UNA ENMIENDA POR CANSANCIO

The aim of the experiment described in this section was to obtain a pure DP forming composition, without any SP content and free of BaMoO4 impurity phase. For all compositions presented herein (Table 4.18), their B-site is half-filled with Co, and the Mo content varies inversely from the Fe content. The BSCFMo0.25-(Co/Fe=2) (Section 4.3.4.1), BSCFMo0.375- (Co/Fe=4) (Section 4.3.3) and BSCFMo0.45-(Co/Fe=10) (Section 4.3.4.2) compositions, which have also Co=0.5 content, are included in this study for comparison reasons.

As can be seen in Figure 4.18 and, there is a clear tendency to higher DP% as the Mo content increases and Fe decreases. Taken as the starting point the BSCFMo0.25-(Co/Fe=2) biphase compound, consisting of 80.2(6) % SP and 18.5(6)% DP, further addition of Mo results in gradually higher DP% content in the biphasic assemblages, as for instance BSCFMo0.375- (Co/Fe=4) and BSCFMo0.45-(Co/Fe=10) specimens containing 71.2(7) % DP and 91.2(3) % DP respectively. 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 20 40 60 80 100 Fe content SP DP BaMoO₄ Phas e (wt %) Mo content 0.30 0.25 0.20 0.15 0.10 0.05 0.00

Figure 4.18: Evolution of % phase by weight with increasing the Mo content and decreasing the Fe content, based on Rietveld refinements against XRD data for the series of BSCFM compositions for which the B-site is half-filled with Co.

Another interesting observation from Figure 4.19, is the evolution in the weight % of the BaMoO4 phase. The formation of the impurity phase is suppressed with increasing the Mo

Page | 192 content, and subsequently decreasing the Fe content, from BSCFMo0.25-(Co/Fe=2) (1.3(1)% BaMoO4) to BSCFMo0.45-(Co/Fe=10), as discussed in Section 4.3.4.3. However, for the higher Mo contents in BSCFMo0.475-(Co/Fe=20) and BSCFMo0.5-(Co/Fe=∞), it increases slightly to 1.2(1)% and 3.2(1)% respectively.

As discussed previously (Section 4.1.3), the driving force for the formation of the DP phase is the B-site ordering induced by the adequate differences in charge and size of the B-site cations. Upon substitution with Mo6+, B-site ordering occurs between the larger and lower charged Co2+ (0.745 Å)ions and the smaller and higher charged Mo6+ (0.59 Å), resulting in a Co-rich and a Mo-rich octahedral site. Complete B-site order would be therefore expected for Mo=0.5, however addition of Mo≥ 0.475 results in the formation of BaMoO4 impurity phase. The BSCMo0.5-(Co/Fe=∞) specimen with nominal composition Ba0.5Sr0.5Co0.5Fe0Mo0.5O3-δ (or BaSrCoMoO6) is an SP-free compound within error, but contains some small amount of BaMoO4. This demonstrates that the formation of a pure DP compound is not favoured under the synthesis conditions. According to the literature, the synthesis of the isostructural DP compounds BaCoMoO6 and Ba2CoWO6, also crystallising in Fm3m, is accompanied with the formation of the Ba(Mo,W)O4 impurity phase [119] and to the best knowledge of the author, there is no report for impurity-free cubic perovskite report[249-251], whilst the Sr-analogue, Sr2CoMoO6 crystallises in tetragonal space group[252-254]. This demonstrates that the formation of BaMoO4 is favoured under the synthesis conditions in this study. This is likely due to the fact that the synthesis is conducted in air and hence some oxidation of Co2+ to Co3+ might occur and hence resulting in the expulsion of the excess Mo6+ as BaMoO4. It is worth noting the synthesis of pure BaMoO4 is reported to be made also at 1000oC in air[255], similarly with the conditions used for the synthesis of BSCFM compounds. Inert or slightly reducing synthesis conditions might prevent the formation of BaMoO4 phases and result in impurity-free BaSrCoMoO6.

Page | 193 Table 4.18: Summary of structural parameters extracted from Rietveld refinements against XRD data for BSCFM for which the B-site is half-filled woth Co, showing phase fractions (Phase %), lattice parameters (Lat.par.), crystallite sizes (Cr.sz), nominal and refined compositions.

Phase(%) SP DP BaMoO4 Lat.par. (Å) a (SP) a (DP) a, c (BaMoO4) Cr.sz.(nm) SP DP DP-odd Nominal composition Refined composition BSCFMo0.25- (Co/Fe=2)** 80.2(6) 18.5(6) 1.3(1) 3.97726(8) 7.97945(60) 5.549(1), 12.824(5) >800(i) 79(5) 46(11) Ba0.5Sr0.5Co0.500Fe0.250Mo0.250O3− 98.7%Ba0.5Sr0.5Co0.535(1)Fe0.220(4)Mo0.244(6)O2.84(1) +1.3% BaMoO4 BSCFMo0.375- (Co/Fe=4)(#1)* 28.8(7) 71.2(7) 0 3.98476(14) 7.98108(8) --- 98(3) 403(41) 70(4) Ba0.5Sr0.5Co0.500Fe0.125Mo0.375O3- Ba0.5Sr0.5Co0.500(4)Fe0.125(5)Mo0.376(2)O2.93(5) BSCFMo0.409- (Co/Fe=6) 22.7(12) 77.3(12) 0 3.98778(24) 7.98275(9) --- 38(3) 133(4) 49(3) Ba0.5Sr0.5Co0.500Fe0.091Mo0.409O3- Ba0.5Sr0.5Co0.496(2)Fe0.114(1)Mo0.389(7)O2.93(5) BSCFMo0.45- (Co/Fe=10)*** 8.7(3) 91.2(3) 0 3.98671(13) 7.98940(4) --- 317(79) >800(i) >800(i) Ba0.5Sr0.5Co0.50Fe0.05Mo0.45O3− Ba0.5Sr0.5Co0486(6)Fe0.087(8)Mo0.425(5)O2.95(8) BSCFMo0.475- (Co/Fe=20) 7.1(3) 91.7(3) 1.2(1) 3.98485(15) 7.99056(4) 5.528(3), 12.67(1) --- >800(i) >800(i) Ba0.5Sr0.5Co0.500Fe0.025Mo0.475O3- 98.8%Ba0.5Sr0.5Co0485(5)Fe0.084(7)Mo0.429(7)O2.96(1) +1.2% BaMoO4 BSCMo0.5- (Co/Fe=∞) 0.1(1) 96.6(2) 3.2(1) 4.16(2) 7.99429(7) 5.557(1), 12.760(4) ---- 97(6) 103(1) Ba0.5Sr0.5Co0.500Fe0.000Mo0.500O3- 96.7%Ba0.5Sr0.5Co0.552(8)Mo0.448(2)O2.97(2) +3.2% BaMoO4

(i) No broadening of the diffraction peaks was observed. The limit of the measureable domain size is in the region of 800 nm.

Page | 194 The lattice parameters of the major DP phase (Figure 4.19, plotted as aP= aDP/2) are just slightly increased for the compositional range between BSCFMo0.25-(Co/Fe=2) and BSCFMo0.375- (Co/Fe=4) from 7.97945(60) Å to7.98108(8) Å respectively, accompanied by an approximately 4 times increase in the DP content from 18.5(6)% to 71.2(7)% accordingly. Moving to higher Mo content and Co/Fe ratio towards BSCMo0.5-(Co/Fe=∞), results in more significantly increase of the DP lattice parameter to 7.99429(7) Å, whilst the refined DP content of 96.6(2)% indicates an increase of about 1.5 times. This demonstrates that the main effect of introducing Mo in the iso- Co compositions, with subsequent decrease in the Fe content, is the increase of the DP volume, which is likely to have very similar composition. When reaching a maximum of approximately 70% for BSCFMo0.375-(Co/Fe=4), the DP composition is likely affected, which however was not refined according to the refinement protocol used.

0.20 0.25 0.30 0.35 0.40 0.45 0.50 3.980 3.985 3.990 3.995 4.000 Fe content aP (Angs trom) Mo content SP DP 0.30 0.25 0.20 0.15 0.10 0.05 0.00

Figure 4.19: Evolution of perovskite lattice parameters for SP (aP=aSP) and DP (aP= aDP/2) with increasing Mo content and decreasing Fe content, in compositions for which their B-site is half-filled with Co.

Considering the global composition, the variable charged transition metals (Co2+/3+/4+, Fe3+/4+) are reduced upon substitution by d0 Mo6+. In general, cations with lower oxidation state are larger in size than for the same cations higher charged. The ionic radii for Co2+ (0.745 Å), in 6-fold coordination environment as in the perovskite structure and high-spin state, is bigger than the corresponding values for Co3+ (0.61 Å) and Co4+ (0.53 Å). Similarly, Fe3+ cations(0.645 Å) are

Page | 195 larger than the Fe4+ (0.585 Å) counterparts. Since increasing amount of Mo6+ induces the reduction of the transition metals to the lowest oxidation state possible under the synthesis conditions (i.e. Co2+ and Fe3+), the perovskite unit cell is expected to expand.

The unit cell for the minor SP phase also expands from 3.97726(8) Å to 3.98778(24) Å for BSCFMo0.25-(Co/Fe=2) and BSCFMo0.409-(Co/Fe=6) respectively. The slight decrease in the SP lattice constants for the BSCFMo0.45-(Co/Fe=10) and BSCFMo0.475-(Co/Fe=20) is most likely due to experimental errors due to the very small 7-8% refined amount.

4.4.

Summary and Conclusions

According to the combined refinement of X-ray (XRD) and Neutron diffraction (ND) data, BSCFMo0.375-(Co/Fe=4), with refined composition Ba0.5Sr0.5Co0.500(3)Fe0.125(1)Mo0.375(3)O2.94(1), is a DP-rich compound, consisting of 70.81(10) % DP and 28.13(9) % SP by weight. There is also a small amount of BaMoO4 (1.06 (8) %), detectable only for the sample for which the joint XRD/ND refinement was performed. Both SP and DP components in BSCFMo0.375-(Co/Fe=4) are A-site disordered whilst B-site ordering occurs only in the DP in contrast to the SP counterpart, as coming out from both the joint Rietveld refinement and confirmed by the HAADF analysis. The SP component was refined to Ba0.5Sr0.5Co0.547(9)Fe0.253(2)Mo0.200(8)O2.81(3), with determined Co and Fe in their trivalent states. The DP phase was refined to Ba0.5Sr0.5Co0.480(1)Fe0.071(1)Mo0.449(2)O2.98(2), with high Mo contentand Co in its divalent oxidation state, whilst Fe is still present as Fe3+. This demonstrates that the size and charge differences of Co2+ (0.745 Å, in high spin state) and Mo6+ (0.59 Å) favor the B-site ordering in the DP component, resulting in a Co-rich (refined composition Co0.480(1)Fe0.020(1))) and a Mo-rich (refined composition Mo0.449(2)Fe0.051(2)) antisite, similarly to the isostructural BaCoMoO6 .

The overall low Co oxidation state (calculated to 2.47+) in BSCFMo0.375-(Co/Fe=4) compared to BSCF (calculated to 3.3+), induced by substitution by Mo6+, and the subsequent low oxygen non-stoichiometry (δ= 0.7) are believed to be the reason for the decrease in the linear thermal expansion coefficient (TEC= 13.87x10-6 K-1), compared to BSCF (δ= 0.29, TEC= 19.5-20.86 K- 1

Page | 196 Hence, the introduction of the d0 Mo6+ causes the reduction of the dn Co, Fe cations to lower oxidation states and concomitant decrease of the charge carriers, resulting in low electrical conductivity (Chapter 3, Section 3.3.1). The good performance of BSCF has been attributed to its high oxygen non-stoichiometry and high oxygen ionic mobility in the bulk[69]. Hence, against expectation, BSCFMo0.375-(Co/Fe=4) is not a poor cathode even if it has lower electrical conductivity and significantly higher oxygen content than BSCF, suggesting that the cathode material behaves in a different way. The good electrochemical performance of BSCFMo0.3.75- (Co/Fe=4) among the rest BSCFM compositions studied must be due to the interplay of the SP and DP component phases.

As coming out from the evolution of the biphasic assemblage with temperature, there are two competing processes that dominate the BSCFMo0.375-(Co/Fe=4) system, favoring the increase of either the SP or the DP content. As coming out from the variable temperature in-situ experiment in combination with the quenching analysis from the last step of the synthesis procedure (annealing at 1000oC for10h) clearly show that temperature favors the enrichment in the entropically favored disordered SP, as indicated by the increase in both weight% and volume weight SP crystallite size. This was attributed to the breaking of the antiphase boundaries in the DP crystallites upon heating. As coming out from the microstructural analysis, the DP crystallites free of antiphase boundaries and corresponding to the DP-odd reflections, represent 71% of the total volume of the DP crystallites for the quenched sample from the synthesis temperature, whilst they represent the 30% and 37% of the total DP volume for the specimen at room temperature and 900oC respectively, as coming out from the in-situ diffraction experiment at the temperature range 25-900oC. However, when the sample was annealed at higher temperature of 1200oC for 10h, followed by either quenching or slow cooling, the SP content in the biphase assemblage is decreased, indicating that the B-site ordering becomes more favourable resulting in higher DP content (86%). Pure DP was not achieved, implying that the reaction occurs via a mechanism based on nucleation and growth, rather than spinodal decomposition for which metastable structures result in a single phase at high temperature.

The two competing processes of favoring the SP or DP formation are evident throughout the whole BSCFM compositional range. The B-site ordering is favored for high Mo content

Page | 197 resulting in gradually increasing DP% content for the example of BSCFMoMo-(Co/Fe=4) series of compositions. For low Mo content (Mo≤0.25), high overall Co/Fe ratio favors the B-site ordering between Co2+-Mo6+ due to the high Co content, as in the case of BSCFMo0.125- (Co/Fe=10) which is a biphasic compound, containing 12.7% DP, whilst BSCFMo0.125- (Co/Fe=4) and BSCFMo0.125-(Co/Fe=2) are pure SP compounds from the diffraction data. In the case of the BSCFMo0.25 compositional pair, the major phase in BSCFMo0.25-(Co/Fe=4) is DP with refined content of 61.6(4)%, whilst BSCFMo0.25-(Co/Fe=2) is an SP-rich compound with just 18.5% DP. Moreover, the combination of high Mo and low Fe content, favors the B-site ordering and gradual increase in DP% content in compositions which are half-filled with Co, leading to the SP-free within error BSCFMo0.5-(Co/Fe=∞).

The competing process of SP enrichment in the biphasic compounds, becomes evident for high Mo- containing specimens. In the compositional pairs of BSCFMo0.375 and BSCFMo0.409, the SP phase fraction increases with the increase of the Co/Fe ratio. For instance, the SP% weight fraction in BSCFMo0.375-(Co/Fe=10) is 44.1(4)% SP that is about 1.5 times more than BSCFMo0.375-(Co/Fe=4), indicating that the B-site ordering is less favored and that Mo6+ is preferentially accommodated in the SP structure. The study of the BSCFMoMo-(Co/Fe=4) series reveal that even if there is a clear tendency to increasing DP% with increasing Mo content, the fluctuation in lattice parameters of the minor SP phase for Mo≥0.25 in contrast with the DP counterpart, suggest tentatively some non-controlled Mo incorporation into the SP structure.

The formation of the BaMoO4 impurity phase was often encountered in the BSCFM biphasic compounds, with no more than 10% weight fraction in all cases. As coming out from the in-situ diffraction experiment for BSCFMo0.375-(Co/Fe=4), BaMoO4 starts forming at 900oC with refined content of 1.04(8)% and increases with temperature reaching approximately 4% for both samples quenched after 10h annealing at the synthesis temperature (1000oC) and 1200oC. However, the slow cooling from the synthesis temperature seems to favor the incorporation of BaMoO4 in the biphasic assemblage in BSCFMo0.375-(Co/Fe=4).

The expulsion of BaMoO4 from the BSCFM compounds was found to be greatly dependent on the Co/Fe ratio, indicating the maximum Mo that can be introduced into the perovskite biphasic

Page | 198 structure. The formation of 1.3(1)% BaMoO4 in BSCFMo0.25-(Co/Fe=2) highlights that the high charge of Mo6+ cannot be compensated by charge balance and reduction of the transition metals. This is due to its relative high overall Fe content that can only be reduced to Fe3+ at the synthesis conditions, whilst further reduction to its divalent state needs temperatures above 1560oC in air or more extreme reducing conditions. On the contrary, Cocan bereduced to its divalent state upon Mo6+ incorporation into the BSCFM system, since Co2+ is one of the common oxidation states of cobalt in air. For the BSCFMoMo-(Co/Fe=4) series, BaMoO4 starts forming for Mo≥ 0.409, with increasing content from 1.6(1)% to 7.5(1)% for BSCFMo0.409- (Co/Fe=4) and BSCFMo0.45-(Co/Fe=4) respectively, implying that the system cannot counterbalance the high charge induced by the Mo6+ incorporation. The formation of BaMoO4 is further suppressed for as high Mo content as 0.45 in the Co-rich BSCFMo0.45-(Co/Fe=10), with only 0.05Fe in its B-site. Finally, the expulsion of 3.2(1)% BaMoO4 from the end member BSCFMo0.5-(Co/Fe=∞), which does not contain Fe and is an SP-free compound, suggest that the formation of a pure DP compound is not favoured under the synthesis conditions in air, that is likely to favor the oxidation of a small amount of Co2+ to Co3+.

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