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Struktur Vortrag: Mi., 17:40–18:00 M-V17
Neutron diffraction study of the multinary chalcogenides CuFe1−xZnxSnS4
- a potential photovoltaic material
Susan Schorr 1 , Hans-Joachim Hoebler 2 , Michael Tovar 3
1 Hahn-Meitner-Institute Berlin, Dep. Solar Energy Research, Glienicker Str. 100,
14109 Berlin – 2 Institute of Mineralogy, Crystallography and Materials Science, University
Leipzig, Scharnhorststr. 20, 04275 Leipzig – 3 Hahn-Meitner-Institute Berlin,
Dep. Structural Research, Glienicker Str. 100, 14109 Berlin
The multinary chalcogenides Cu2FeSnS4 (stannite) and Cu2ZnSnS4 (kesterite) have
newly attracted attention as possible photovoltaic materials [1] since the availability of
indium is an object of discussion regarding the large-scale production of CuInSe2 solar
cells. Their both structures are are topologically identical, but assigned to different
space groups due to a different distribution of the cations Cu + , Zn 2+ and Fe 2+ . The
stannite structure is consistent with the I-42m symmetry, with Fe located at at the
origin (2a) and Cu at 4d (0, 1 1
, ) [2]. In the kesterite structure, one Cu atom occupies
2 4
1
, ) and 2d
4
3
, ) respectively, leading to the spacegroup I-4 [2]. In both structures Sn is located
4
). The elements Cu and Zn are neighbours in the periodic table, Cu+
the 2a (0,0,0) position, with Zn and the remaining Cu ordered at 2c (0, 1
2
(0, 1
4
at 2b (0,0, 1
2
and Zn 2+ have the same number of electrons, i. e. they have equal atomic form factors
(f (Cu + )=f (Zn 2+ )). Hence these both cations are not distinguishable by conventional
X-ray diffraction. The problem can be solved using neutron diffraction, because of
the different neutron scattering lengths of Cu and Zn (bCu=7.718 fm, bZn=5.67 fm).
Thus the investigation of the restructure process of the cation substructure in the
stannite-kesterite join, CuFe1−xZnxSnS4, is a prime example to demonstrate the power
of neutron diffraction in structural research of complex materials.
Neutron powder diffraction experiments were performed at the Hahn-Meitner-Institute
Berlin at the high resolution powder diffractometer E9 (λ=1.79 ˚A). Structural parameters
and cation site occupancies were determined by Rietveld analysis of the data. From
the latter the average neutron scattering length (bav(exp)) of the cation sites was obtained,
providing the information about the cation restructure process. Based on a
cation distribution model a theoretical average neutron scattering lengths (bav(calc))
was calculated. The condition bav(exp)=bav(calc) could be fullfilled by modelling the
cation distribution. Surprisingly the evaluated Cu + and Zn 2+ distribution in kesterite
(Cu2ZnSnS4) is random on the sites 2c and 2d, which is in disagreement to literature
[2, 3]. This may be caused by the Cu-Zn differentiation problem in X-ray diffraction
as described above. The crossover from stannite (x=0) to kesterite (x=1) in
CuFe1−xZnxSnS4 was found as a three-stage process of cation restructure involving
Cu + , Zn 2+ and Fe 2+ . The Sn 4+ cation does not take part in this process. The knowledge
about the metal distribution in CuFe1−xZnxSnS4 multinary compounds provides
a basis for further studies, i. e. about defect compounds.
[1] H. Katagiri, Thin Sol. Films 480-481 (2005) 426. [2] Hall et al., Can. Mineral. 16
(1978) 131. [3] Bonazzi et al., Can. Mineral. 41 (2003) 639 and references within.