DOI: 10.1002/adma.((please add manuscript number))
Remarkable oxide ion conductivity observed at low temperatures in a complex
superstructured oxide
R.J. Packer and S.J. Skinner*
[*]
Dr. S. J. Skinner, Corresponding-Author,
Department of Materials
Imperial College London
Prince Consort Road
London
SW7 2BP
UK
E-mail: s.skinner@imperial.ac.uk
Keywords: Ionic conductivity, superstructure, CeNbO4+d, diffusion
Conventionally, oxide ion conduction in ceramics is viewed as requiring high symmetry
crystal structures, preferably cubic or near cubic, such as the fluorite1, 2 and perovskite type
oxides2. In materials with these structure types it is accepted that ion conduction is isotropic
and that this property is essential for the application of these materials in useful devices such
as SOFCs and permeation membranes. Indeed the development of fast oxide ion conductors
has been predicated on high symmetry systems. More recently fast ion conduction has been
demonstrated in anisotropic oxides such as La2NiO4+d (LNO)3-7 and GdBaCo2O5+d (GBCO)811
. In these anisotropic systems excellent performance as ionic conductors and as fuel cell
electrodes10,
12, 13
has been demonstrated for the bulk materials, suggesting that the ion
conduction in the anisotropic direction is not adversely affected by the relatively slow
transport perpendicular to this conduction plane3. Each of these compositions has lower
symmetry than the typical fluorite- and perovskite-type oxide ion conductors, with La2NiO4+d
and related layered materials adopting crystallographically tetragonal and/or orthorhombic
structures depending upon composition. With La2NiO4+d in the orthorhombic structure the
1
diffusivity of the oxygen species has been shown, from atomistic simulations, to involve
oxygen interstitial species14, a result that has been confirmed experimentally from neutron
diffraction studies of the Pr analogue7. In both the LNO and GBCO families the materials
adopt crystallographic structures with relatively low symmetry that cannot be considered as
distorted cubic systems. Clearly these materials show that fast ion transport is feasible in
lower symmetry systems and occurs at levels that are competitive with the high symmetry
three dimensional ionic conductors. Having demonstrated fast oxide ion conduction in noncubic systems (GBCO) and those with oxygen excess (interstitial) contents (LNO), efforts
have subsequently focussed on widening the range of new fast oxide ion conductors and
evaluating the potential of structurally complex materials. Here we report on remarkable low
temperature fast ion conductivity in a complex modulated, superstructured, interstital oxide,
CeNbO4+d.
CeNbO4+d has been reported to exist in one of four polymorphs depending on excess
oxygen stoichiometry, with discrete phases reported for the CeNbO4, CeNbO4.08, CeNbO4.25
and CeNbO4.33 compositions15. Structurally CeNbO4+d adopts the parent fergusonite
monoclinic structure at temperatures below 800oC, and on oxidation of Ce3+ to Ce4+, a long
range ordered oxygen sublattice exists resulting in a series of complex polymorphs, including
both commensurate and incommensurate superstructures, as evidenced by electron
diffraction15.
Previously, as an extension of studies on anisotropic oxide ion conducting
La2NiO4+d-type oxides, the high temperature scheelite type CeNbO4+d phase was investigated
and found to be a mixed ionic-electronic conductor at temperatures greater than 800 oC, with
oxygen diffusion coefficients determined through isotopic labelling studies as being of the
order of 10-8 cm2s-1, somewhat lower than competing ionic conductors,16, 17 but still exhibiting
fast oxide ion conductivity. Given the desire to operate electrochemical devices, such as solid
oxide fuel cells, at lower temperatures this study of CeNbO4+d was extended to the lower
2
temperature fergusonite structured oxides, with the overall aim of stabilising the scheelite
phase at lower temperatures by cation substitution.
Oxygen tracer diffusion coefficients, D*, and surface exchange coefficients, k*, Fig. 1,
were determined over the technologically relevant temperature range for solid state
electrochemical devices of 600 – 850 oC, and found to exhibit non-Arrhenius behaviour, with
a significant increase in diffusion coefficient on decreasing temperature. This increase in
oxygen diffusion coefficient coincides with the temperature of the scheelite-fergusonite phase
transition. Whilst there is a degree of scatter in the diffusion data, more so as the material
approaches the phase transition, this is not unexpected in isotopic labelling/SIMS
measurements as the data quality relies on the interaction of the ion beam with the sample.
These interactions can result in roughening of the surface, and can be affected by defects such
as closed porosity which will vary from sample to sample. However it is likely that some of
the scatter observed in the phase transition region (1073 – 1150K) can be attributed to the
kinetics of equilibration and associated with the change in oxygen stoichiometry. From
thermal analysis studies of these compositions (see supplementary information, S1) it is
evident that the oxygen stoichiometry will vary as a function of temperature, but that in the
low temperature regime the non-stoichiometry is always 4+ and hence the material adopts
either the CeNbO4.08 or the CeNbO4.25 structure types, both of which are modulated
superstructures. It is also assumed that these compositions are not line phases but represent
phase fields with a range of oxygen stoichiometries. The exact thermodynamic limits of these
phases are yet to be determined, although previous work by Vullum et al18 has indicated a
stability limit at high pO2. As these measurements were not made at high pO2, this stability
limit is not considered relevant here. Considering these factors and despite the data scatter it is
noteworthy that the activation energy of the low temperature diffusion process is ~0.99eV,
consistent with the activation energy of oxygen diffusion in many oxide ion conductors19.
From Figure 1 it appears that the oxygen transport in the high temperature tetragonal phase is
3
temperature independent, however we believe that the phase change and associated oxygen
stoichiometry change is more likely to give an anomalously elevated value for the data
obtained at exchange temperatures of between 1120 and 1150K due to incomplete phase
transformation. Given that the lower temperature phase is structurally significantly more
complex than the high temperature phase, this remarkable fast ion conduction is unexpected.
Corroborating these diffusion data, and eliminating the possibility of this enhancement
being due to experimental artefacts, is the simultaneous calculation of the surface exchange
coefficients, Fig. 1(b), which follow a relatively linear temperature dependence through the
phase transition region, with a calculated activation energy of ~1.22eV. From Fig. 2 it is
evident that the oxygen diffusivity of monoclinic CeNbO4+d (T < 1073K) is competitive and
comparable with La0.7Sr0.3CoO3-d (LSC)20,
21
, reported as having the highest oxide ion
diffusion coefficients of all reported mixed conducting oxides. It is of course of note that the
activation energy of the LSC phase is slightly lower than that calculated for the CeNbO4+d
material, however this level of ionic diffusion is rarely observed and is significant in such a
complex material. With such remarkable ionic conduction properties it is essential that a full
understanding of the diffusion mechanism and structural chemistry is developed.
To aid in the interpretation of these data, samples of CeNbO4+d were investigated by
powder neutron diffraction to highlight any superstructure present that would result from
oxygen ordering. Weak diffraction peaks that are typical indicators of superstructure would be
unlikely to be observed in powder X-ray diffraction data and hence the greater sensitivity of
powder neutron diffraction to oxygen ions has been exploited in this study. Initially the
experimental data were found to fit well to a simple monoclinic cell as reported by Santoro et
al22 with a = 5.5451(6)Å, b = 11.422(1)Å, c = 5.1696(6)Å and  = 94.622(8)o, but close
examination of the powder diffraction pattern indicated several peaks that could not be
accounted for with the monoclinic model and neither were these peaks accounted for by
fitting to secondary phases. Considering the electron diffraction data available15 a model
4
structure was generated based on the proposed supercell and a Le Bail refinement23 to the
powder diffraction data performed using the GSAS package24, 25 giving refined parameters of
a = 14.376(1) Å b = 22.799(2) Å c = 11.819(1) Å,  = 90.27(1)o,  = 74.94(1)o and  =
89.98(1)o in spacegroup P1. In Fig. 3 it is clear that this accounts for all of the experimental
intensity. As the  and  angles refined to values close to 90o it is suggested that the
superstructure of this phase is higher symmetry monoclinic (rather than triclinic) and that a
full structure refinement currently under way will determine the commensurate nature of this
phase. It should also be noted that the oxygen excess in this phase is accommodated as
oxygen interstitials and it is only in the oxygen excess materials that the superstructure is
present. Studying the structure of this proposed model suggests that there are pseudosinusoidal oxygen pathways, Fig. 4, through the superstructured oxide that could provide a
facile diffusion route for oxide ions in the anisotropic (ab) direction. This suggests that some
long range ordering of interstitial oxygen may be beneficial for fast ion conduction,
particularly if that ordering is modulated.
Hence it is clear that there is strong evidence for fast oxide ion conduction in a low
temperature complex oxide and that the presence of superstructure and ordering of oxygen
ions when oxygen interstitial species are present is related to this behaviour. This discovery
indicates that there are potentially many more oxides available that would provide lower
temperature oxygen diffusion materials for fuel cells and membranes opening up a new
operating temperature range for devices.
Experimental
Samples of CeNbO4+d were prepared using a standard solid state route as detailed elsewhere
[15,16] and confirmed to be single phase by powder X-ray diffraction (Philips PW1700 series,
Cu K radiation). Powders were then ball milled in acetone using stabilized zirconia milling
media for 24 hrs to break up agglomerates and reduce particle sizes. The milled powder was
5
then air dried before 1 g of powder was uniaxially pressed into 13mm diameter disks,
isostatically pressed at 300 MPa and then sintered into dense bodies at 1500oC for 18 hrs.
Density of the sintered bodies was determined using the Archimedes technique and all
samples were found to be in excess of 95% of theoretical density.
Isotopic exchanges were performed on a series of samples over the temperature range
600-900 oC. Each of the dense samples were polished with sequentially finer SiC papers,
followed by the use of diamond polishing media down to a 0.25m finish. These samples
with mirror finishes were placed in the isotopic exchange apparatus as described elsewhere[21,
26].
Each exchange involved a pre-anneal in research grade oxygen followed by the
exchange anneal in 200 mbar of
18
O2. After each anneal the sample was quenched to room
temperature. To determine the diffusion and surface exchange coefficients of the exchanged
samples secondary ion mass spectrometry was used, with
18
O diffusion profiles collected
using an Atomika 6500 instrument with either Ar or N2 primary ion beams. Each diffusion
profile was fitted to the Crank solution to Ficks 2nd law of diffusion for a semi-infinite
medium, with the data corrected for the natural isotopic background concentration and the
isotopic concentration of the gas phase as discussed elsewhere[27, 28].
Powder neutron diffraction data were collected on the Polaris time-of flight instrument
at the ISIS spallation source, Rutherford Appleton Laboratories, Oxford, UK.
Acknowledgements
The authors would like to thank the EPRSC in the UK for providing funding for a studentship
(RJP) through the Doctoral Training Account and also the STFC for the award of instrument
time on Polaris at the Rutherford Appleton Laboratories, Didcot, UK.
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
6
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7
(a)
(b)
Figure 1. (a) Oxygen tracer diffusion and (b) surface exchange coefficients determined by
SIMS for CeNbO4+d highlighting the remarkable low temperature fast oxide ion diffusion.
Line in (a) at T>1073K is only intended as a guide and highlights that the data in the outline
box are likely to display anomalously high diffusion coefficients associated with incomplete
phase transformation.
8
-5
CeNbO4+d
Log D* [cm2s-1]
La0.7Sr0.3CoO3
-6
La0.6Sr0.4Co0.2Fe0.8O3-d
-7
-8
-9
8.0
8.5
9.0
9.5
10.0
10.5
11.0
10000/T K-1
Figure 2 – Comparison of the oxygen tracer diffusion coefficients determined for CeNbO4+d
(low temperature) with literature reports for La0.7Sr0.3CoO3-d19 and La0.6Sr0.4Co0.2Fe0.8O3-d20.
Dashed lines are intended only as a guide.
9
Figure 3 – Neutron diffraction data recorded for CeNbO4+d in static air and refined using the
Le Bail method23. Experimental data – crosses, solid line - fit, with phase markers and
difference plot also displayed below the curves. A final χ2 value of 2.968 was achieved.
10
Figure 4 – Proposed CeNbO4+d superstructure highlighting the pseudo-sinusoidal oxygen
pathways resulting from the commensurate modulation as identified by Thompson et al15 .
Red – oxygen, yellow – cerium, green – niobium.
11
Supplementary Information
(a)
(b)
Figure S1 – Thermogravimmetric analysis data of as-prepared CeNbO4+d phases on heating
(a) in flowing air, indicating the oxidation of the materials and the reduction of the oxidized
phases on passing through the phase transition temperature. Note: CeNbO4.25 does not oxidize.
(b) cooling data indicating the slight reoxidation of all phases on transition back to the
monoclinic phase.
Experimental
Thermogravimetric analysis data were obtained from loose powder samples of the CeNbO4+d
compositions using a Netzsch Jupiter STA449 instrument with a heating rate of 20o/min for
both heating and cooling steps. Samples were supported in Pt crucibles. A constant flow of
dried air was passed over the samples during both cycles.
12
The table of contents entry should be fifty to sixty words long, written in the present tense,
and refer to the chosen figure.
Fast oxide ion conductivity is observed in a structurally complex material, CeNbO4+d, at
remarkably low temperatures indicating that a new approach to bulk ionic conductors may be
required, opening new research directions for future electrochemical devices such as solid
oxide fuel cells.
Keyword (see list): Ceramics, Fuel Cells,
R.J. Packer and S.J. Skinner*
Title: Remarkable oxide ion conductivity observed at low temperatures in a complex
superstructured oxide
-5
CeNbO4+d
Log D* [cm2s-1]
La0.7Sr0.3CoO3
-6
La0.6Sr0.4Co0.2Fe0.8O3-d
-7
-8
-9
8
9
10
11
12
10000/T K-1
13