Defect chemistry and transport properties of
Ba
x
Ce
0.85
M
0.15
O
3-
J. Wu,
a)
L.P. Li,
b)
W.T.P. Espinosa,
c)
and S.M. Haile
Material Science, California Institute of Technology, Pasadena, California 91125
(Received 17 November 2003; accepted 30 April 2004)
The site-incorporation mechanism of M
3+
dopants into A
2+
B
4+
O
3
perovskites controls
the overall defect chemistry and thus their transport properties. For charge-balance
reasons, incorporation onto the A
2+
-site would require the creation of negatively
charged point defects (such as cation vacancies), whereas incorporation onto the
B
4+
-site is accompanied by the generation of positively charged defects, typically
oxygen vacancies. Oxygen-vacancy content, in turn, is relevant to proton-conducting
oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant
oxygen sites. We propose here, on the basis of x-ray powder diffraction studies,
electron microscopy, chemical analysis, thermal gravimetric analysis, and alternating
current impedance spectroscopy, that nominally B-site doped barium cerate can exhibit
dopant partitioning as a consequence of barium evaporation at elevated temperatures.
Such partitioning and the presence of significant dopant concentrations on the
A-site negatively impact proton conductivity. Specific materials examined are
Ba
x
Ce
0.85
M
0.15
O
3-
(x 0.85 - 1.20; M Nd, Gd, Yb). The compositional limits
for the maximum A-site incorporation are experimentally determined to be:
(Ba
0.919
Nd
0.081
)(Ce
0.919
Nd
0.081
)O
3
, (Ba
0.974
Gd
0.026
)(Ce
0.872
Gd
0.128
)O
2.875,
and
Ba(Ce
0.85
Yb
0.15
)O
2.925
. As a consequence of the greater ability of larger cations to
exist on the Ba site, the H
2
O adsorption and proton conductivities of large-cation
doped barium cerates are lower than those of small-cation doped analogs.
I. INTRODUCTION
Charge transport in doped transition metal perovskites
(e.g., LaMnO
3
and derivatives) by both oxygen ions and/
or electrons has been widely studied and such materials
find applications in a broad range of electrochemical and
other devices.
1,2
More recently, high proton conductivity
has been established in doped A
2+
B
4+
O
3
perovskites
such as BaCeO
3
, SrCeO
3,
and SrZrO
3
.
3–6
The particu-
larly high conductivity of rare-earth (or yttrium) doped
BaCeO
3
has led to extensive studies of its applicability as
an electrolyte for reduced-temperature solid-oxide fuel
cells and for hydrogen sensors.
7,8
Proton incorporation in BaCeO
3
has been generally
recognized to occur by two steps:
2Ce
Ce
x
+O
o
x
+M
2
O
3
→ 2M'
Ce
+V
••
o
+ 2CeO
2
,
(1)
H
2
O (gas) + V
••
o
+O
o
x
→ 2OH
•
o
, (2)
where M is the trivalent dopant species. In the first step,
introduction of M
3+
ions on the Ce
4+
site creates oxygen
vacancies within the perovskite structure. In the second,
the exposure of the doped material to humid atmosphere
leads to the incorporation of hydroxyl groups onto for-
merly vacant oxygen sites, and of protons at other oxy-
gen sites. To facilitate reaction (1) and encourage dopant
incorporation onto the tetravalent site, samples are pre-
pared with general composition BaCe
1-z
M
z
O
3-
.
In contrast to the simple dopant incorporation mecha-
nism implied in Eq. (1), in which the entirety of the
dopant species resides on the B-site of the perovskite
oxide, it has been shown (through a careful phase dia-
gram study) that Nd in particular can be partitioned over
a)
Address all correspondence to this author.
e-mail: jwu@caltech.edu
b)
Present address: Department of Physics and Astronomy,
Brigham Young University, Provo, Utah 84602.
c)
Present address: Boeing Rocketdyne Propulsion and Power,
6633 Canoga Avenue FB19, Canoga Park, California 91309.
DOI: 10.1557/JMR.2004.0302
J. Mater. Res., Vol. 19, No. 8, Aug 2004 © 2004 Materials Research Society 2366