4th EucheMs chemistry congress

thursday, 30-Aug 2012
s852
chem. Listy 106, s587–s1425 (2012)
solid state Chemistry Materials chemistry/New materials
solid state Chemistry and Nanochemistry
o - 5 2 3
vAryinG the nAnoStruCture of ternAry
GerMAniuM teLLurideS And itS infLuenCe on
therMoeLeCtriC ProPertieS
t. roSenthAL 1 , t. SChrÖder 1 , M. SChneider 1 ,
C. Stiewe 2 , o. oeCKLer 3
1 LMU Munich, Department of Chemistry, Munich, Germany
2 DLR, Köln, Germany
3 IMKM, Leipzig, Germany
Multinary tellurides – especially nanostructured ones – have
turned out to be promising novel thermoelectrics with high figures
of merit ZT = S2σT/k. In multinary systems, the charge carrier
concentration is flexible, allowing one to optimize the Seebeck
coefficientS. Real-structure effects and nanostructures enhance
phonon scattering and thus reduce thermal conductivity k, which
means an increase of ZT.
At ambient conditions, stable compounds of the
“homologous” series (GeTe) (Sb Te ) exhibit trigonal layered
n 2 3
structures. In the rocksalt-type high-temperature phase,
Ge/Sb/vacancies are randomly distributed on the cation sites. The
GeTe content n determines both the vacancy concentration in the
cubic phase and the thickness of the rocksalt-type slabs in the
trigonal phase. In addition to the thermal treatment, this has a
strong impact on the nanostructures formed when the diffusion
required for the phase transition occurs incompletely. For n = 12
or 19, ZT values up to 1.3 are reached. [1] Se and Sn doping result
in more pronounced nanostructures at lower n combined with new
features probably related to phase separation. In-substituted
samples exhibit less short-range vacancy ordering resulting in a
less pronounced nanostructure, while Li can be used to stabilize
the cubic phase by filling vacancies. Additional Sb sheets in
layered phases lead to translation periods of up to 10 nm in (often
metastable) compounds like (MTe)(Sb Te )(Sb ) (M = Ge, Sn). 2 3 2 4 [2]
The average domain size of GeBi Te quenched under
2 4
high-pressure (12GPa) depends on the quenching rate. The
associated grain and domain boundary effects strongly influence
the electrical conductivity σ, which changes form metallic to
semiconducting with decreasing domain size and more randomly
oriented domains. [3]
references:
1. T. Rosenthal et al., Chem. Mater. 2011, 23, 4349.
2. M. N. Schneider et al., Chem. Eur. J. 2012, 18, 1209.
3. T. Schröder et al., Phys. Rev. B 2011, 84, 184104.
Keywords: Tellurides; Nanostructure; Thermoelectric;
solid state Chemistry and Nanochemistry
4 th EucheMs chemistry congress
o - 5 2 4
the roLe of doMAin Size, StruCture And
trAnSforMAtion in defininG P-x-t hyStereSiS
in hydroGen SorPtion By trAnSition MetAL
oxideS (ton) And SuLPhideS (tSn) to forM
hxton And hxtSn BronzeS when ACtivAted By
SPiLLover
P. SerMon 1 , A. BerzinS 2
1 Brunel University, Wolfson Materials Centre, Uxbridge,
United Kingdom
2 Johnson Matthey Research Centre, Blounts Court, Sonning
Common, United Kingdom
A non-stoichiometric bronze is a solid formed by
intercalation via electron and cation insertion into the host oxide
or sulphide. Hydrogen bronzes (H TO and H TS ) are of
x n x n
particular interest since they contain the smallest guest ion and
the consequent high charge density tends to polarize the
surrounding host lattice. Following early studies on H WO and
x 3
H MoO a considerable number of well-characterised hydrogen
x 3
bronzes have been prepared from transition metal oxides and
chalcogenides with either a layer structures or three dimensional
matrices containing tunnels; proton mobilities therein approach
those in water.
We describe significant rates and extents of H sorption by
2
transition metal oxides and sulphides in the presence (but not in
the absence) of Pt at 323K and 5-101kPa, where anion removal
by bulk reduction was not thought to be significant. Preliminary
in-situ X-ray diffraction has revealed the development and
decomposition of H TO and H TS phases as a function of T and
x n x n
p . The results are relevant to the optimisation of (i) catalysts,
H2
(ii) reversible electrode materials, (iii) electrochromic devices and
(iv) hydrogen storage.
XRD is an ideal non-invasive technique for probing
transformations of such solids. -WO under present conditions is
3
normally monoclinic (and only orthorhombic or tetragonal at
higher temperatures). By varying the partial pressure of hydrogen
in the presence of activating Pt at a temperature of 323K, WO3 undergoes reversible phase changes to a tetragonal structure
(a = 0.375nm; c = 0.379nm) that exists at 1-40kPa H , and 2
thereafter a cubic phase (a = 0.397nm) is found. Upon reducing
the p tetragonal domains are reformed, one of which is identical
H2
to that found in adsorption. Upon flushing for 12h with N2 a
orthorhombic structure (a = 0.725nm; b = 0.750nm; c=0.384nm)
is found.
Data are presented on reversible sorption of hydrogen by
Pt/MoO , Pt/WO , Pt/Mo W O , Pt/Na WO and Pt/MoS .
3 3 y 1-y 3 y 3 2
AUGUst 26–30, 2012, PrAGUE, cZEcH rEPUbLIc