 0.022(1)

2.2.2. Stability under N2 2.2.2. Stability under N2 Heat treatment of the SrFe1−xBxO3-δ samples at 950 °C under◦ N2 showed similar results to those Heat treatment of the SrFe1−xBxO3-δ samples at 950 C under N2 showed similar results to observedthose observed on sulfate on sulfate doping, doping, with with the theborate-conta borate-containingining samples samples remaining remaining cubic cubic after after this this heat heat 3+ treatmenttreatment in nitrogennitrogen (Figure(Figure 12 12).). In In line line with with the the reduction reduction of theof the Fe oxidationFe oxidation state state to Fe to3+ ,Fe there, there was wasa shift a shift in the in peakthe peak positions positions to lower to lower angles, angles, indicating indicating a larger a larger unit unit cell. cell.

Figure 12. X-ray diffraction patterns for (a) SrFe0.95B0.05O3-δ,(b) SrFe0.95B0.05O3-δ after heating under Figure 12. X-ray diffraction patterns for (a) SrFe0.95B0.05O3-δ, (b) SrFe0.95B0.05O3-δ after heating under N2, N2,(c) SrFe0.9B0.1O3-δ,(d) SrFe0.9B0.1O3-δ after heating under N2. (c) SrFe0.9B0.1O3-δ, (d) SrFe0.9B0.1O3-δ after heating under N2.

2.2.3.2.2.3. Thermogravimetric Thermogravimetric Analysis Analysis These samples were then analysed by TGA (heat treatment under N ). In contrast to the These samples were then analysed by TGA (heat treatment under N2). 2In contrast to the air air synthesised SrFe S O samples, there was no mass loss due to CO observed in these synthesised SrFe1−xSxO1−3-δx samples,x 3-δ there was no mass loss due to CO2 observed in2 these SrFe1−xBxO3-δ systems,SrFe1−xB thusxO3- δindicatingsystems, thusno carbonate indicating present. no carbonate present. 2.2.4. Conductivity Data 2.2.4. Conductivity Data For the SrFe1−xBxO3-δ samples (x = 0.05, 0.1), the conductivity data showed signiﬁcantly lower For the SrFe1−xBxO3-δ samples (x = 0.05, 0.1), the conductivity data showed significantly lower conductivities at lower temperatures compared with the undoped system (Figure 13). However, above conductivities at lower temperatures compared with the undoped system (Figure 13). However, 600 ◦C, the conductivities were comparable with the x = 0.1 sample, in particular showing a small above 600 °C, the conductivities were comparable with the x = 0.1 sample, in particular showing a improvement in the conductivity compared with undoped SrFeO3-δ. Notably, these temperatures are small improvement in the conductivity compared with undoped SrFeO3-δ. Notably, these in the range where operation as a cathode in a solid oxide fuel cell would be. temperatures are in the range where operation as a cathode in a solid oxide fuel cell would be.

Crystals 2017, 7, 169 11 of 13 Crystals 2017, 7, 169 11 of 13

1.8

1.7

1.6

1.5 ) -1 cm

(S 1.4 σ

log 1.3

1.2

1.1

1 0.8 1 1.2 1.4 1.6 -1 1000/T (K )

Figure 13. Plot of log σ vs. 1000/T for SrFeO3-δ (●), SrFe0.95B0.05O3-δ (○), SrFe0.9B0.1O3-δ (■) in air. Figure 13. Plot of log σ vs. 1000/T for SrFeO3-δ ( ), SrFe0.95B0.05O3-δ ( ), SrFe0.9B0.1O3-δ () in air. # Conductivity values (at 700 °C) for the borate- and sulfate-doped samples are shown in Table 5, Conductivity values (at 700 ◦C) for the borate- and sulfate-doped samples are shown in Table5, and compared to the equivalent data for silicate-doped SrFeO3-δ (from reference [23]). The data show and compared to the equivalent data for silicate-doped SrFeO δ (from reference [23]). The data show similar values for all samples at this typical solid oxide fuel cell3- operating temperature. similar values for all samples at this typical solid oxide fuel cell operating temperature.

Table 5. Conductivity data in air at 700◦ °C for SrFe1−xMxO3-δ where M = Si [23], S and B. Table 5. Conductivity data in air at 700 C for SrFe1−xMxO3-δ where M = Si [23], S and B. Si (x) S (x) B (x) Si (x) S (x) B (x) 0 0.05 0.1 0.15 0 0.025 0.05 0.075 0.05 0.1 0 0.05 0.1 0.15 0 0.025 0.05 0.075 0.05 0.1 Conductivity 700 °C (S cm−1) 26 21 35 18 26 25 30 25 30 32 Conductivity 700 ◦C (S cm−1) 26 21 35 18 26 25 30 25 30 32 3. Experimental 3. Experimental High-purity SrCO3, Fe2O3, (NH4)2SO4, and H3BO3 were used to prepare SrFe1−xS/BxO3-δ samples. StoichiometricHigh-purity mixtures SrCO3 ,of Fe the2O3 powders, (NH4)2 SOwere4, andintimately H3BO3 groundwere used and toinitially prepare heated SrFe 1to−x S/B900 x°CO3- (4δ samples.°C/min) for Stoichiometric 12 h. Samples mixtures were then of theballmilled powders (350 were rpm intimately for 1 h, Fritsch ground Pulverisette and initially 7 heatedplanetary to ◦ ◦ 900Mill)C and (4 reheatedC/min) forto 1000, 12 h. 1050 Samples and were1100 then°C for ballmilled 12 h with (350 ballmilling rpm for 1of h, samples Fritsch Pulverisettebetween heat 7 planetary Mill) and reheated to 1000, 1050 and 1100 ◦C for 12 h with ballmilling of samples between treatments. For the SrFe1−xBxO3-δ (x = 0.05, 0.1) samples, a higher temperature (1200 °C) heat treatment ◦ heatwas treatments.required to Forachieve the SrFe single1−x BphasexO3-δ samples.(x = 0.05, In 0.1) order samples, to ensure a higher maximum temperature oxygen (1200 content,C) heat all treatmentsamples underwent was required a final to achieve heat treatment single phase at 350 samples. °C for 12 In h order in air. to ensure maximum oxygen content, all samples underwent a ﬁnal heat treatment at 350 ◦C for 12 h in air. Additionally, portions of the SrFe1−xSxO3-δ (x = 0, 0.025, 0.05, 0.075) samples were heated to 900 °C underAdditionally, oxygen for portions 12 h with of theslow SrFe cooling1−xSx Oat3- 50δ (x°C =/h 0, to 0.025, 350 °C, 0.05, with 0.075) the samples samples then were maintained heated to ◦ ◦ ◦ 900at thisC undertemperature oxygen for for 12 12 h h followed with slow by cooling cooling at 50at 50C °C/h /h to to 350 roomC, withtemperature. the samples To thentest the maintained stability at this temperature for 12 h followed by cooling at 50 ◦C/h to room temperature. To test the stability under low p(O2) conditions, both sulfate and borate-doped samples were heated under N2 to 950 °C ◦ underfor 12 lowh. p(O2) conditions, both sulfate and borate-doped samples were heated under N2 to 950 C for 12Powder h. X-ray diffraction data were used in order to determine lattice parameters and phase Powder X-ray diffraction data were used in order to determine lattice parameters and phase purity. For SrFe1−xSxO3-δ samples heated in air, X-ray diffraction data were collected on a Panalytical purity.Empyrean For SrFediffractometer1−xSxO3-δ samplesequipped heated with ina Pixcel air, X-ray 2D detector diffraction (Cu data K were radiation). collected For on the a Panalytical remaining Empyrean diffractometer equipped with a Pixcel 2D detector (Cu Kα radiation). For the remaining SrFe1−xSxO3-δ and SrFe1−xBxO3-δ samples, a Bruker D8 diffractometer with Cu K radiation was used. SrFe1Samples−xSxO3-δ wereand SrFealso 1analysed−xBxO3-δ usingsamples, thermogravimetric a Bruker D8 diffractometer analysis (Netzch with STA Cu 449 Kα1 F1radiation Jupiter was used. Thermal Analyser with mass spectrometry attachment). Samples were heated to 950 °C in N2 (10 °C/min) and held for 30 min to reduce the iron oxidation state to Fe3+. Pellets for conductivity measurements were prepared as follows: powders of SrFe1−xSxO3-δ and SrFe1−xBxO3-δ heated in air were initially ball milled (350 rpm for 1 h), before pressing into compacts and sintering at 1100 °C (SrFe1−xSxO3-δ) and 1200 °C (SrFe1−xBxO3-δ) for 12 h in air. Four Pt electrodes

Crystals 2017, 7, 169 12 of 13

Samples were also analysed using thermogravimetric analysis (Netzch STA 449 F1 Jupiter Thermal ◦ ◦ Analyser with mass spectrometry attachment). Samples were heated to 950 C in N2 (10 C/min) and held for 30 min to reduce the iron oxidation state to Fe3+. Pellets for conductivity measurements were prepared as follows: powders of SrFe1−xSxO3-δ and SrFe1−xBxO3-δ heated in air were initially ball milled (350 rpm for 1 h), before pressing into ◦ ◦ compacts and sintering at 1100 C (SrFe1−xSxO3-δ) and 1200 C (SrFe1−xBxO3-δ) for 12 h in air. Four Pt electrodes were attached with Pt paste and the samples were heated at 900 ◦C for 1 h in air. Samples were then furnace cooled to 350 ◦C and held at this temperature for 12 h to ensure full oxygenation. Conductivities were measured using the four probe dc method.

4. Conclusions The results presented in this paper demonstrate the ﬁrst reports of successful incorporation of sulfate and borate into SrFeO3-δ. This doping strategy results in a change from a tetragonal to a cubic cell, which is maintained even after heating under N2, where undoped SrFeO3-δ transforms to the oxygen vacancy ordered brownmillerite structure. Conductivity data in air show that the borate/sulfate-doped samples have comparable conductivities (Table5) to undoped SrFeO 3-δ at solid oxide fuel cell operating temperatures. Given these initial promising results, further studies are warranted to investigate the performance of these doped systems as solid oxide fuel cell cathodes.

Acknowledgments: We would like to thank EPSRC for funding (studentship for Abbey Jarvis). The raw datasets associated with the results shown in this paper are available from the University of Birmingham archive: http://epapers.bham.ac.uk/3011/. Author Contributions: P.R.S. conceived and designed the experiments; A.J. performed the experiments; A.J. and P.R.S. analysed the data; A.J. and P.R.S. wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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