Venta predio Ocaña, Barrio Cristo Rey, Norte de Santander a la Alcaldía del mismo municipio

The structural modulation accompanying the oxidation of CeNbO_4+δ with heating, enables the use of diffraction methods as simple diagnostic tool for studying the redox behaviour of these phases, and example x-ray diffraction patterns for each

of the oxidised phases of the parent material can be found in [253]. In-situ x-ray diffraction measurements (Alba Synchrotron, Spain, Chapter 3) were collected on 10% and 20% acceptor doped Ce_1–xAxNbO4+δ (A=Ca, Sr) with dynamic heating

at 10°C min–1 under static air, to examine their redox behaviour in relation to the parent material and identify any initial differences generated by dopant type and concentration (Figure 5.1). Table 5.1 presents a summary of the hyperstoichiometric phases formed by Ce_1–xSrxNbO4+δ, and shows similarly to CeNbO4+δ, three prin-

ciple processes i. a large oxidation step from ≈350 °C ii. reduction of the oxygen excess and formation of the tetragonal polymorph and iii. re-oxidation on cooling.

Three phases coexist upon heating; δ=0, δ=0.08 and δ=0.25 the overall hyperstoi- chiometry is intermediate between δ=0.08 and δ=0.25. However the intensity of the δ=0.25 reflections are much weaker (Figure 5.1) for Ce_0.8Sr_0.2NbO_4+δ, which sug- gests the average value of delta is lower in comparison to Ce0.9Sr0.1NbO4+δ which

may reflect the higher dopant content and subsequently greater compensatory Ce4+. Both phases show the evolution of the tetragonal polymorph as a discrete phase upon heating, however on cooling the tetragonal reflections diverge to first form an un- known phase visually representative of a stoichiometry intermediate between δ=0.08 and δ=0.25. For Ce_0.8Sr_0.2NbO_4+δ the temperature range of stability of the tetrag- onal phase is wider, which shall be discussed in greater depth in Section 5.3. With further cooling Ce_1–xSrxNbO4+δ forms a δ=0+0.08 phase mixture which remains

stable to room temperature. Both 10% and 20% strontium doped Ce_1–xSrxNbO4+δ

show similar behaviour overall, which might be expected given the marginal in- crease of strontium content that accompanies the targeted dopant level. Whilst no dynamic in-situ x-ray diffraction data has been collected on CeNbO_4+δ, the calcu- lated content from thermogravimetry (Chapter 2, Section 2.3) with a heating rate of 10 °C min–1 was δ=0.25 and δ=0.08 on heating and cooling respectively, therefore strontium additions appear to supresses the hyperstoichiometry [263].

Figure 5.1: Dynamic in-situ x-ray diffraction (Alba Synchrotron) of Ce_1–xSrxNbO4±δ, heated at

Phase Sr=0.1 Sr=0.2 Heating δ=0 25-497 °C 25-473 °C

δ=0.08 345-755°C 365-708°C δ=0.25 390-712°C 430-623 °C Cooling δ=0+0.08 607-25 °C 587-25 °C

Table 5.1: Summary of the hypostoichiometric phases identified by dynamic in-situ x-ray diffraction (Alba Synchrotron) collected on Ce_1–xSrxNbO4+δ heated at 10 °C min–1 under static

air, data collection approximating 2 minutes every 30°C.

Unusually, Ce1–xCaxNbO4+δ did not present any evidence of oxidation, instead

displaying a gradual increase in symmetry analogously to LaNbO₄ [209] and to heating the CeNbO_4+δ phases under vacuum or under reductive atmospheres [263]. Further analysis was therefore performed using laboratory in-situ methods. A semi- dynamic program was implemented by heating Ce_1–xCaxNbO4+δ at 10 °C min–1

with data collection for 2 minutes every 100°C in an attempt to best replicate Figure 5.1 and minimise isothermal heating which may lead to different redox behaviour. Figure 5.2 presents evidence for oxidation; from 400°C both Ce_0.9Ca_0.1NbO_4+δ and Ce_0.8Ca_0.2NbO_4+δ form a δ=0.25 type stoichiometry, which remains single phase up to the onset temperature of the tetragonal transition at 700 °C. It can be seen that Ce_0.9Ca_0.1NbO_4+δoxidises over a broader temperature range of 3-400°C and is almost entirely tetragonal by 700 °C, suggesting that higher calcium concentrations raise the transition temperature.

Figure 5.2: Semi-dynamic in-situ x-ray diffraction (laboratory source) collected on Ce1–xCaxNbO4+δ heated at 10°C min–1, data collection for 2 minutes every 100°C.

Upon forming the δ=0.25 stoichiometry, the parent cell of CeNbO₄ shows an ap- preciable contraction from 324.43 ˚A to 310.95 ˚A at 25 °C as cerium oxidises to the tetravalent state [253]. This is similarly represented at high temperature by a notable high angle shift, particularly the (200) reflection which occupies an inter-

mediate position between the (040) and (002) reflections and which readily distin- guishes the pattern of δ=0.25 from the δ=0 stoichiometry (Figure 5.4). However the reflection positions of the oxidised phase of Ce_0.9Ca_0.1NbO_4+δ and in particu- lar Ce_0.8Ca_0.2NbO_4+δ, display only a small shift relative to the starting δ=0 phase upon heating. Because formation of the δ=0.25 stoichiometry in the parent material is accompanied by oxidation of 50% of the cerium sites to the tetravalent state, for- mation of this phase by Ce_1–xCaxNbO4±δ is expected to result in a similar relative

contraction of the parent cell to that observed between CeNbO4 and CeNbO4.25,

regardless of the compensatory Ce4+ if the contraction is fairly linear with Ce4+ content which it is for the parent material (∆Ce4+ of 10% gives ∆V of 2.47 ˚A3). Whilst the change in cell volume is specifically non-linear with increasing Ce4+ and Ca2+ content in as-prepared Ce_1–xCaxNbO4±δ (Chapter 4), the relationship be-

tween cell volume and Ce4+ content upon oxidation would have to be particularly non-linear for these phases to form the same stoichiometry as the parent material of δ=0.25 due to the already contracted cell of Ce_1–xCaxNbO4±δ. This implies Figure

5.2 corresponds to the oxidation of phases structurally similar to CeNbO_4.25 but of lower oxygen stoichiometry.

Figure 5.3: A: The cell volume of Ce1–xCaxNbO4±δ and CeNbO4+δ with dynamic heating

(corresponding cell constants can be found in Appendix B, Table B.1). B: Comparison of the patterns at 600°C. C: Comparison of the cell volume of Ce1–xCaxNbO4±δ and CeNbO4+δ at

room temperature as a function of speculated Ce4+ content.

Interestingly upon oxidation, both Ce_0.9Ca_0.1NbO_4+δand Ce_0.8Ca_0.2NbO_4+δafford patterns and cell volumes that are remarkably similar to CeNbO_4.25. This may suggest that the structures contain similar net Ce4+ contents, which is responsible for the contraction of CeNbO₄ phases with or without excess oxygen. However as Figure 5.3.C shows comparison of the cell volume as an indicator of cerium oxidation is not absolute and may be complicated by other structural factors for example the dopant and thermal expansion characteristics at high temperature.

On cooling both compositions form phases that are dissimilar to both the starting materials and those formed on heating. Whilst the patterns are still representative of the δ=0.25 stoichiometry, the cell volume increases relative to the phases formed on heating as per Figures 5.3.A and 5.4, indicating a lower oxygen content. With continued cooling Ce0.9Ca0.1NbO4+δ becomes biphasic, reforming the δ=0 phase

which coexists with the oxidised phase; the (121) and (002) reflections of which converge and overlap significantly at room temperature. However the pattern of Ce0.8Ca0.2NbO4±δ post oxidation is almost identical to the δ=0 stoichiometry, with

the exception that the (121) and (002) reflections show a small high angle shift, and a corresponding contraction of the parent cell suggesting δ>0.

Figure 5.4: Comparison between the different phases formed on heating and cooling Ce_1–xCa_xNbO_4±δ semi dynamically at 10°C min–1.

A possible cause for the lack of oxidation observed in the data collected at the Alba Synchrotron facility may lie in the sample environment; data collection required sample loading in packed, sealed capillaries, with the top of the sample that is exposed to air falling outside of the beam. It is possible that Ce_1–xCaxNbO4+δ

samples were packed more densely relative to those doped with strontium. Data collected with the laboratory source required depositing a fine layer of powder on a platinum foil with the entire sample surface exposed to static air. This in its self may lead to different oxidation behaviour, for example the catalytic influence of Pt cannot be ruled out. The semi-dynamic laboratory program was therefore performed on Ce_0.8Sr_0.2NbO_4+δ to compare results (Appendix B, Figure B.1), and whilst this program has fewer data points the results are broadly in agreement, showing the same behaviours as that presented in Figure 5.1. The exception that the δ=0.25 evolves from a slightly lower temperature.

5.2.2

Simultaneous TG-DSC

Thermogravimetric analysis was performed on as-prepared Ce1–xAxNbO4+δ, heated

dynamically at 10 °C min–1 under static air, with isothermal heating at 800°C for 45 minutes. Similarly to the thermogravimetric data collected on CeNbO_4+δ two consecutive reduction re-oxidation processes can be identified for each composition. The TG curve begins with a mass loss from ≈200 °C, which is complete at ≈320 °C signifying the onset of the large oxidation step. A similar initial mass loss has also been observed in the TG curve of CeNbO_4+δ, however inconsistently [263]. It is possibly attributed to the reduction of δ=0.08 secondary phase commonly formed during quenching, however such a large reduction is inconsistent with diffraction patterns of the as-prepared materials which show the presence of this phase to be negligible (<1-2%) which would generate a mass loss of ≈0.01%. Furthermore the reduction of compensatory Ce4+ accompanying the loss of oxygen from regular lattice sites is not supported by any discontinuities in the cell constants from x-ray diffraction. Instead it is assumed to result predominantly from the desorption of surface species such as H₂O or CO₂. With further heating the oxidation process peaks at ≈530-570 °C, and assuming Ce1–xAxNbO4+δ at the onset of this process

is oxygen stoichiometric (δ=0), corresponds to the calculated hyperstoichiometry provided in Table 5.2.

Figure 5.5: Simultaneous TG-DSC collected on Ce1–xAxNbO4+δ heated at 10°C min–1under

static air, A-D represent Sr=01, Sr=0.2, Ca=0.1 and Ca=0.2 respectivly.

onset of the monoclinic-tetragonal transition which is evident by a corresponding endotherm and the previous structural data presented in Figures 5.1 and 5.2. This work suggests the tetragonal phase of Ce_1–xAxNbO4+δ may be slightly oxygen defi-

cient under oxidising conditions, the mass of Ce_1–xSrxNbO4+δ reduces further with

isothermal heating above the transition temperature indicating slow reduction ki- netics. However the calculated deficiencies fall within reasonable error allowances. On cooling Ce_1–xAxNbO4+δ re-oxidises from ≈660-700 °C, affording a stoichiome-

try approximately equal to δ=0 for all compositions except Ce0.9Ca0.1NbO4±δ. The

discrepancy with the structural work which predicts Ce_1–xSrxNbO4+δ to have an

approximate stoichiometry of δ ≈0.04, may arise from either i. ambiguity of the ini- tial mass loss and errors associated with thermogravimetry ii. the differing sample environments between the techniques iii. an oxygen deficient phase coexisting with the hyperstoichometric phase, however this is not supported by x-ray diffraction.

Oxidation Reduction Reduction Oxidation Endotherm

Peak (Isothermal) Cooling Heating (T°C)

Sr=0.1 δ=0.17 δ=-0.01 δ=-0.02 δ=0 668.9, 701.2

Sr=0.2 δ=0.12 δ=-0.02 δ=-0.03 δ=-0.01 607.9, 660

Ca=0.1 δ=0.23 δ=-0.02 δ=-0.02 δ=0.04 686.6

Ca=0.2 δ=0.12 δ=-0.02 δ=-0.02 δ=0.01 643.3

Table 5.2: Summary of the redox processes identified by simultaneous TG-DSC collected on Ce1–xAxNbO4+δ heated at 10 °C min–1 under static air.

In agreement with Figure 5.1, the oxygen contents of Ce_0.9Sr_0.1NbO_4±δ and Ce0.8Sr0.2NbO4±δ are intermediate between a δ=0.08 and δ=0.25 phase mixture,

and that of Ce_0.8Sr_0.2NbO_4±δ is lower reflecting the weaker reflection intensity of the δ=0.25 phase. The calculated hyperstoichiometry of Ce_0.9Ca_0.1NbO_4±δ is com- parable to the δ=0.25 phase indicated by Figure 5.2, however the oxygen content of Ce_0.8Ca_0.2NbO_4±δ is much lower reflecting the smaller contraction of the par- ent cell which is expected to accompany the simultaneous oxidation of Ce3+ to Ce4+ and incorporation of excess oxygen. This further provokes the assumption that Ce_0.8Ca_0.2NbO_4±δ forms a new oxidised phase of intermediate stoichiometry previously unobserved in the parent material. The (new) phases formed on cooling Ce1–xCaxNbO4+δ also represent intermediary stoichiometries between δ=0 and that

formed on heating, the oxygen content of which is lower for Ce_0.8Ca_0.2NbO_4±δ.