Journal Name
ARTICLE
This journal is © The Royal Society of Chemistry 2016 J. Name ., 2016, 00 , 1-3 | 1
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
SPS-assisted synthesis and thermoelectric characterization of
the Magnéli phase V6O11
M. Joos,
a
G. Cerretti,
a
I. Veremchuk,
b
P. Hofmann,
e
H. Frerichs,
a
D. H. Anjum,
f
T. Reich,
c
I.
Lieberwirth,
d
M. Panthöfer,
a
W.G. Zeier
e
and W. Tremel
a
The discovery of a large thermopower in layered cobalt oxides led to a surge of interest in oxides for thermoelectric
application. Whereas conversion efficiencies of p-type oxides compete with those of non-oxidic materials, n-type oxides show
significantly lower thermoelectric performances. In this context, Magnéli oxides have gained attention as promising n-type
thermoelectrics. A combination of crystallographic shear and intrinsic disorder leads to relatively low lattice thermal
conductivities and metall-like electrical conductivities. However, advances in the application of Magnéli phases are mostly
hindered by synthetic and processing problems, especially when metastable or nanostructured materials are needed. In this
work, we describe the rapid preparation and simultaneous consolidation of bulk amounts of the Magnéli phase V6O11 by spark
plasma sintering (SPS). Synchrotron powder X-ray diffraction (PXRD) indicated V6O11 to be the predominant phase
(96.64 wt. %) in samples prepared by SPS-assisted synthesis, and the structure of V6O11 was determined from a single phase
sample synthesized by ampoule reaction with synchrotron X-ray powder diffraction data. V6O11 exhibits a low electrical
resistivity of 1.85⋅10
-5
Ω m, a Seebeck coefficient of -39 V⋅K
-1
and a low thermal conductivity of 2.7 W⋅(mK)
-1
with a
maximum of 0.025 at 760 K, a value that is moderatzy high for n-type vanadium oxides and metal oxides in general.
Additionally, V6O11 demonstrated superior electronic properties compared to V2O5, the only other vanadium oxide so far
characterized on its thermoelectric properties. The study represents a proof of concept for the development of promising,
cheaper, and more efficient thermoelectric materials.
Introduction
Multivalent transition metal oxides (TMOs) exhibit a cornucopia of
interesting and also useful properties beyond those of conventional
semiconductors employed in electronic and optoelectronic
devices.
1–4
The ground state in TMOs is dictated by the valence state
of the transition metal cations, and the interplay between spin, orbital,
charge, and lattice degrees of freedom has been explored widely in
this class of materials with partially filled d shells.
5–7
Vanadium oxides are prototype examples of multivalent TMOs with
strongly correlated electrons.
8,9
They can undergo reversible metal-to-
insulator phase transitions accompanied by changes in their
crystallographic, magnetic, optical, electronic and electrical
properties. Electron-lattice interactions and electron-electron
correlation tailor the properties of vanadium oxides for a variety of
applications as chemical sensors,
10
electrode materials for lithium
batteries,
11,12
capacitors and supercapacitors,
13
for electronic and
optical
2
devices and also in catalysis.
14,15
These examples show the potential of vanadium oxides for the design
of novel information storage and energy conversion devices. For
engineering the properties of TMOs it is desirable to decouple
electronic and thermal transport. This decoupling can be achieved
through an intrinsic periodic arrangement of structural building blocks
with different electron and phonon transport characteristics that occur
in the so-called Magnéli phases. Vanadium-based Magnéli phases
represent a series of compounds with sub-stoichiometric oxygen
composition and the generic formula V
O
2−1
=V
2
O
3
+
( − 2) VO
2
(3 ≤ ≤ 9), whose structure is derived from rutile and
corundum, in which crystallographic shear (CS) planes occur due to
the removal of oxygen.
16–21
Most phases of the Magnéli series are
unstable at high temperatures, getting either oxidized or reduced to
neighbouring V
O
2−1
phases,
22–25
and they are expected to yield the
compositional end phases VO2 and V2O3 eventually.
As CS planes act as phonon-scattering centers, they reduce the
thermal conductivity while maintaining a high electron mobility and
a large thermopower, which are important requirements for
thermoelectric applications.
26,27
When moving from VO2 to V2O3, the
end members in the V-O phase diagram of the V
O
2−1
Magnéli
series, the d band occupation (i.e. degree of reduction) across the
series increases, leading either to insulating (V3O5), semiconducting
(V6O11, V8O15, V9O17) or metallic (V7O13) behavior at room
temperature.
28–31
Advances in the application of Magnéli phases are mostly hindered by
synthetic and processing challenges, especially when metastable and
a
Johannes Gutenberg University Mainz, Duesbergweg 10-14, D-55128 Mainz,
Germany
b
Max Planck Institute for the Chemical Physics of Solids, Nöthnitzer Str. 40, D-
01187 Dresden, Germany
c
Institut für Kernchemie, Fritz-Straßmann-Weg 2, 55128 Mainz, Germany
d
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz,
Germany
e
Physikalisch-Chemisches Institut, Justus-Liebig-Universität Gießen, Heinrich-Buff-
Ring-17, 35392 Gießen, Germany
f
Imaging and Characterization Core Lab, King Abdullah University of Science and
Technology, Thuwal 23955-6900, Saudi Arabia
†Further data can be reviewed in the supplementary information.
See DOI: 10.1039/x0xx00000x