Neutron Powder Diffraction as a Characterization Tool of Solid Oxide Fuel Cell Materials

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Neutron powder diffraction as a characterizationtool of solid oxide fuel cell materials

J.A. Alonso a,*, M.J. Martnez-Lope a, A. Aguadero b, L. Daza b,c

a Instituto de Ciencia de Materiales de Madrid, C.S.I.C. Cantoblanco, E-28049 Madrid, Spainb

Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT),Av. Complutense 22, 28040 Madrid, Spain

c Instituto de Catalisis y Petroleoqumica (CSIC), Marie-Curie 2, Campus Cantoblanco,

28049 Madrid, Spain

Received 27 February 2007; accepted 17 March 2007

Abstract

Neutron diffraction is a powerful tool for the characterization of materials and, particularly, oxides.Oxide materials find applications in solid oxide fuel cells (SOFCs) as solid electrolytes as well as anode

and cathode materials. As a structural probe, neutrons are specially suitable for the crystallographic study

of oxides, given the comparable scattering factors of O and other heavier elements, allowing its precise

localization in the crystal structure. Many problems can be addressed by neutrons, related to the octahedral

tilting in perovskites, phase transitions, orderedisorder phenomena, presence of anionic vacancies, etc.

Neutrons make possible an accurate determination of the thermal factors and provide a visualization of

the diffusion paths in ionic conductors. Neutrons allow the localization of light atoms such as hydrogen,

and make possible the distinction between neighbouring elements, typically Fe and Mn. In this work we

will describe some recent applications of this technique in the field of solid electrolytes and electrode

materials, including some examples from our group.

2007 Elsevier Ltd. All rights reserved.

Keywords: Neutron powder diffraction; Crystal structure; Superstructure; Perovskite structure; Oxygen vacancies;

Octahedral tilting; Fluorite structure; Apatite structure; Pyrochlore structure; LAMOX; BIMEVOX; SOFC electrolytes;

SOFC cathodes; SOFC anodes; Oxygen-ion conductivity; Proton conductor

* Corresponding author. Tel.:

34 913349071; fax:

34 913720623. E-mail address: [email protected] (J.A. Alonso).

0079-6786/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.progsolidstchem.2007.03.004

Progress in Solid State Chemistry 36 (2007) 134e150www.elsevier.com/locate/pssc
mailto:[email protected]://www.elsevier.com/locate/psschttp://www.elsevier.com/locate/psscmailto:[email protected]


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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

2. Electrolyte materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1362.1. Oxide-ion conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

2.2. Proton conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

3. Cathode materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

4. Anode materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

1. Introduction

Neutron powder diffraction (NPD) is a powerful tool for the characterization of materials, in

general, and oxides, in particular. Oxide materials, and especially transition metal oxides, con-

stitute a wide family of materials with varied and fascinating properties, spanning from the

high-temperature superconductivity in cuprates to the colossal magnetoresistance in manganese

perovskite or in double perovskites of iron and molybdenum. In the field of energy conversion,

oxide materials find applications in solid oxide fuel cells (SOFC) as solid electrolytes, given the

conductivity of oxide anions and protons in certain materials with fluorite or perovskite struc-ture, as well as cathode and anode materials, in oxides with sufficiently high ionic and elec-

tronic conductivity and catalytic activity to favor the reduction of the oxygen and the

oxidation of the fuel, respectively. The NPD technique has shown to be especially effective

in the characterization of different aspects of these materials. As a structural analysis probe,

neutrons are well suited for the crystallographic study of oxides, since the scattering factor

for oxygens is comparable to those of other heavy elements, allowing its precise localization

in the crystal lattice. As an illustration, Fig. 1 shows a representation of the crystal structure

of LaCrO3 (perovskite used in SOFCs as interconnect) where the diameter of the atoms is pro-

portional to its scattering factor as viewed by X-rays and neutrons; for X-rays the oxygens are

almost invisible, whereas for neutrons they have a scattering factor comparable to La and Cr.The interaction of neutrons with matter happens directly with the atomic nuclei (not with the

electrons, as X-rays) in such a way that the collected information is complementary to that

obtained with other diffraction techniques. A second implication of the nuclear interaction is

the absence of a form factor for the scattering length, allowing the collection of structure factors

up to very high diffraction angles (exploration of a wide region of the reciprocal space), making

it possible to precisely determine the atomic positions and the thermal factors of the atoms in

the crystal. An interesting feature is the low absorption of neutrons by matter; the penetrant

character of neutrons enables the study of samples in situ within different devices (furnaces,

cryostats, high-pressure chambers, electrochemical devices), allowing to follow in real time

the behavior of the samples in the actual working conditions. In this paper we will describesome representative results of different research groups together with some results recently

obtained in our laboratory; we do not pretend to be exhaustive but only to illustrate the power

of NPD for some paradigmatic examples.

135J.A. Alonso et al. / Progress in Solid State Chemistry 36 (2007) 134e150


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2. Electrolyte materials

2.1. Oxide-ion conductors

Oxide-ion conductors are solid oxides that contain highly mobile oxide ions. Some of

them are additionally electronic insulators and they find applications as oxide-ion electro-lytes; others present mixed oxide-ion/electronic conductors and can be used as electrodes.

In general, oxide-ion conduction is poor below 1000 C in commercially available materials.

The materials based upon ZrO2 (zirconia) with fluorite structure have been extensively used

in SOFCs, given their high ionic conductivity of O2 anions and chemical stability in oxidiz-

ing or reducing conditions at the working temperatures, typically of 1000 C. Oxides with

this structure have high ionic conductivity when the host cations (such as Zr4) are replaced

by lower-valent cations such as Y3. The missing charge is balanced by the formation of

oxygen vacancies in the oxide-ion sublattice, resulting in an impressive ionic conductivity;

this mechanism is schematized in Fig. 2. Moreover, the conductivity of ZrO2 doped with

Sc2O3 is almost twice as high as those of other zirconia-based electrolytes, as the conven-tional yttria-stabilised zirconia (YSZ). However, the limited availability and high cost of

scandium have limited interest in its applications in SOFCs. The compound with maximum

conductivity presents a phase transition related to an orderedisorder transition of the oxygen

vacancies. The presence of Sc3, with an ionic radius inferior to Zr4, produces a unique

phase relationship, including an intermediate phase with rhombohedral symmetry. Neutron

diffraction is an ideal technique to study this kind of transformations, which involve subtle

shifts of the oxygen atoms [1]. The oxygen vacancies prefer a sixfold coordination and

induce a great distortion in the second oxygen neighbours: the observed rhombohedral distor-

tion can be, therefore, a consequence of the long-range ordering of the oxygen vacancies, as

identified by NPD [1]. A small addition of 2 mol% yttria to scandia stabilised zirconia resultsin stabilisation of the cubic phase and so avoids the major phase changes that occur on thermal

cycling of scandia substituted zirconias, which might be expected to be detrimental to long term

electrolyte stability [2].

Fig. 1. Idealized representation of the LaCrO3 crystal structure, where the diameters of the atoms are proportional to the

scattering factors for (a) X-ray diffraction and (b) neutron diffraction.

136 J.A. Alonso et al. / Progress in Solid State Chemistry 36 (2007) 134e150


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The fast ion conduction properties of some perovskite related oxides in the system La0.9Sr0.1Ga0.8Mg0.2O2.85 are also well known: high resolution ND data [3] have shown that its crystal

structure at room temperature is monoclinic instead of the standard orthorhombic symmetry

exhibited by the undoped LaGaO3 phase. In both systems the main distortion of the ideal

perovskite structure is related to the tilting of the GaO6 octahedra, and the observed changein symmetry is associated with the type of tilting: the undoped compound shows an in-phase

tilting (Fig. 3a) when we consider successive octahedra along the c crystallographic axis,

whereas it is in anti-phase (Fig. 3b) in the complex system. The oxygen vacancies in this elec-

trolyte, also located by NPD, are placed at the apical positions of the octahedra.

Recently, Lacorre et al. described a family of solid oxides based on the parent compound

La2Mo2O9 [4] with a different crystal structure from all known oxide electrolytes, which

exhibits fast oxide-ion conducting properties. Like other ionic conductors, this material un-

dergoes a structural transition around 580 C resulting in an increase of conduction by almost

two orders of magnitude, with a conductivity of 6 102 S cm1 at 800 C, comparable to that

of YSC. The crystal structure of La2Mo2O9 was studied by NPD [5]; the conducting high-temperature form b-La2Mo2O9 has a cubic structure (space group P213) which is derived

from that ofb-SnWO4. Partial site occupation by oxygen atoms, strongly anisotropic thermal

factors, and short-range order with a distance characteristic of OeO pairs have been evidenced.

Fig. 4 depicts the coordination environment of Sn2 in b-SnWO4 compared to that of La3 in

b-La2Mo2O9: the space of the lone pair in the former compound is occupied by O3 oxygens,

located at large multiplicity sites accounting for the O2-ion conductivity. An original concept

is proposed for the origin of oxide-ion conduction in this compound, which could be applied to

the design of new oxide-ion conductors [6,7]: the so-called LPS (lone-pair substitution) concept

could be used to seek for novel families of anion conductors, issued from already known com-

pounds with lone-pair elements. It is based on the similarity in volume between an electroniclone pair and oxide or fluoride anions, and on the vacancy created by the replacement of a lone

pair by a non-lone-pair cation with a higher oxidation state [7]. Various substitutions on La2Mo2O9 have been attempted: on the lanthanum site (La2xAx)Mo2O9 with A Sr, Ba, K, or

Fig. 2. Crystal structure of YSZ (Zr1xYxO2x/2) with fluorite structure: Zr4 and Y3 cations are coordinated to eight

oxygen anions and the charge valance is achieved via oxygen vacancies (represented by squares), allowing the fast

oxide-ion conduction.



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Bi; on the molybdenum site La2(Mo2xBx)O9 with BRe, S, W, Cr and V; and on the oxygen

site with fluorine [8]. Several members of the so-called LAMOX series are studied throughX-ray and neutron diffraction, and conductivity measurements: large O2 thermal factors and

local static disorder agree well with the anionic nature of the conductivity. Partly vacant sites

with short inter-site distances suggest a most probable conduction path with tri-dimensional

character [8].

Two-dimensional oxide-ion conduction in a layered oxide has also been investigated, partic-

ularly in the oxides Bi4V2xMxO11y first studied by Abrahams et al. [9,10]. Substitution of

aliovalent cations M for V in Bi4V2O11 suppresses a Tt of 570C, changing the transition

from long-range order below Tt to short-range order below a lower Tt . This oxide is an excel-

lent solid electrolyte for applications in an oxidizing atmosphere, but it is subject to reduction

by the fuel of a SOFC. From neutron diffraction data, crystal structure determinations of var-ious Bi4V2xMxO113x compounds were carried out using Rietveld-type full profile refinement

procedures. All these compounds belong to the g-phase of Bi4V2O11 and the substitution rates

deduced from the refined parameters are strongly related to oxygen vacancies of the structure

Fig. 3. Crystal structure of (a) orthorhombic LaGaO3 perovskite along the c axis showing the in-phase tilting of GaO6octahedra along the c direction, and (b) monoclinic distortion of La0.9Sr0.1Ga0.8Mg0.2O2.85, characterized by an anti-

phase tilting of the (Ga0.8Mn0.2)O6 octahedra along c.



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[11]. In Ni-doped samples (so-called BINIVOX) the defect structure of g-Bi2V0.9Ni0.1O5.35quenched from high temperature has been refined using X-ray and neutron powder diffraction

data [12]. The defect structure shows that oxide ion vacancies are exclusive to the equatorial

positions around the Ni/V atom. Consideration of site occupancies, inter-site contact distances,

and geometrical constraints yields two likely coordination environments for V/Ni, i.e. distorted

tetrahedra and distorted octahedra. The percentage of V/Ni sites that are distorted tetrahedramay be calculated as 68% with the remainder as distorted octahedra.

Other system that has attracted much interest is that of complex oxides with apatite structure

[13]. Neutron diffraction has enabled to investigate the microscopic origin of the huge difference

in ionic conductivities of two tightly-related oxides with apatite structure, with compositions

La9.33Si6O26 (s 1.2 104 S cm1 at 700 C) and La8Sr2Si6O26 (s 2.9 10

7 S cm1 at

700 C) [14]. The presence of cationic vacancies in the former material generated a structural

disordering over the oxygen sites that conform the ionic conducting channels; the oxygens are

shifted from the usual positions (0,0,1/4), to close sites (0,0,0.38) which are interstitial positions

within the conduction channels; this effect improves the mobility of oxygens along the channels

and the ionic transport properties by several orders of magnitude.

Other ND investigations on different oxoapatites containing excess oxygen [15] also enabled

to carefully examine the anionic sublattice, observing different positions for the interstitial

atoms in the conduction channels for germanates and silicates, given the different structural

flexibility of the tetrahedral groups in both types of compounds, thus explaining the larger

interstitial oxygen contents in germanates [13,15].

2.2. Proton conductors

Electrolytes based on proton conducting materials have a wide range of technological appli-cations in fuel cells, batteries, gas sensors, hydrogenation/dehydrogenation of hydrocarbons,

electrolysers, etc. In a certain intermediate temperature range (approximately 500e800 K),

no protonic conductors showing high conductivity exist, which motivates many material

O1

O2

O3

Sn La

lone pair

Fig. 4. Schematic representation of Sn2 coordination in b-SnWO4 compared to that of La3 in b-La2Mo2O9: the space

of the lone pair in the former compound is occupied by O3 oxygens, placed at large multiplicity sites accounting for the

O2-ion conductivity.



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researchers to improve the proton conducting properties within this temperature interval,

because this is the most important and desirable temperature range for both chemical and

energy conversion processes. For example, conversion of hydrogen using SOFCs with a proton

conducting electrolyte in the mentioned temperature range would reduce the cost of expensive

electrocatalysts [16].Oxygen-deficient perovskite oxides are oxide ion conductors in a dry environment. In a humid

atmosphere, however, some of them absorb water to fill up oxygen vacancies, incorporate mobile

protons into the structure, and thereby become proton conductors. Two main families of oxygen

defective perovskites have been the topic of extensive research, the rare-earth doped zirconates,

based on AZrO3 (ABa, Sr), and rare-earth doped cerates, based on ACeO3.

Zirconate-based ceramics, SrZr1xYxO3a for example, have high chemical stability as well

as high proton conductivity so that they are expected to be applied to fuel cells and chemical

pumps. Proton conduction in ceramic oxides involves hopping from a reticular oxygen to an

adjacent one. Therefore, the local structure is a key feature of these materials, and in fact it

was investigated in many theoretical studies, also in relation with the mechanism of proton dif-

fusion. Many computational approaches have been devoted to reveal the structure around a

hydrogen atom, in particular in zirconate-based proton conductors. The results of all these stud-

ies are consistent with respect to the hydrogen site; a hydrogen atom is bound to an oxygen

atom with an OeH distance of approximately 1.0 A, and the OeH is directed roughly along

the bisector of the two edges of the two adjacent MO6 octahedra. Indeed, experimental studies

have been done using neutron diffraction, which is generally the most direct method to charac-

terize crystal structures including light elements. Rather surprisingly, however, none of them

verified the bisector site. Sata et al. [17] studied D2O and H2O dissolved Sc-doped SrTiO3

single crystals by neutron diffraction; they concluded that the protons were located betweentwo adjacent oxygen atoms. The OeH line leaned toward the Ti (Sc) site from the OeO

straight line by an angle of 5 3.5.

A high-resolution NPD study on the protonic conductor SrZr0.95Sc0.05O3a [18] with and

without dissolved heavy water at a temperature of 10 K (space group Pnma) show that the deu-

terons are located outside the ZrO6/ScO6 octahedra near oxygen atoms, as shown in Fig. 5. The

protons may be bound to the oxygens of the ScO6 octahedra. The combination of the Rietveld

method [19] and the maximum entropy method (program PRIMA [20]) was very useful to

analyze the obtained diffraction data, modifying incomplete structure models and determining

disordered structures.

In general, concentration and mobility of protons determine the protonic conductivity; com-plete hydration (qualitatively) is possible for up to 75 mol% of In3 substituted BaZrO3 [21].

Such a high proton concentration may improve the proton conductivity. A Rietveld analysis of

low temperature (10 K) NPD data collected on as-prepared and deuterated BaZr1xInxO3dsamples confirmed cubic symmetry (space group Pm-3m) for all compositions. The level of

oxygen vacancies refined in the as-prepared samples were in good agreement with the values

expected to conserve charge neutrality, whilst an increase in oxygen occupancy, reflecting

the incorporation of OD-species, was obtained for the deuterated materials. An expansion of

the unit cell parameter, a, was observed as a function of In3 doping as well as after the deu-

teration reaction.

More complex perovskites from the system Sr3Ca(Zr1xTa1y)O8.5x/2 offer a high concen-tration of oxygen vacancies and show promise as good proton conductors for SOFCs and related

applications. The oxygen-ion vacancies can be filled by OeH groups, by exposing the sample

to a wet 5% H2/Ar atmosphere at intermediate temperatures (350e400C) [22]: in particular,



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the complex triple perovskite of formula Sr3CaZr0.5Ta1.5O8.75 has been synthesized and,

although an X-ray diffraction study indicates a tetragonal unit cell for an as-prepared sample,

a subsequent investigation by high-resolution neutron diffraction reveals a slight monoclinic dis-

tortion. Thermogravimetric analysis (TGA) shows that water is taken up to fill 55% of the vacant

oxygen sites when a vacuum-treated sample is subjected to wet 5% H2/Ar atmosphere at 380

Cfor 24 h [23].

BaCeO3-based compounds present the highest protonic conductivity, although an improve-

ment of the chemical stability and proton conductivity is required to implement these materials

for the construction of effective electrochemical devices. Introduction of protons in the solid

oxide network is achieved by doping the B octahedral site with lower valence cations and

with the incorporation of hydroxyls in the so-obtained oxygen-deficient solid according to

the reaction:

H2O Oxo V

o5 2OH

o

The crystal structure and phase transitions of BaCeO3, as well as neodymium-doped and

yttrium-doped barium cerate were investigated by neutron diffraction as a function of dopant

amount and of temperature. A classical work by Knight [24] from time-of-flight NPD data col-

lected at 4 K allowed to determine the preferential site for proton insertion. The heavy atoms in

BaCeO3 occupy sites of high pseudosymmetry, thus making detailed, accurate structural refine-

ments all but impossible with X-ray diffraction. By contrast, the sensitivity of neutron diffrac-

tion to both the light atom positions and site occupancies has made this technique invaluable in

the structural characterization of this compound. Experimentally, measuring these features in

doped BaCeO3 is very demanding due to the large DebyeeWaller factors associated with

the oxygen atoms. Using time-of-flight NPD, where a wide Q range is available, helps to reducethe correlation between the site occupancy factor and the DebyeeWaller factor.

Undoped BaCeO3 undergoes a sequence of phase transitions [24] where it first loses an in-

phase tilt at 563 K transforming from Pmcn, abb, to Incn, a0bb, it then gains a tilt, which

Fig. 5. Location of deuterons in D-doped SrZr0.95Sc0.05O3a perovskite (space group Pnma). D atoms are outside the

ZrO6/ScO6 octahedra near oxygen atoms, the OeD bond is leaning from the OeO straight line.



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is a rare occurrence in perovskites, at 673 K becoming F-32/n, aaa, before finally trans-

forming into the aristotype phase at 1173 K. For both Y- and Nd-doping, the width of the

Incn phase is increased over the undoped parent phase and that of the F-32/n phase is reduced.

The light atom sensitivity of neutron diffraction extends to the proton itself and a structural

site for the proton in doped BaCeO3 was proposed by Knight [25] using high-resolution neutronpowder data collected on both BaCe0.9Y0.1O2.95 and BaCeO3 at 4.2 K. Neutron scattering

lengths are isotope-dependent, and the optimum experiment would utilise the change in sign

of the scattering length for protium- and deuterium-stabilised samples. In the former case, a neg-

ative peak would be observed in the difference nuclear density synthesis, as protium has a neg-

ative scattering length of3.739 fm, and, in the latter case, a positive peak should be found at

the same site, as deuterium has a positive scattering length of 6.671 fm. This particular feature

is illustrated in Fig. 6, showing two difference Fourier maps modelized for BaCe0.9Y0.1O2.95structure containing either protium or deuterium. Knight [25] found a site in the H-doped com-

pound, which was stable under Rietveld refinement, and absent in the undoped phase. This site

Fig. 6. Difference Fourier maps modelized for BaCe0.9Y0.1O2.95 doped with (a) protium (H), showing a negative peak at

(0.864,0.235,0.633) and equivalent positions and (b) deuterium (D), showing a positive peak at the same positions.



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made a chemically sensible OeH bond length of 0.93 A to oxygen on general positions. The

proposed position for the proton in BaCe0.9Y0.1O2.95 at 45.2 K is very similar to that described

for the zirconate perovskites, shown in Fig. 5, with the OeH bond roughly directed along the

bisector of the two edges of the two adjacent (Ce,Y)O6 octahedra but slightly leaned toward the

Ce (Y) site from the Oe

O straight line.It is precise to remark that this kind of diffraction experiments allows one to determine only

the average structure of a crystalline matrix modified by the hosted species; hence, the study of

local deviations from an ideal periodicity in the environment of the dopant or in the site of

insertion of the proton, performed by XAFS (X-ray absorption fine structure) is a necessary

complement of long-range structural investigations on proton conducting ceramics and an im-

portant validation tool for theoretical studies on the proton-transfer mechanism.

Another appealing example is the fast-proton conductor (H3O)SbTeO6, with pyrochlore-like

crystal structure, showing a conductivity value of 101 S cm1 at 30 C under saturated water

vapor partial pressure. The crystal structure is constituted by a network of randomly distributed

SbVO6 and TeVIO6 octahedra linked by their corners, conforming large interconnected cages

were the H3O units were located for the first time, through Fourier synthesis from neutron pow-

der diffraction data [26]. Fig. 7 shows the final Rietveld refinement of the structure. Hydronium

ions are located off-center in these cavities, linked by weak hydrogen bonds to the O1 frame-

work oxygens. The relatively large thermal factors of O and H at the H3O units, of 2.5 and

3.7 A2, respectively, together with the large multiplicity of these sites could account for the

huge mobility of protons in this pyrochlore and suggest that both kinds of atoms are not static

at fixed positions but could be dynamically fluctuating between crystallographically equivalent

sites; Fig. 8 gives an idealized image of the diffusion path of protons across the interconnected

cavities of the pyrochlore.

3. Cathode materials

The characterization of ABO3 perovskite-type cathode materials, in terms of crystal struc-

ture, has been traditionally carried out by NPD, addressing issues related to the tilting of the

Fig. 7. Neutron powder diffraction pattern of (H3O)SbTeO6 pyrochlore, refined by the Rietveld method. The crosses are

the experimental points, the solid line is the calculated pattern and the difference is at the bottom. The vertical marks

correspond to the position of the allowed Bragg reflections.



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octahedra, oxygen occupancy and valence state of the transitions metals at the B positions (Mn,

Fe, Co.

) [27]. As an example, Zhou et al. [28] found that a reduction in symmetry to trigonalP-3c1 allows a better refinement of the neutron data than the conventional R-3c established by

X-ray diffraction in certain materials of the (La,Sr)FeO3d family. They also found an incre-

ment of symmetry from rhombohedral to cubic in samples of composition La0.6Sr0.4FeO3d,

with oxidation states for Fe of 3.04 to 3.36 in samples quenched from 1500 to 700 C, re-

spectively. By contrast, in a slowly cooled specimen they did not find oxygen vacancies,

whereas for the sample quenched from 1500 C an oxygen deficiency ofd 0.2 was obtained.

The classic electrode materials in SOFCs have been the hole-doped manganese perovskites,

in samples of stoichiometry (La,A)MnO3, where A is an alkali-earth element. This kind of

material has also triggered an extraordinary interest upon the report of colossal magnetoresis-

tance properties [29], exhibiting a substantial change in electrical resistance upon the applica-tion of an external magnetic field. This phenomenon is possible, thanks to the simultaneous

occurrence of half-metallic and ferromagnetic behavior, for moderate A-doping levels,

accounted for by a double-exchange mechanism. An exhaustive study of these phases by neutron

diffraction has allowed to establish with great precision their complex phase diagrams as a func-

tion of compositions and temperature or pressure, and to unveil intricate structural behavior re-

lated to the charge ordering of the Mn cations in different oxidation states (as a reference, see

Refs. [30,31]). A particular case is constituted by the self-doped perovskites that nominally con-

tain excess oxygen like LaMnO3d; detailed investigations by NPD demonstrated that actually

these materials contain cationic vacancies, whereas the oxygen sublattice is complete [32].

Besides the perovskites with more conventional three-dimensional structure, recently thelayered perovskites of K2NiF4 type, containing alternating perovskite and NaCl layers, have

attracted much attention [33,34]. Given their mixed conduction properties they have been con-

sidered as potential cathodes in intermediate-temperature SOFCs, with working temperatures of

Fig. 8. Interconnected cavities in the pyrochlore structure, giving rise to a well-established path for diffusion of H

across the solid, thus explaining the huge proton conductivity of (H3O)SbTeO6, comparable to that of Nafion at

room temperature.



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700e800 C. In the system La2Ni1xCuxO4d, the localization of interstitial oxygen in the

NaCl-type layers has helped to interpret the physical properties in relationship with the Cu

contents [35]; Fig. 9 illustrates the location of insterstitial oxygens in large multiplicity sites.

NPD investigations have allowed to find a correlation between the electronic conductivity and

the Nie

O bond distances. In a closely related system of composition La2xSrxNiO4d, a shorten-ing of the NieO1 distances in the system as the Sr content increases is correlated with the incre-

ment of the total conductivity [36]. An in situ NPD study of the thermal evolution of the crystal

structure of La2NiO4d [37] and La2Ni0.6Cu0.4O4d [38] and allowed us to establish a relationship

between the shortening of the apical NieO2 distances and a semiconducting-to-insulator transi-

tion undergone by the sample under the usual operation conditions of a cathode in a SOFC.

4. Anode materials

The conventional anode for the zirconia-based SOFCs are Ni/YSZ cermets, which displayexcellent catalytic properties for fuel oxidation and current collection, however, suffer from

problems arising from carbon deposition while using hydrocarbon fuels, low tolerance to sulphur

and poor redox cycling, causing volume instability. In order to overcome the disadvantages of

Fig. 9. Representation of La2(Ni,Cu)O4d structure, where the interstitial O4 atoms have been located from NPD data in

the LaO layer (with NaCl structure). The anisotropic thermal factors are indicated by 80% probability ellipsoids.



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the traditional Ni/YSZ cermet anode, alternative oxides are being investigated as potential an-

odes for SOFCs.

Early attempts to identify an alternative anode material consisted of doping transition metal

ions into zirconia or a rare-earth into ceria; these materials demonstrated to have an insufficient

catalytic activity of the anode. An interesting approach to the anode problem involves the iden-tification of a mixed oxide-ion/electron conductor (MIEC) that is stable in the anodic atmo-

sphere and more catalytically active for fuel oxidation than is doped ceria. The perovskite

oxides, in particular doped LaMnO3, are an interesting class of materials for this purpose as

they are stable in both oxidative and reductive atmospheres. They can also be substituted on

the A and B sites with alkali earth and transition metal elements, respectively, which allows

interesting modifications of their electronic as well as their catalytic properties.

Recently, Tao and Irvine reported a redox stable La1xSrxCr1yMnyO3d fuel electrode for

SOFCs, with comparable performance to traditional cermet anodes [39]. The introduction of

A-site deficiency is thought to decrease the reactivity of perovskite with the electrolyte and sev-

eral compositions including (La0.75Sr0.25)0.95Cr0.5Mn0.5O3d (LSCM) were also investigated

and reported [40]. The structure of LSCM has been investigated by X-ray diffraction and neu-

tron diffraction to further understand its properties under SOFC operating conditions. Samples

were prepared with nominal A-site deficiency; however, NPD demonstrates that the A-site

deficiency is actually minimal or even null, with spinel impurity compensating for low content

of A-site species. This was not apparent from XRD. The perovskite oxide LSCM exhibits

a rhombohedral structure with space group R-3c; conductivity and thermal expansion data in

air exhibit an anomalous change starting at 400 C; in fact an in situ high-temperature

NPD experiment unveiled a phase transition from rhombohedral R-3c to cubic Pm-3m, over

the temperature range from 500 to over 1100

C [13]. Fig. 10 shows the coexistence of bothphases in the mentioned temperatures interval.

A new family of perovskite titanates with formula La4Sr8Ti12xMnxO38d has been investi-

gated as fuel electrode materials for SOFCs. These phases present a rhombohedral (R-3c) unit

cell. Mn substitution does not have a large impact on the bulk conductivity of the phases stud-

ied, which remains close to the values observed in other related titanates, although the grain

boundary contributions are largely improved [41]. Perovskite titanates with nominal

Fig. 10. Temperature evolution of the fraction of rhombohedral and cubic phases of (La0.75Sr0.25)0.95Cr0.5Mn0.5O3dstudied by in situ NPD. Modified from Ref. [13].



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stoichiometry ABO3d often exhibit quite interesting properties, but their structural character-

ization is not always rigorous. An NPD study combined with transmission electron microscopy

and other techniques has allowed to demonstrate how excess oxygen can be incorporated in a

titanate perovskite based lattice in a related family of layered perovskites La4Srn4TinO3n2,

showing that such layered perovskites are able to accommodate extra oxygen beyond theparental ABO3 perovskite in crystallographic shears [42].

Recently, double perovskites (containing two different cations at the B positions showing

a long-distance ordered arrangement, as shown in Fig. 11) have been proposed as MIEC

systems for anode operation on H2 or CH4 as the fuel [42]. The system Sr2Mg1xMnxMoO6has been suggested following a strategy based on different observations: (1) the perovskite

structure can support oxide-ion vacancies to give good oxide-ion conduction; (2) a perovskite

containing a mixed-valent cation from the 4d or 5d block can provide good electronic conduc-

tion; (3) the ability of Mo(VI) and Mo(V) to form molybdenyl ions allows a sixfold-coordinated

Mo(VI) to accept an electron while losing an oxide ligand; and (4) if the two octahedral-site

cations of the double perovskite are each stable in less than sixfold oxygen coordination, the

perovskite structure can remain stable on the partial removal of oxygen. The partner cations

Mg(II) or Mn(II) were selected since they are not further reduced by the fuel and are stable

in either fourfold or sixfold oxygen coordination [43].

To this respect, NPD techniques have played an important role in the characterization of

many double perovskite oxides, helping to elucidate the true crystal symmetry, to analyze

the tilting of the BO6 octahedra or to assess the presence and long-distance ordering of oxygen

vacancies. In particular, Sr2MnMoO6 and Sr2MnWO6 have been recently reported as mono-

clinic (space group P21/n) after a high-resolution NPD study [44]: Sr2MnMoO6 was previously

described from XRD data to exhibit the cubic structure depicted in Fig. 11, with a

7.98 A

[45,46]. Fig. 12 illustrates the goodness of the fit of NPD data for Sr2MnMoO6 either in the

cubic Fm-3m space group, where many reflections are split or not allowed by the symmetry,

or in the actual P21/n space group, where all the features of the NPD pattern are explained.

The monoclinic space group P21/n allows for two different octahedral sites, which can be

Fig. 11. Cubic superstructure for the double perovskite Sr2MnMoO6 defined in the Fm-3m space group, as described in

Ref. [45] from a XRD study.



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occupied by the B and B0

atoms (BMn and B0

Mo or W). In this case, the Mn and Mo or Watoms are fully ordered in each site, in such a way that each MnO6 octahedra is linked to six

B0O6 octahedra. The B and B0 ions are arranged alternately forming a rock salt sublattice. In

both Sr2MnMoO6 and Sr2MnWO6 unit cells the monoclinic distortion is very small, with b

angles very close to 90; this effect has been widely observed by NPD in many 1:1 ordered

perovskites with a strong pseudo-orthorhombic character.

5. Conclusions

Neutron diffraction finds application as a powerful characterization tool of the different

material oxides that constitute a solid oxide fuel cell. During the last 20 years, neutrondiffraction has become the technique of choice to investigate structural features related to

the position and thermal factors of oxygen atoms, to locate, quantify and determine the

long-range ordering of oxygen vacancies, to evaluate the tilting and distortion of octahedra

115 120 125

-400

-200

0

200

400

600

P21/n

intensity

(a.u.

)

2 (deg)

115 120 125

130

130

-400

-200

0

200

400

600

Sr2MnMoO6 Fm-3m

intensity

(a.u.)

2 (deg)

Fig. 12. Rietveld refinement of NPD data for Sr2MnMoO6 in (a) a cubic Fm-3m space group, showing many unex-

plained reflections, and (b) the actual monoclinic P21/n space group.



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in perovskite structures, to follow in situ crystallographic transitions and determine the ac-

tual symmetry in slightly distorted structures, to locate hydrogen atoms in proton conductors

and to help to identify models for the ionic conduction in these fascinating systems. Neutron

diffraction has been an invaluable tool to establish correlations between subtle structural de-

tails and the properties of interest, helping the material scientists to design improved mate-rials with optimised properties.

Acknowledgements

We thank the financial support of the Spanish Ministerio de Educacion y Ciencia to the

MAT2004-0479 project and of Madrid Community to the ENERCAM-CM S-0505/ENE/

0304 project. A.A. also wants to acknowledge CIEMAT for a grant. We thank the Institute

Laue Langevin, Grenoble, France, for making all facilities available.

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Neutron Powder Diffraction as a Characterization Tool of Solid Oxide Fuel Cell Materials

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