Effect of Sr content on the crystal structure and electrical properties of the system La2-xSrxNiO4+δ (0 ≤ x ≤ 1)

Article abstract of DOI:10.1039/b606316k

Materials formulated as La_2-xSr_xNiO _4+δ (0 ≤ x ≤ 1) have been prepared and investigated by high-resolution neutron powder diffraction in order to correlate the structural variation induced by the incorporation of Sr into the crystal lattice with the electronic and thermal properties of each material. The evolution of the electrical conductivity and thermal expansion coefficients with temperature have been determined in order to study the potential use of these compounds as cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFC). These oxides show a good thermal expansion coefficient (TEC = 11-13 × 10 ^-6 K^-1), and high electronic conductivity up to 273 S cm^-1. It is noticeable that a great enhancement of the electrical conductivity with the Sr content is concomitant with the shortening of the Ni-O1 bond length. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b606316k

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Effect of Sr content on the crystal structure and electrical properties of the
system La2−xSrxNiO4+d (0 ≤ x ≤ 1)†
A. Aguadero,*a M. J. Escudero,a M. Pe´rez,a J. A. Alonso,b V. Pomjakushinc and L. Dazaa,d
Received 4th May 2006, Accepted 28th June 2006
First published as an Advance Article on the web 19th July 2006
DOI: 10.1039/b606316k
Materials formulated as La2−xSrxNiO4+d (0 ≤ x ≤ 1) have been prepared and investigated by
high-resolution neutron powder diffraction in order to correlate the structural variation induced by the
incorporation of Sr into the crystal lattice with the electronic and thermal properties of each material.
The evolution of the electrical conductivity and thermal expansion coefficients with temperature have
been determined in order to study the potential use of these compounds as cathodes for
intermediate-temperature solid oxide fuel cells (IT-SOFC). These oxides show a good thermal
expansion coefficient (TEC = 11–13 × 10−6 K−1), and high electronic conductivity up to 273 S cm−1. It
is noticeable that a great enhancement of the electrical conductivity with the Sr content is concomitant
with the shortening of the Ni–O1 bond length.
La2NiO4 phases,12 the chemical replacement of La by Sr in the
series La2−xSrxNiO4+d was thus studied to monitor structural and
transport effects.
1
Introduction
Solid oxide fuel cells (SOFCs) are promising green energy sources
because of their high energy conversion efficiency with very low
emission of air pollutants.1,2 For a viable commercialisation of
the SOFC technology, one of the most important challenges is
the reduction of the temperature operation below 800 ◦C. This
requires new materials, especially cathode materials with increased
electrocatalytic activity, and characterized by an enhanced oxygen
transport in addition to the required electronic conductivity.
Recently, K2NiF4 type oxides have attracted considerable atten-
tion as alternative cathode materials for intermediate-temperature
solid oxide fuel cells (IT-SOFC) due to their interesting trans-
port properties. They present high catalytic activity for oxygen
reduction, good ionic and electronic conductivity over a wide
temperature range and an adequate thermal expansion coefficient
that match with those of the other cell components.3,4 The crystal
lattice of K2NiF4 structures can be described as a stacking of
perovskite layers, ABO3, alternating with AO rock salt layers.
The ionic conductivity of these materials is mainly associated
to the excess oxygen (d) accommodated in interstitial form in
their AO layers.5 La2NiO4+d is a widely studied compound6–10
with very appealing transport properties.11 However, it lacks
adequate electronic conductivity, which can be improved by
doping the A position with alkaline-earth ions. In this regard,
calcium and strontium are suitable elements with comparable
cationic sizes to that of lanthanum. Since Ca-doping was shown
to have little influence on the electrical conductivity of substituted
During recent years great efforts have been devoted to under-
stand the different electrical behaviour of La2−xSrxNiO4+d from the
that of the isostructural high-temperature superconducting copper
13–15
oxides La2−xSrxCuO4+d.
However, there have been much fewer
studies conducted to determine the viability of these materials
as mixed ionic electronic conductor (MIEC) cathodes for IT-
SOFC. The aim of this work is to investigate the evolution of the
structure of the system La2−xSrxNiO4+d (0 ≤ x ≤ 1) as a function of
the strontium content and its influence on the electric conductivity
and thermal expansion; our conclusions are helpful for selecting
the most suitable composition as a cathode material for IT-SOFCs.
2
Experimental
La2−xSrxNiO4+d (x = 0, 0.10, 0.25, 0.50, 0.75, 1) oxides were
synthesized via a nitrate–citrate route. Stoichiometric amounts of
analytical grade La2O3, Ni(NO3)2·6H2O (99%) and Sr(NO3)2 were
dissolved in nitric acid. Citric acid was added in a large excess
(3.3 mol per mol of La2−xSrxNiO4+d) with continuous stirring.
The obtained solution was dehydrated and slowly heated until
self-combustion of the precipitate. The obtained precursors were
calcined at 600 ◦C for 30 min and finally fired in air between 950
and 1200 ◦C for 8 h.
In order to assess phase purity, X-ray diffraction (XRD) anal-
yses were performed in a Philips “X Pert-MPD” diffractometer
˚
using Cu-Ka radiation, k = 1.5406 A. The diffraction patterns
were recorded in the 2h range 20–80◦ in steps of 0.04◦. The oxygen
content of the samples was determined by iodometric titration
against a standardized potassium thiosulfate solution. Neutron
powder diffraction (NPD) data were collected at room temperature
for all the compounds using the high-resolution diffractometer
HRPT16 at the SINQ spallation source at the Paul Scherrer Institut
(Villigen, Switzerland). The high intensity mode (Dd/d ≥ 2 × 10−3)
aCentro de Investigaciones Energe´ticas Mediambientales y Tecnolo´gicas
(CIEMAT), Av. Complutense 22, 28040, Madrid, Spain E-mail:
ainara.aguadero@ciemat.es
bInstituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049,
Madrid, Spain
cLaboratory for Neutron Scattering, ETHZ & PSI, CH-5232, Villigen PSI,
Switzerland
dInstituto de Cata´lisis y Petroleoqu´ımica, CSIC, Cantoblanco, 28049,
Madrid, Spain
˚
was selected, with a neutron wavelength k = 1.494 A within the
† The HTML version of this article has been enhanced with colour images.
angular 2h range 5–165◦. 3 g of each sample were contained in a
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vanadium can. The collection time was 2 h per pattern. Diffraction
data were analysed by the Rietveld method, using the FULLPROF
program,17 with the use of its internal tables for scattering lengths.
The line shape of the diffraction peaks was generated by a
pseudo-Voigt function and the background refined to a fifth-
degree polynomial. In the final run the following parameters were
refined: background coefficients, zero point, half width, pseudo-
Voigt and asymmetry parameters for the peak shape, scale factor
and unit-cell parameters. Positional and occupancy factors for
oxygen atoms and isotropic thermal factors were also refined for
NPD data. The coherent scattering lengths for La, Sr, Ni and O
were 8.240, 7.020, 1.030 and 5.803 fm, respectively.
Dense compounds are required for the measurements of thermal
expansion coefficient and electrical conductivity. The prepared
powders were pressed into pellets (10 mm in diameter and 1.4 mm
think) using poly(ethylene glycol) as a binder and 3 tons of uniaxial
pressure during 5 min. The sintering behaviour of green compacts
and thermal expansion of dense ceramics were studied using a
Linseis L75/1550C dilatometer with a heating rate of 5 ◦C min−1.
The electrical conductivity of the system was measured in air
by the DC four-probe method. The measurements were carried
out in the temperature range 350–950 ◦C. The dense ceramics
were placed onto an alumina support and four platinum wires
contacts were pressed onto the sample using a spring loaded
alumina tube. A constant current load of 500 mA was applied by a
potentiostat/galvanostat (EG & G Model 263) and the potential
drop recorded by a Fluke 179 True RMS Multimeter.
revealed the existence of small NiO impurity peaks. Due to the
relatively strong scattering of Ni atoms, the small amount of NiO
detected is quite prominent in the neutron powder diffraction
pattern. The maximum weight fraction value reached was of
around 18% in LaSrNiO4+d and 8% in La1.25Sr0.75NiO4+d. For Sr
contents lower than 0.75 the NiO fraction is negligible, suggesting
that Sr solubility in the system sharply decreases for Sr content
≥0.50 per formula unit.
Iodometric titration shows that the samples with low Sr content
(x = 0.1, 0.25) are able to accommodate some oxygen excess, while
samples with x ≥ 0.5 are sub-stoichiometric or oxygen deficient.
The d values are 0.17, 0.14, 0.036, −0.031 for x = 0, 0.1, 0.25 and
0.5, respectively. This is in agreement with the results obtained
by previous researchers18 and is due to the hole-doping effect
induced by the introduction of Sr into the La sublattice, implying
the oxidation of Ni2+ to Ni3+ to preserve the charge neutrality
and reducing the trend to incorporate interstitial oxygen into the
crystal structure.
The crystal structure refinement was performed from high-
resolution NPD data collected at room temperature and wave-
˚
length k = 1.494 A. For the x = 0 sample, the orthorhombic
Fmmm model proposed by Jorgensen et al.6 was considered.
La was located at 8i (0,0,z) positions, Ni at 4a (0,0,0) sites,
and the four crystallographically independent oxygen atoms, O1–
O4, at 8e (1/4,1/4,0), 8i (0,0,z), 32p (x,y,z) and 16j (1/4,1/4,z),
respectively. The O4 site corresponds to interstitial oxygen atoms.
The insertion of these additional oxygen atoms into the lattice
causes a displacement of the axial O2 from its normal location
to a new axial O3 position. This is indeed observed, since the
refinement of oxygen site occupancies allows us to detect a large
vacancy concentration at the O2 positions. In agreement to this,
the occupancy of one O4 interstitial position would be expected to
displace four neighbouring O2 atoms into the O3 axial site. Fig. 2
shows the local defect structure created by the insertion of the
O4 interstitial oxygens. The refinement of the La2NiO4+d structure
3
Results and discussion
The compounds La2−xSrxNiO4+d (x = 0, 0.10, 0.25, 0.50, 0.75, 1)
were successfully obtained as black, well crystallized powders. As
shown in Fig. 1, XRD data showed no evidence of impurities
or starting materials in the prepared powders. However more
exhaustive analysis of the structure by neutron powder diffraction
Fig. 1 XRD patterns of La2−xSrxNiO4+d (0 ≤ x ≤ 1), collected with Cu-Ka radiation.
4378 | Dalton Trans., 2006, 4377–4383 This journal is The Royal Society of Chemistry 2006
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Fig. 2 View of the K2NiF4 structure showing the localization of the
interstitial atoms (O4) in the LaO rock salt layer. In the NiO6 octahedra, O1
represents the equatorial atoms whereas O2 and O3 are the axial oxygens.
Table 1 Structural parameters after the Rietveld refinement of NPD data
˚
for La2NiO4+d at RT. Space group Fmmm, a = 5.4563(2) A, b = 5.4630(2)
3
2
˚
˚
˚
A, c = 12.7049(3) A and V = 78.70(2) A with v = 1.78, Rp = 2.75, Rwp
=
3.61, Rexp = 2.71 and RI = 3.70
2
˚
Atom
Site
x
y
z
B/A
f occ
La
Ni
O1
O2
O3
O4
8i
4a
8e
8i
32p
16j
0
0
0.25
0
0
0
0.25
0
0.3592(2)
0
0
1.01(3)
1.05(4) 0.5
1.36(5)
1.8(1)
1.8(1)
1.1(5)
1
1
0.1752(9)
0.64(2)
0.36(2)
0.09(2)
−0.055(5) −0.059(5) 0.172(2)
0.25 0.25 0.223(2)
based on this model led to a good fit between the calculated and
observed NPD profiles (RBragg = 3.70). Table 1 summarises the
structural parameters obtained for this x = 0 sample.
The subsequent analysis of the NPD patterns along the series
for x > 0 evidenced the existence of a structural transition from
the orthorhombic Fmmm to the tetragonal I4/mmm symmetries
upon the incorporation of Sr into the crystal lattice. In order to
correlate the different structures obtained, the F4/mmm setting
was used19 instead of the standard I4/mmm space group setting.20
In F4/mmm, La, Sr and O2 atoms are located at 8e (0,0,z) sites,
Ni at 4a (0,0,0), O1 at 8c (1/4, 1/4, 0), O3 at 32m (x,y,z) and the
interstitial O4 atoms at 16e (1/4,1/4,1/4) positions. We observed a
similar behaviour to that described by Jorgensen et al.6 concerning
the displacement of the O3 oxygen atoms with the insertion of
the interstitial O4 oxygens. In this regard, the complementary
occupancies of the axial O2 positions and the displaced axial O3
oxygens were constrained to 1.00, and the isothermal factors were
also constrained.
Fig. 3 Variations of lattice parameters for the system La2−xSrxNiO4+d (0 ≤
x ≤ 1) as a function of strontium content: (a) a and b parameters; (b) c
parameter; (c) unit cell volume, V.
the volume cell. This is in agreement with the expected oxidation
of Ni2+ to Ni3+ upon chemical doping with Sr. The decrease in
the volume cell also reveals the minor tendency to accommodate
excess of oxygen of this materials as the Sr content increases. In
our case, the introduction of the interstitial position (O4) in the
refinement was only required for Sr content lower than 0.5 per
formula unit.
The analysis of the diffraction data of the Sr-rich members
of the series, La1.25Sr0.75NiO4+d and LaSrNiO4+d reveals a very
poor fitting between the experimental and the calculated NPD
For the samples La2−xSrxNiO4+d (x = 0.1,21 0.25 and 0.5)
the refinement in this tetragonal model led to good reliability
RBragg factors (∼3%). As is shown in Table 2 and Fig. 3, in this
compositional range a and b unit-cell parameters decrease with
the Sr content while c increases leading to an overall decrease of
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Table 2 Structural parameters after the Rietveld refinement of NPD data for La2−xSrxNiO4+d (x = 0.1, 0.25 and 0.5) at RT. Space group F4/mmm, La,
Sr and O2 at 8e (0,0,z) sites, Ni at 4a (0,0,0), O1 at 8c (1/4,1/4,0), O3 at 32m (x,y,z) and O4 at 16e (1/4,1/4,1/4) positions
La1.9Sr0.1NiO4+d
La1.75Sr0.25NiO4+d
La1.5Sr0.5NiO4+d
˚
a = b/A
5.45243(7)
12.6927(2)
377.24(1)
5.4288(2)
12.7032(5)
374.39(2)
5.40079(9)
12.7331(3)
371.41(1)
˚
c/A
3
˚
V/A
La (Sr)
z
0.3616(1)
0.50(2)
0.62(3)
0.79(3)
0.1742(3)
0.3614(2)
0.41(4)
0.68(4)
0.70(6)
0.176(2)
0.1(1)
−0.041(5)
0.171(2)
0.1(1)
0.3616(2)
0.42(2)
0.43(3)
0.41(4)
0.1736(2)
2
2
2
˚
˚
˚
B/A
B/A
B/A
z
Ni
O1
O2
2
˚
B/A
0.48(7)
1.15(4)
O3
x = y
−0.5233(1)
0.1742(2)
0.48(7)
z
2
˚
˚
B/A
B/A
2
O4
5(1)
1(1)
v2
5.41
4
5.31
2.28
3.19
12.7
5.98
8.29
2.32
3.47
11.70
5.53
7.49
2.19
3.14
Rp (%)
Rwp (%)
Rexp (%)
RI (%)
profiles due to the appearance of extra features or shoulders
in the experimental profile (Fig. 4(a)). Attempts to refine these
data in less symmetric space groups did not improve the fitting.
Preliminary studies in LaSrNiO4 revealed the coexistence of
three tetragonal K2NiF4 phases with different Sr contents and
slightly different unit-cell parameters.22 In the present case, the
introduction of two tetragonal phases allowed us to satisfactorily
improve the profile fitting. Fig. 4(b) illustrates the final refinement
for La1.25Sr0.75NiO4+d, successfully performed with a three-phase
model consisting on two tetragonal K2NiF4 phases and one NiO
minor impurity phase. Table 3 lists the structural parameters
obtained for both tetragonal phases present in La1.25Sr0.75NiO4+d
and LaSrNiO4+d samples. It is worth remarking that the materials
formulated as La2−xSrxNiO4+d (x = 0.5, 0.75 and 1) were refined
with an oxygen sub-stoichiometric model according with the
results obtained from iodometric analysis; hence O4 was not
defined and the axial positions O2 and O3 were not split.
It is evident that the coexistence of two K2NiF4 phases for
high x > 0.5 values is due to Sr segregation giving rise to Sr-
rich and Sr-poor phases. The Sr content of each phase could not
be refined from the neutron profiles given the similar scattering
lengths of La and Sr atoms. However, we can estimate the Sr
contents of each particular sample from the unit-cell parameters
vs. composition diagrams published elsewhere.23 Therein, for
the samples with nominal compositions La1.25Sr0.75NiO4+d and
LaSrNiO4+d, the majority phases present a lower Sr content, with
an approximate value of x = 0.6 and 0.5, respectively. The minority
phases in both samples have an approximate Sr content of x = 0.8
and x = 0.6, respectively. This means that the sample with nominal
composition LaSrNiO4 has, in fact, a lower Sr content than that
with nominal x = 0.75, suggesting that the incorporation of Sr has
reached a plateau under the synthesis conditions for x = 0.75. In
fact, Fig. 3 shows that the a parameter and the volume display a
broad minimum and maximum at x = 0.75.
Table 4 lists the interatomic distances obtained for each sample.
The Ni–O1 and Ni–O2 bond lengths are plotted vs. Sr content
in Fig. 5. The Ni–O1 bond length (Fig. 5(a)) decreases with the
Sr content up to x = 0.75, which in fact corresponds to the Sr-
richest sample. This decrease is concomitant with the progressive
oxidation of Ni2+ to Ni3+, and can be related to the observed
Fig. 4 Observed (points) and calculated (full curve) neutron diffraction
profiles for La1.25Sr0.75NiO4+d using (a) a single phase F4/mmm model, and
(b) a two phase F4/mmm model. In both cases, the second row of tick
marks indicate the Bragg reflections of a NiO impurity.
4380 | Dalton Trans., 2006, 4377–4383
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Table 3 Structural parameters after the Rietveld refinement of NPD data for La2−xSrxNiO4+d (x = 0.75 and 1) with two tetragonal F4/mmm phases in
each material at RT. La, Sr and O2 at 8e (0,0,z) sites, Ni at 4a (0,0,0) and O1 at 8c (1/4,1/4,0), positions
La1.25Sr075NiO4+d (1)
La1.25Sr075NiO4+d (2)
LaSrNiO4+d(1)
LaSrNiO4+d (2)
˚
a = b/A
5.3919(2)
12.7135(6)
369.62(3)
5.4048(6)
12.529(2)
365.99(8)
5.3969(3)
12.7331(9)
370.87(4)
5.4019(6)
12.632(2)
368.62(7)
˚
c/A
3
˚
V/A
La (Sr)
z
0.3612(3)
0.41(5)
0.22(6)
0.59(7)
0.1715(4)
1.2(1)
0.3609(8)
0.6(2)
1.6(3)
0.7(3)
0.1646(9)
0.3655(4)
0.15(9)
0.1(1)
0.1(1)
0.1695(5)
0.3549(6)
0.3(1)
1.0(2)
1.1(2)
0.169(2)
2
2
2
˚
˚
˚
B/A
B/A
B/A
z
Ni
O1
O2
2
˚
B/A
0.7(3)
1.0(1)
1.8(4)
Mole fraction (%)
v2
Rp (%)
Rwp (%)
Rexp (%)
RI (%)
68(2)
29.2
6.07
8.51
1.58
3.39
24(1)
29.2
6.07
8.51
1.58
4.44
45(2)
2.43
5.77
7.28
4.67
6.60
37(2)
2.43
5.77
7.28
4.67
8.35
Table 4 Interatomic distances for the system La2−xSrxNiO4+d (0 ≤ x ≤ 1) system determined from Rietveld refinement of power neutron diffraction
Bond length/x
0
0.1
0.25
0.5
0.75(1)
0.75(2)
1(1)
1(2)
La(Sr)–O1 × 4
La(Sr)–O2
La(Sr)–O2 × 4a
La(Sr)–O2 × 2
La(Sr)–O3
La(Sr)–O3
La(Sr)–O3
La(Sr)–O4
La(Sr)–O4
Ni–O1 × 4
Ni–O2 × 2
Ni–O3
2.632(1)
2.34(1)
2.766(2)
2.763(2)
2.42(2)
2.46(3)
2.48(2)
2.59(1)
2.19(1)
1.9303(1)
2.22(1)
2.22(2)
2.608(1)
2.379(4)
2.7639(7)
2.605(2)
2.35(3)
2.756(5)
2.598(2)
2.393(4)
2.7374(6)
2.598(3)
2.412(7)
2.728(1)
2.586(7)
2.46(2)
2.721(2)
2.564(4)
2.495(8)
2.735(1)
2.647(5)
2.34(2)
2.718(2)
2.413(2)
2.4992(3)
2.44(2)
2.54(3)
2.3923(9)
2.384(2)
1.9277(0)
2.211(4)
2.2475(0)
1.9194(0)
2.24(3)
2.19(2)
1.9095(0)
2.211(3)
1.9063(0)
2.180(6)
1.9109(2)
2.06(1)
1.9081(1)
2.158(6)
1.9099(2)
2.14(2)
a For the x = 0 compound, defined in Fmmm, the multiplicity of La(Sr)–O2 distances is 2.
shrinkage of the a and b parameters. The Ni–O2 distances for
x = 0, 0.1 and 0.25 must be averaged with Ni–O3, since O2 and
O3 are both axial positions slightly split as a consequence of the
introduction of the O4 interstitials. An overall decrease of the Ni–
O2 axial bond lengths (Fig. 5(b)) is also observed, indicating the
mentioned charge transfer effect upon substitution of La3+ by Sr2+.
Aiming to determine the mechanical compatibility of our
materials with the other cell components, thermal expansion
measurements of the dense ceramics were carried out in air in
undoped La2NiO4+d compound to values well above 100 S cm−1
for most of the Sr-doped La2−xSrxNiO4+d (0.5 ≤ x ≤ 1). The total
electrical conductivity clearly increases with the Sr content up to
273 S cm−1 at 600 ◦C for the sample with x = 0.75. Although
the structural data have been collected at room temperature, and
the conductivity measurements cover the range 350–950 ◦C, we
are tempted to correlate the observed increase in conductivity
with x to the contraction of the Ni–O1 equatorial bond lengths
previously described. This can be understood as a result of the
increment of charge carriers in the conduction band upon hole
injection concomitant to the Sr chemical doping. We think that
the hole injection effect together with the more basic character of
Sr2+ vs. La2+ leads to more covalent, shorter Ni–O chemical bonds
with a higher transfer integral which gives rise to an improved
charge transport across the solid. The total conductivity is slightly
lower for the sample with nominal composition LaSrNiO4 than
that of the x = 0.75 compound, since the actual Sr content
is higher in this latter sample, as demonstrated in the neutron
study. Fig. 7 shows a metal–insulator transition around 450 ◦C for
La2NiO4+d, as described elsewhere in our previous work,24 whereas
La2−xSrxNiO4+d samples (0.1 ≤ x ≤ 1) present a metallic behaviour
in the studied temperature range.
◦
the temperature range 25–1000 C. Fig. 6 illustrates the thermal
expansion coefficients (TEC) obtained at 600 and 800 ◦C. It is
remarkable that the observed values tend to increase with the
introduction of Sr, exhibiting maximum values for the x = 0.75
compound. This might be ascribed to the increase of the shortest
La–O2 distance with the Sr content, responsible for the cohesion
between the perovskite and the rock-salt layer (see Table 4).
The total electrical conductivity (r) of sintered ceramics was
determined under air using the four-probe method in the temper-
ature range 350–950 ◦C. Fig. 7 shows the r vs. 1000/T curves. The
electrical conductivity requirement for SOFC cathode material is
about 100 S cm−1 at the operating temperature. It is r◦emarkable
that the conductivity increases from 50 S cm−1 (at 1000 C) for the
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Fig. 7 Electrical conductivity for the system La2−xSrxNiO4+d (0 ≤ x ≤ 1)
in air at different temperatures.
rich members of the series (x = 0.75, 1) require the presence of
two K2NiF4 type phases to explain the broadening of the high-
angle reflections, accounting for a Sr-segregation for x > 0.50.
The structural parameters, in particular Ni–O bond lengths and
(La,Sr)–O distances can be correlated with the physical properties,
which allow us to propose the materials of the series La2−x
SrxNiO4+d (0 ≤ x ≤ 1) as appealing cathode-material candidates for
IT-SOFC. The electronic conductivity values obtained is adequate
for that required for cathode materials at SOFC. The highest
conductivity was 273 S cm−1 at 600 ◦C for La1.25Sr0.75NiO4+d.
Their thermal expansion coefficients are close to that of the
electrolytes usually utilized at SOFC (8YSZ, CGO) (11–13 ×
10−6 K−1). Although this magnitude increases with the Sr content,
leading to values up to 17 × 10−6 K−1, high thermal stability
is still retained. The presence of a majority Sr-rich phase and a
minority Sr-deficient phase in the last members of the series could
account for the change in conductivity behaviour observed for
Fig. 5 Variation of Ni–O bond lengths for the system La2−xSrxNiO4+d
(0 ≤ x ≤ 1) as a function of strontium content: (a) Ni–O1; (b) Ni–O2.
La1.25Sr0.75NiO4+d and LaSrNiO4+d
.
Acknowledgements
We thank the financial support of CICyT to the project MAT2004-
0479 and MAT2005-02933 and of the Madrid community to
the project ENERCAM-CM S-0505/ENE/0304. We also wish
to acknowledge CIEMAT for the scholarship. This work was
partially performed at the spallation neutron source SINQ, Paul
Scherrer Institute, Villigen, Switzerland.
Fig. 6 Evolution of the thermal expansion coefficient (TEC) for the
system La2−xSrxNiO4+d (0 ≤ x ≤ 1) as a function of strontium content
calculated from dilatometric data in air at 600 and 800 ◦C.
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