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Materialien/Werkstoffe Poster: Do., 13:00–15:30 D-P409
Synthesis, Crystal Structures, Metal Atom Distribution and Magnetic Properties
of the monoclinic Compounds Cr5−xTixSe8 (x ≈ 2, 3, 4)
Joseph Wontcheu 1 , Winfried Kockelmann 2 , Wolfgang Bensch 1
1 Institute of Inorganic Chemistry, University of Kiel, Olshausenstr. 40-60, 24098 Kiel,
Germany – 2 Rutherford Appleton Laboratory, ISIS, Chilton, OX11 0QX, UK
The non-stoichiometric chromium titanium selenides Cr5−xTixSe8 (x ≈ 2, 3, 4) were
prepared applying high-temperature solid-state reactions. The crystal structures have
been refined with X-ray and neutron powder diffraction data with the Rietveld method.
The samples crystallize in the non-conventional monoclinic space group F 2/m with
four formula units per unit cell. The structures of all phases are related to the NiAstype
structure with ordered metal vacancies in every second metal atom layer. The
cation distributions on four inequivalent crystallographic sites were determined. The
results showed a considerable disorder but they nevertheless revealed the site preference
of Ti atoms for the 8h and 8i sites for the three investigated compounds. Based on
the results these compounds are formulated as (Cr2.21Ti1.79)[Cr0.91Ti0.03∆3.06]Se8
for Cr3.11Ti1.82Se8, (Cr1.36Ti2.64)[Cr0.73Ti0.19∆3.08]Se8
for Cr2.09Ti2.83Se8, and (Cr0.22Ti3.78)[Cr0.63Ti0.4∆2.89]Se8
for Cr0.85Ti4.26Se8 (note: () denotes atoms in the fully occupied layers, [] atoms in the
partially layers and ∆ denotes the vacancies). The MSe6 octahedra share edges within
layers and faces between layers. Consequently, long M-M separations occur within the
layers and remarkably short M-M contacts result between M atoms in octahedra sharing
faces. The irreversibility in the field-cooled/zero-field-cooled magnetization at 10
kOe depends strongly on the Ti concentration x. In all cases, χ(T) obeys the Curie-
Weiss law in the high-temperature range. The effective magnetic moments are 4.03,
3.91 and 3.90 µB per Cr atom for Cr3Ti2Se8, Cr2Ti3Se8 and CrTi4Se8, respectively,
slightly larger than the expected value 3.87 µB for spin-only Cr 3+ . This is somewhat
unusual for Cr 3+ , but was frequently observed for chromium chalcogenides [1, 2].
[1] W. Bensch et al., J. Solid State Chem. 145 (1999) 235.
[2] W. Bensch et al., J. Solid State Chem. 158 (2001) 198.