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To overcome such microstructural issues, Ce_1–xAxNbO4±δ powders were first re-

duced under 5% H2/N2(g) (5 hours at 800°C) followed by sintering under argon gas

so as to prevent re-oxidation. Alternatively the powders may be reduced in-situ by sintering under a reductive atmosphere; however this was avoided to prevent induc- ing fractures. For example the surface of the green body would likely reduce first creating a compositional gradient; due to the high degree of compaction, reduction of the interior of the body would probably occur during the intermediate stage of sintering. Alternatively reductive pre-annealing followed by reductive sintering may prevent this, however further cerium reduction over the course of the sintering time may still induce fracturing.

Figure 6.13: SEM micrographs of Ce1–xAxNbO4±δ reduced under 5% H2/N2(g)at 800°C for 5

hours followed by sintering under Ar_(g) at 1500°C for 18 hours.

CeN bO_+δ Sr=0.1 Ca=0.1 Sr=0.2 Ca=0.2

(µm) (µm) (µm) (µm) (µm)

18.20 ± 2 7.51 ± 1 10.26 ±1 15.18 ± 2 37.53 ± 3

Table 6.7: The grain size of Ce1–xAxNbO4±δ sintered under Ar(g) at 1500°C for 18 hours

Similarly to the air sintered samples (Figure 6.3), the micrographs of argon sintered Ce1–xAxNbO4±δ presented in Figure 6.13 show only minor microcracking. However

as it can be seen from Table 6.4, there is a notable reduction in average grain size. Once again Ce_0.9Sr_0.1NbO_4±δand Ce_0.9Ca_0.1NbO_4±δ have a smaller grain size relative to the parent material, and both Ce0.8Sr0.2NbO4±δ and Ce0.8Ca0.2NbO4±δ

have a larger average grain size relative to compositions with 10% dopant additions. This observation implies a reduction in grain boundary mobility, either by further dopant segregation to the boundary, or possibly, by additional secondary phase particles which may act to pin them. A number of studies on other materials with mixed valence cations sintered under inert or reductive atmospheres have reported such phenomena. For example grain boundary segregation with inhibited grain growth was found for doped SnO₂ and SrTiO₃ [356][357].

Reduction of compensatory Ce4+ to Ce3+ during reductive annealing may lead to further dopant grain boundary segregation. This would increase the solute cloud at the boundary and increase the retarding force that inhibits grain growth. This is supported by Table 6.5 which provides the average atomic percentage of each constituent at the grain interiors of argon sintered Ce_1–xAxNbO4±δ, and which are

considerably more depleted of alkaline earth compared to air sintered specimens. It is possible that the reason CeNbO_4+δ can accommodate significantly more alkaline earth compared to LaNbO₄, despite the virtually identical ionic radii of La3+ (1.16 ˚

A) and Ce3+ (1.143 ˚A) is due to the oxidation of cerium to the 4+ valence state (0.97 ˚A). The change in both cerium radius and coordination may lead to coopera- tive effects that act to stabilise the dopant, and which are partially removed upon reduction. Whilst any segregation effects may be minor after the short annealing duration and furthermore not so easily distinguishable by diffraction methods; the diffraction work from Chapter 4 did not show the presence of secondary phases af- ter annealing Ce_1–xAxNbO4±δ in 5% H2/N2(g) for 3 hours at 800 °C. Furthermore

EDXS of the grain interior after longer thermal treatment (5 hours) did not indicate significant compositional changes (Section 6.3.1) (Appendix C, Table C.6). Assum- ing solute drag is the cause for reduced grain growth, this work does not distinguish at what point the segregation occurred, but it does suggest that high temperature sintering may have induced segregation and subsequent precipitation of secondary phases from reduced Ce_1–xAxNbO4–δ.

Composition Ce (at%) A2+ (at %) Nb (at%) O (at%)

Ce0.9A0.1NbO4 15 1.6 16.7 66.6 Sr=0.1 18.81 ± 2.79 0.87 ± 0.24 18.94 ± 1.52 61.38 ± 4.39 Ca=0.1 20.05 ± 2.56 0.77 ± 0.18 19.56 ± 1.12 59.62 ± 3.67 Ce0.8A0.2NbO4 13.3 3.3 16.7 66.6 Sr=0.2 15.51 ± 1.23 2.22 ± 0.26 17.98 ± 0.7 64.30 ± 1.93 Ca=0.2 17.08 ± 1.01 3.09 ± 0.20 19.24 ± 0.60 60.59 ± 1.75

Table 6.8: Average atomic percentage of each constituent calculated by EDXS at the grain interior in Ce_1–xAxNbO4±δ first reduced under 5% H2/N2(g) at 800°C for 5 hours followed by

sintering under Ar_(g) at 1500°C for 18 hours. Standard deviations are calculated over many spectra from randomly positioned sample sites.

Figure 6.13 highlights regions with visually identifiable secondary phases, which were non-uniformly distributed, but significantly more notable compared to the surface of Ce_1–xAxNbO4±δ sintered under air. For example Ce0.9Ca0.1NbO4±δ

which was previously shown to have clean grain boundaries contained unwanted phases. Because the starting powders were of the same synthetic batch to those used for the analysis presented in Section 6.2, this implies that over the course of the two thermal treatments any dopant segregation was followed by phase precipitation.

The morphologies of the secondary phases also differ considerably from those of the air sintered samples, and are more particulate-like and less dispersed around the grain boundaries, which may be a consequence of the different sintering atmosphere or suggest an absence of a liquid phase. The latter would further support the inhib- ited grain growth particularly of Ce_0.8Sr_0.2NbO_4±δ, despite the greater deficiency of dopant in the grain interior. EDXS of the secondary phases for all compositions, show a greater ratio of alkaline earth to niobium compared to samples sintered under air, indicating a composition closer in agreement with A₂Nb₂O₇.

The greater alkaline earth content and probable volume fraction of secondary phases may be attributable to greater segregation and precipitation of strontium and cal- cium during the annealing processes. However a liquid phase might still be expected unless the alkaline earth content of the unwanted phase increased such that it was greater than or equal to A₂Nb₂O₇, in which case the melting temperature would increase to ≥1610 °C and ≥1576 °C for A=Sr and A=Ca respectively [358][340]. Alternatively a liquid phase may be initially present, but through segregation and dissolution over the course of the anneal, the composition was altered leading to de-wetting. An increase in the solid-liquid interfacial energy could also cause the liquid phase, if present, to confine to triple point regions between grain boundaries instead of dispersing between grains.

Composition Spectrum Ce (at%) A2+ (at %) Nb (at%) O (at%)

Sr=0.1 1 3.33 14.89 19.28 62.52 2 3.44 13.90 18.92 63.75 Sr=0.2 1 6.44 15.61 21.08 56.88 2 2.87 16.15 19.81 61.18 Ca=0.1 1 4.35 23.90 22.63 49.12 2 7.00 15.40 18.87 58.74 Ca=0.2 1 4.12 19.91 23.85 52.12 2 4.36 17.22 21.94 56.47

Table 6.9: Atomic percentage of each constituent from the secondary phases presented in Figure 6.13 determined by EDSX.

Sintering of Ce1–xAxNbO4±δ under air was consistently performed in one furnace to

avoid temperature differentials, whilst non-oxidising anneals were performed in an alternate furnace, which may have been calibrated differently which may perhaps provide a simpler explanation for what is interpreted as an absence of a residual liquid phase. According to Table 6.5, the grain interior of Ce_0.8Ca_0.2NbO_4±δ was

the least deprived of dopant which would suggest less segregation and therefore less solute drag and which may account for the larger grain size. Specifically why this is observed for Ce_0.8Ca_0.2NbO_4±δ and not Ce_0.9Ca_0.1NbO_4±δ for example, is not known, but could be due to temperature irregularities with respect to sample position in the furnace. The grain size of the parent material also decreased after inert atmosphere sintering. CeNbO_4+δ will likely contain some impurities, however this should be low (<0.5%). Instead it is suggested that a combination of sintering atmosphere and reductive pre-annealing may alter the mechanisms of mass transport during sintering, how specifically, is beyond the scope of this work, but has also been suggested in the literature for other ceramics [356][359].