.
Angewandte
Communications
DOI: 10.1002/anie.201106111
Oxide Ion Conductors
Remarkably High Oxide Ion Conductivity at Low Temperature in an
Ordered Fluorite-Type Superstructure**
Xiaojun Kuang, Julia L. Payne, Mark R. Johnson, and Ivana Radosavljevic Evans*
Oxide ion conductors are technologically important materials
because of their potential applications in oxygen sensors and
pumps, as dense membranes for oxygen permeation, catalysts,
and as electrolytes for solid oxide fuel cells (SOFCs).[1–4] To be
efficient in various applications, candidate materials should
possess a conductivity of at least 10À2 ScmÀ1 at device-
operating temperatures; currently commercially used yttria-
stabilized zirconia (YSZ) reaches this target at 7008C.[1]
Given the drive towards lowering device-operating temper-
atures, there is a strong impetus and a great challenge for
materials chemists to develop materials with enhanced ionic
mobility and superior low-temperature oxide ion conductiv-
ity.[5,6] A better understanding of generic structural features
and pathways which facilitate ionic mobility at lower temper-
ature is a key step in reaching this goal.
stable in the narrow range between 730 and 8248C.[11] There
has been considerable interest in stabilizing the highly
conducting d-Bi2O3 phase by isovalent or aliovalent cation
substitution to preserve oxide ion conductivity at lower
temperatures. For example, 20% substitution of Er into Bi2O3
results in oxide ion conductivity of 2 ꢀ 10À2 ScmÀ1 at 5008C
and 0.4 ScmÀ1 at 7008C.[12] Double cation substitution has
yielded even higher conductivities at low temperatures (300–
5008C); the best examples include Dy-W,[13] Pr-V,[7] and the
recently reported La-Re[8] co-substitutions. On first use,
Bi0.85Pr0.105V0.045O1.545 and Bi12.5La1.5ReO24.5 show the high-
est oxide ion conductivity among the doped d-Bi2O3 materi-
als, with s ꢀ 10À3–10À2 ScmÀ1 at 300–4008C, approaching the
Cu-doped layered Bi2VO5.5 (BICUVOX), which itself has the
disadvantage of two-dimensional, anisotropic conductivity.
Although the relative chemical instability of Bi oxides under
reducing conditions has so far hampered their applications in
SOFCs, the use of bilayer electrolytes can overcome this
issue.[14] In addition to high oxide ion conductivity, bismuth-
based oxides show electrocatalytic activity and therefore also
have great potential for applications in electrochemical
oxygen separation.[15,16]
[7]
[8]
Here we report a remarkably high oxide ion conductivity
at low temperatures (300–5008C) in an ordered pseudo-cubic
3 ꢀ 3 ꢀ 3
d-Bi2O3
superstructure
with
composition
Bi1ÀxVxO1.5+x (x = 0.087 and 0.095). Its conductivity is the
highest we know of in a singly substituted d-Bi2O3-based
material
and
comparable
to
the
unstable
[7]
Bi0.85Pr0.105V0.045O1.545 and Bi12.5La1.5ReO24.5 on their first
use, that is, before their conversion into a stable tetragonal
form and an associated drop of conductivity of almost two
orders of magnitude.[8,9] By contrast and unusually, our
materials crystallize as stable ordered superstructures, and
do not undergo phase transitions to lower symmetry and
lower conductivity polymorphs. Our ab initio molecular
dynamics (AIMD) simulations reveal the structural features
and mechanisms which facilitate the high oxide ion mobility
at low temperatures, and provide conceptual insight readily
applicable to other materials and structure types.
A common structural feature in the best d-Bi2O3-based
oxide ion conductors reported so far is that doping stabilizes
simple cubic structures with a ꢀ 5.5 ꢁ and space group
[7–8,13]
ꢀ
Fm3m.
By comparison, doped d-Bi2O3 materials which
possess long-range superstructures usually have lower con-
ductivities.[17–19] Simple cubic doped materials, however, are
often only metastable and convert to lower symmetry forms
with significantly lower conductivities, which is a major
obstacle to their practical use.
Initial X-ray and electron diffraction studies of the 3 ꢀ 3 ꢀ
3 fluorite superstructures in the Bi2O3-V2O5 system carried
out by Zhou,[20] suggested the existence of a phase with
composition Bi18V2O32 (Bi1ÀxVxO1.5+x with x = 0.100); the
closely related Bi16V2O29 (x = 0.111) was also found to be a
3 ꢀ 3 ꢀ 3 fluorite supercell, but distinguishable from Bi18V2O32
based on peak positions in its diffraction pattern. In our
syntheses (see Methods in ESI), single-phase materials were
formed for compositions with x = 0.087 and 0.095
(Bi0.913V0.087O1.587 and Bi0.905V0.095O1.595), by firing the starting
oxides at 8258C for 12 h, after initial calcinations at 700, 750,
and 8008C (for 12 h at each temperature with intermediate
grinding); we will occasionally refer to these two very similar
compositions jointly as “Bi18V2O32”. Bi16V2O29 started appear-
ing as a second phase for 0.095 < x < 0.100 and V-doped g-
Bi2O3 was present for 0.074 < x < 0.083 (see Figure S1 in the
Supporting Information).
The high-temperature cubic fluorite-type bismuth oxide,
d-Bi2O3, with intrinsic oxygen vacancies, shows the highest
oxide ion conductivity measured in any material (around
1 ScmÀ1 at 7508C);[10] however, it is only thermodynamically
[*] X. Kuang, J. L. Payne, I. Radosavljevic Evans
Department of Chemistry, University of Durham
Science Site, Durham DH1 3LE (UK)
E-mail: ivana.radosavljevic@durham.ac.uk
M. R. Johnson
Institut Laue Langevin
38042 Grenoble (France)
[**] Financial support was provided by the EPSRC through grant EP/
F030371. J.L.P. thanks the Durham University for a PhD student-
ship. The authors thank the ILL for computational facilities and
Didier Richard and Eric Pellergrini for recent developments in LAMP
and nMoldyn codes.
Impedance measurements on Bi0.905V0.095O1.595 and
Bi0.913V0.087O1.587 (Figure 1) were carried out on heating to
Supporting information for this article is available on the WWW
690
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 690 –694