Cite this: DOI: 10.1039/c0xx00000x
Absence of Ni on the outer surface of Sr doped La2NiO4 single crystals
Mónica Burriel,*a Stuart Wilkins,b John P. Hill,b Miguel Angel Muñoz-Márquez,c
Hidde H. Brongersma, a, d John A. Kilner,a,c Mary P. Ryan,a and Stephen J. Skinner *a
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
outermost surface layer and near-surface region of La2-xSrxNiO4+δ
A combination of surface sensitive techniques was used to
single crystals.
determine the surface structure and chemistry of
To date the outermost surface structure and surface chemistry of
La2-xSrxNiO4+δ. These measurements unequivocally showed
K2NiF4-based nickelates (with the A2BO4 structure) has only
that Ni is not present in the outermost atomic layer,
55
been explored using a theoretical simulation approach. Read et
suggesting that the accepted model with the B-site cations
al.9, 13 evaluated the surface energy and crystal morphology of
exposed to the environment is incorrect.
low index surfaces of La2-xSrxNiO4+δ finding that by the
incorporation of Sr into La2NiO4+δ electron holes are created. The
Solid oxide fuel cells (SOFC) offer a low carbon electrochemical
Ni3+ species are postulated as being active sites responsible for
route to energy conversion with high efficiency and low
emissions, an attractive alternative to combustion of hydrocarbon 60 the catalytic activity of Ni-containing perovskite-related oxides.
Read also concludes that the (111) and (001) surfaces are
fuels. Nickelate compounds with the K2NiF4-type structure are
terminated by Ni+2-O species, that the (110) surface is terminated
promising mixed conducting cathodes for intermediate
by all three ion species, and the (100) surface is O2- terminated.
temperature solid oxide fuel cells (IT-SOFC) 1-4. Their fast
Although the surface energies are similar, this (100) surface is
oxygen transport properties are related to their ability to
accommodate hyperstoichiometric oxygen in interstitial positions 65 suggested as the highest stability surface for the undoped
La2NiO4+δ phase, in which the NiO6 octahedra on the outermost
in the rocksalt layers of the structure.1 Recent molecular
plane are distorted from the bulk symmetry. The higher activity
dynamics studies of La2NiO4+δ by Chroneos et al. 5 have shown
of the transition metal cations, in comparison to that of the
that the oxygen migration process takes place by an interstitialcy
alkaline-earth metals or lanthanide metals, in A2BO4 oxides has
mechanism involving the migrating interstitial oxygen (Oi) and a
70
also been predicted from density functional theory (DFT)
neighbouring apical oxygen (Oa). In addition, there is evidence of
simulations for the LaSrCoO4 (010) surface. In this case the O2
the orientational dependence of oxygen surface exchange in
reduction was calculated to be more energetically favourable on
La2NiO4+δ thin films 6. Substitution of La with the larger Sr atom
Co sites (B-sites) than on La/Sr sites (A-sites), on the basis of the
can increase the stability of the La2NiO4+δ structure7 and also
difference in adsorption energies 14. Furthermore, DFT
enhances the electronic conductivity with the shortening of the
8
Ni–O1 (equatorial O) bond length. Additionally, upon chemical 75 simulations on single perovskite LaMnO3 (001) surfaces predict
the cubic MnO2 surface (B-site) with adsorbed O atoms to be the
doping with Sr, oxidation of Ni2+ to Ni3+ is expected together
most stable one 15 and ab-initio studies on several perovskite
with a decrease in the cell volume, which reveals the minor
SOFC cathode materials (LaBO3 (001) with B=Mn, Fe, Co and
tendency to accommodate excess of oxygen.
9
Ni) suggest the LaO (A-site) termination to be catalytically
As highlighted by Read et al. , understanding of the surface
chemistry of oxides is essential for a wide variety of industrial 80 inactive due to its high O vacancy formation and strong O
adsorption energies 16.
applications, from catalytic oxidation of NH3 to the oxygen
Typically surfaces are difficult to analyze due to their
reduction and incorporation of species in fuel cell electrodes.
sensitivity to carbonation, and the issue of segregation of species
Even though the surface properties of these materials play a
to outermost layers. Traditionally XPS has been used to
critical role in the ultimate device efficiency, as the surface
oxygen exchange is often the rate-limiting step, the structure and 85 determine the surface chemistry of species, but the technique is
inherently limited to the near-surface, as the chemical
chemistry of the surfaces have never been thoroughly
information provided is always an average over a distinct volume.
investigated. In the case of lanthanum manganite (La1-xSrxMnO3,
Conventional XPS experiments at normal take-off angles for the
LSM), the state-of-the art cathode material, a significant effort is
detected photoelectrons and using laboratory X-ray sources
presently being directed towards the understanding of the atomicscale processes governing the cathode performance 10-12. In this 90 provide signals integrated over the first 5-10 nm; a distance
defined by the photoelectron inelastic mean free path (IMFP, λ)
work we have focused on understanding the structure and related
which depends on the photoelectron kinetic energy and
properties of the lanthanum nickelate oxides by combining three
consequently on the binding energy to a specific element.
complementary and specialized techniques: X-ray scattering
Because 95% of the photoelectrons will escape in 3λ, if the takethrough crystal truncation rod (CTR) experiments, Low Energy
95
off angle is grazing to the surface the detection depth can be
Ion Scattering (LEIS) and angle-resolved X-ray photoelectron
reduced down to sub-nanometric distances depending on the
spectroscopy (AR-XPS) and have obtained information on the
experimental set-up. To experimentally determine and fully
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elucidate the surface structure and chemistry of La2-xSrxNiO4+δ
we have used a combination of surface sensitive techniques:
CTR experiments to probe the surface structure and LEIS
measurements to probe the surface chemistry. Complementary
near-surface (0.5-7 nm) and chemical redox information on the
same samples was provided by using angle-resolved-XPS.
Single crystals of La1.67Sr0.33NiO4+δ composition were grown
by the floating-zone technique (described in detail elsewhere 17).
A large La1.67Sr0.33NiO4+δ single crystal was cleaved along the
(001) and (110) crystallographic planes into several smaller
crystals. One of the single crystal samples was used for the CTR
measurements. The functional form of the CTRs is indicative of
the precise form of the surface termination and is extremely
sensitive to the atomic arrangement of the uppermost layers.
Two further samples were used for LEIS measurements. LEIS is
a surface analysis tool based on the scattering of low energy ions
that provides elemental characterisation of the outermost surface
layer of atoms in a solid material. Most importantly it yields
quantitative, matrix independent, elemental characterisation of
this layer and near surface depth profiles 18. The advent of LEIS
now offers the opportunity of correlating surface exchange
kinetics with chemical processes at materials atomic surfaces,
giving unprecedented levels of information on electrochemical
systems with isotopic discrimination (e.g. 18O/16O ratios). 19,20
LEIS experiments were performed on 2 single crystals: one
“as-cleaved” and one “heat treated” (72 hours in air at 450 ºC).
For each of the samples two cleaved surfaces were measured:
(001) and (110) which had been previously treated with atomic
oxygen during 32 min to remove hydrocarbon contaminants from
the surface. Each surface was measured using two different noble
gas ions: He+/3 keV and 22Ne+/5 keV (further experimental
details included in ESI). Figure 1 shows the 22Ne+ LEIS energy
spectrum obtained for the (001) face of the La1.67Sr0.33NiO4+δ
single crystal after heat treatment. The original data are plotted
together with the extracted background (originated from sputtered
ions and fit to an exponential function) and with the background
corrected spectrum. The spectrum shows the peaks corresponding
to the elemental composition of the outermost surface layer. The
main peaks can be clearly attributed to single scattering of the Sr
and La elements. Critically we find that there is no peak at the
expected energy position for Ni indicating its absence from the
surface monolayer. In addition to the main Gaussian peaks the
contribution of the in-depth distribution of La appears at the
lower energy side of the corresponding peak suggesting a high
near-surface concentration of this species. Similar conclusions are
drawn from the corresponding data recorded for the (110) heattreated surface (ESI, Fig. S1) and for the (110) and (001) faces of
the as-cleaved sample. All the data are consistent with the
presence of La and Sr, and the absence of Ni, at the surface.
Preference of the A-site cations at the surface of had been
previously measured by LEIS for polycrystalline BaZrO321 and
Sm1-xSrxCoO3- δ22 perovskites. To obtain the sensitivity and
detection limit of each element (Ni, La and Sr) high-purity NiO,
SrO and La2O3 powders were used as reference materials (LEIS
energy spectra and calculations included in ESI, Fig. S2). Ni has
the lowest sensitivity, with its detection limit calculated to be
10% of the surface fraction. In the He+ spectra, in addition to the
La and Sr peaks, peaks corresponding to lighter elements such as
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lattice O, together with some surface contamination peaks (Na, K
and F), were observed.
X-ray scattering measurements were carried out at beamline
X22C at the National Synchrotron Light Source (Brookhaven
National Laboratory) with 12 keV X-rays using a six-circle
diffractometer for surface scattering experiments. The
experiments were carried out in an α = β mode, where α denotes
the incidence angle, that is, the angle between the incident X-ray
and the crystal surface, and β denotes the exit angle. The surface
normal was parallel to the crystallographic c-axis. The CTR
scattering intensity distributions along the (00L), (-20L) and
(-11L) directions were measured in an air atmosphere at room
temperature and 450 ºC, to investigate more closely the system
under near-operating fuel cell conditions. A Lorentzian squared
fit was applied to each scan and optimised by the least square
method. Footprint, Lorentz factor and Debye-Waller corrections
were applied before comparing experimental and modelled
truncation rod data.
The crystal truncation rod data for the (001) surface along the
(00L) direction are shown in Figure 2, along with the fits to the
data for the two possible ‘perfect’ terminations: La1.67Sr0.33O2 and
NiO2. The data demonstrate that the surface is clearly (La,Sr)O
terminated, and not NiO2 terminated, in good agreement with the
LEIS results. Figure 3a is a representation of the (La,Sr)2NiO4
structure, consisting of a stacking sequence of (La,Sr)2O2
bilayers and NiO2 monolayers along the c-axis. A 3D view of the
crystal highlighting the two possible ideal terminations for the
(001) face (top surface) is represented in Figure 3b: NiO2
termination at the left and (La,Sr)O termination at the right side.
Similar results (i.e., best fit for the (La,Sr)O termination) were
obtained both at high temperature and room temperature for the
(00L) direction, and for the two other measured CTR directions,
namely (-20L) and (-11L) (ESI, Fig. S3). On closer examination
some deviation of the fit to the perfect (La,Sr)O surface
termination is observed. This can be attributed to a combination
of several surface effects, such as the variation in surface
stoichiometry (Sr:La and (Sr+La):Ni ratios) and to strain
relaxation in the surface region. Attempts at introducing
compositional and strain variations within the CTR model
structure evidence a complex combination of chemical
differences, rumpling and relaxation of the interplanar spacing in
the surface region. Therefore a simple isotropic strain relaxation
model cannot be applied.
To complement these results and to provide information on the
near-surface composition, angle-resolved XPS measurements
were also performed on the (001) faces for both the “as-cleaved”
and the “heat treated” single crystals to obtain a compositional
depth-profile. La 3d and 4d, Ni 2p and 3p, Sr 3d, O 1s and C 1s
photoelectron peaks were recorded in with a monochromatic
AlKα source using a pass energy of 30 eV (further experimental
details included in ESI). Figure S4 (ESI) shows that in both
crystals samples (as-cleaved and annealed for 72 hours in air at
450 ºC) a mixture of Ni3+ and Ni2+ is present and the amount of
Ni decreases as the surface is approached, in good agreement
with LEIS and CTR results. Additionally, in the as-cleaved
sample, the amount of Ni3+ is greater than in the annealed,
especially when comparing the 4 nm regions. For a proper
determination of the Ni3+/Ni2+ ratio, a careful study of the Ni 3p
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line has been performed for both samples (Fig. 4). The Ni 3p line
shows coexistence of Ni3+ and Ni2+ that appear at approximately
71 and 67 eV respectively, which is consistent with the reported
literature results.23 After the heat treatment both components are
reduced and, as expected, Ni3+ noticeably decreases its
contribution from being 34% to 17% of the total Ni component
(for a depth of 3.5 nm). The average oxidation state varies
therefore from +2.34 in the as-cleaved to +2.17 in the annealed
crystal, following a similar behavior to that observed for
La2NiO4+δ powder by X-ray absorption spectroscopy of the nearedge region (XANES). 24 Table 1 shows the thickness-dependent
A:B site ratios, i.e. (Sr+La):Ni ratios, calculated from the fitted
XPS spectra taken on the (001) faces at different angles. The peak
areas used for this calculation correspond to the Ni 3p, La 4d and
Sr 3d photoelectron peaks since their kinetic energy is very
similar (i.e. 1418, 1380 and 1354 eV respectively) and therefore,
the probed depth will be closely the same. For both samples the
surface clearly shows an A-site enrichment, which decreases in
depth towards the bulk theoretical A:B site ratio of 2. For the ascleaved crystal the expected theoretical ratio is nearly reached for
a depth of 7 nm, while as for the heat treated sample an A-site
enrichment 2.25 times larger than the theoretical one (A:B ratio =
4.5) is measured at the same depth. However, it has to be taken
into account that the depth profiling performed with XPS is a topdown analysis; this means that the outermost surface layer
contribution is always present in the recorded data. By increasing
the take-off angle the collected photoelectrons will be closer to
the surface, therefore shallower regions close to the surface will
be analysed. It should be pointed out that the concentration ratios
calculated at each depth correspond to the average composition of
an analysed volume from the surface to that depth. This means,
that if the average A:B ratio at 7 nm is 2.32, and the external
surface is A-site enriched, the exact composition at that depth
would be smaller than 2.32.
Table 1. (La+Sr):Ni ratio calculated from the fitted XPS spectra
taken on the (001) faces at different angles
Depth (nm)
0.6
1.8
3.5
4.8
5.9
6.6
7.0
bulk (theoretical)
(La+Sr)/Ni
As cleaved
Heat treated single
single crystal
crystal
5.3 ± 1.2
6.7 ± 1.9
8.5 ± 1.6
5.5 ± 1.1
6.5 ± 1.0
5.2 ± 0.9
3.9 ± 0.5
4.3 ± 0.7
3.4 ± 0.4
4.6 ± 0.6
3.1 ± 0.4
4.7 ± 0.5
2.3 ± 0.2
4.5 ± 0.4
2.0
2.0
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These complementary techniques show that Ni does not appear
at the outermost crystallographic surface. This is counter to the
hypothesis that Ni3+ species at the surface facilitates the catalytic
activity of these materials. In Kröger-Vink notation, the overall
exchange reaction at the gas/film interface can be written as:
1
𝑂 ↔ 𝑂𝑖′′ + 2ℎ∙ (Equation 1)
2 2
if the oxygen is incorporated into the crystal through the
formation of oxygen interstitials or
1
𝑂 + 𝑉𝑂∙∙ ↔ 𝑂𝑂𝑥 + 2ℎ∙ (Equation 2)
2 2
if the oxygen is incorporated into oxygen lattice vacancies. In
both cases the surface exchange mechanism will involve a
number of elementary reactions including O2 transport from the
gas phase to the surface, adsorption on the surface, dissociation
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and charge transfer, and finally, incorporation into the surface
layer. Although many different mechanisms are possible, they all
include a charge transfer step involving the formation of electron
holes either through the oxidation of B site ions (nickel holes) or
by the formation of O- small polaron species. This possibility
would involve the formation of oxygen holes on the surface,
which could occur via different compensation mechanisms
involving O- species, such as:
1
𝑂2 + 𝑉𝑂∙∙ +𝑂𝑂𝑥 ↔ 2𝑂𝑂∙ (Equation 3)
12
𝑂2 + 2𝑂𝑂𝑥 ↔ 2𝑂𝑖′ + 𝑉𝑂∙∙ (Equation 4)
21
𝑂2 + 𝑂𝑂𝑥 ↔ 𝑂𝑖′ + 𝑂𝑂∙ (Equation 5)
12
𝑂 + 2𝑂𝑂𝑥 ↔ 𝑂𝑖′′ + 2𝑂𝑂∙ (Equation 6)
2 2
In the case of formation of Ni holes, as the Ni species are not
directly accessible at the outermost surface, it is necessary to
consider other possibilities, such as surface point defects and step
edges. In Figure 3b these 2 possibilities are shown: oxygen could
be incorporated through vacant apical surface sites in the
(La,Sr)O layer and/or through vacant equatorial sites at step
edges in direct contact with Ni species. The existence and
significance of the surface oxygen vacancies is in good
agreement with the thermodynamic calculations performed by
Read et al. 9, who found that the formation of oxygen vacancies
has a lower energy at the surface than in the bulk and would also
explain the high activation energies of the surface exchange
(kinetic process) for Sr-doped La2NiO4+δ measured by Li et al.25.
These authors relate it to oxygen vacancies being only minority
defects in the nickelates, and being more energetically favourable
on equatorial sites than on apical sites 26, as opposed to the case
of perovskite materials, in which oxygen vacancies are the
majority defects and which possess considerably lower activation
energies. The authors suggested that both for undoped and Srdoped La2NiO4+δ the rate limiting step for the surface exchange
would involve oxygen vacancies and would thus be limited by the
availability of oxygen vacancies on the surface. It should be
pointed out that Figure 3c constitutes an ideal representation of
La2-xSrxNiO4+δ surface and that the real equilibrium surface
structure is expected to be modified from the bulk one by
rumpling and/or relaxation of the interplanar spacing in the
surface region, as evidenced by the CTR results. Moreover A-site
enrichment (several nm in depth) has been measured by XPS,
which means that either the stacking sequence of (La,Sr)2O2 and
NiO2 layers varies in the close-surface region, or a different Asite rich surface phase is formed.
As expected, for the sample that was not annealed a mixture of
Ni2+ and Ni3+ is measured by XPS, as by the incorporation of Sr
into La2NiO4+δ electron holes are created for charge
compensation. It is also observed that the ratio between Ni2+ and
Ni3+ species does not vary in depth (within the experimental
error), which implies that the differences in La/Sr composition
could be compensated by differences in the oxygen content
(interstitials and/or vacancy concentration) in the top surface
region. When the sample is annealed, oxygen will partially
desorb to reach the new equilibrium state accompanied by the
reduction of Ni3+ to Ni2+ to maintain electroneutrality. The XPS
Ni 3p spectra show a larger Ni2+ component for the annealed
sample, which clearly evidences a reduction in the oxidation state
of the Ni species in the near surface region.
Conclusions
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In this study we have clearly shown that for La1.67Sr0.33NiO4+δ, a
promising mixed conducting cathode, Ni is not present on the
outermost atomic layer of the low-index (110) and (001) faces of
a single crystal. This significant discovery suggests that the
accepted model with the B-site cations exposed to the
environment is incorrect and other possibilities which explain the
catalytic activity of the surface have been proposed. The LEIS
results have been complemented with the chemical redox
information provided by AR-XPS and correlated to the
crystallographic data of the crystal surface provided by CTR
measurements. All of these methods are in good agreement.
Hence, in light of these compelling new data, it is essential that
the simulation of these surfaces is revisited. More generally it is
clear that the unique monolayer sensitivity provided by LEIS,
complemented by other state-of-the-art surface techniques, can
provide a complete picture of the surface and near-surface
chemistry of this important class of materials. It is anticipated
that the new surface information provided by these techniques
will contribute significantly to the understanding of the surface
processes in mixed conducting materials, with great importance
in the catalysis and solid state ionics fields.
Acknowledgements
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This research was supported by a Marie Curie Intra European
Fellowship within the seventh European Community Framework
Programme and by KAUST (King Abdullah University of
Science and Technology) Academic Excellence Alliance (for
M.B). Use of the National Synchrotron Light Source, and work
carried out at Brookhaven National Laboratory, was supported by
the U.S. Department of Energy, Office of Science, Office of
Basic Energy Sciences, under Contract No. DE-AC0298CH10886. The authors also thank D. Prabhakaran (University
of Oxford) for providing single crystals and M. Fartmann
(TASCON GmbH), T. Grehl, P. Brüner (ION-TOF GmbH), S.
Fearn and H. Téllez (Imperial College) for LEIS measurements
and support with LEIS data analysis.
Notes and references
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7.
8.
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10.
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12.
13.
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17.
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23.
a
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Department of Materials, Imperial College London, Exhibition Road,
London SW7 2AZ, United Kingdom. Fax: +44 (0)20 7594 6757; Tel: +44
(0)20 7594 6782; E-mail: m.burriel@imperial.ac.uk,
s.skinner@imperial.ac.uk
b
Brookhaven National Laboratory, Upton, NY 11973, USA
c
CIC energiGUNE, Albert Einstein 48, 01510 – Miñano (Álava), Spain
d
Eindhoven University of Technology, Eindhoven, The Netherlands
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
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