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Struktur und Dynamik Poster: Mi., 14:00–16:30 M-P139
Charge Ordering in transition metal oxides with low d-level occupation
A. C. Komarek 1 , M. Isobe 2 , M. Hölzel 3,4 , A. Senyshyn 3,4 , A. Cousson 5 , F.
Bouree 5 , G. Andre 5 , W.-D. Stein 1 , T. Lorenz 1 , M. Braden 1
1 II. Physikalisches Institut, Uni Köln, Zülpicher Str. 77, 50937 Köln – 2 Institute
for Solid State Physics, The University of Tokyo – 3 Darmstadt University of Technology,
Institute for Material Science, Petersenstrasse 23, D-64287 Darmstadt –
4 Munich University of Technology, FRM-II, Lichtenbergstr. 1, D-85747 Garching –
5 LaboratoireLeon Brillouin, CEA/CNRS, F91191 Gif-Sur Yvette Cedex, France
We present a structural study of three different transition metal oxide systems by
means of neutron and single crystal X-ray diffraction. We found charge ordering in
the Y1−xCaxTiO3 titanate system and in the K2V8O16 hollandite system and confirm
the charge order in the LiV2O5 vanadate to persist to low temperatures. Having
one electron in the 3d shell, the rare earth titanates RTiO3 represent an interesting
system to study the complex interplay of magnetic and structural degrees of freedom.
In addition we have analysed the metal-insulator (MI) transitions driven by hole doping
when trivalent R-ions get replaced by divalent earth-alkaline ions. Upon cooling
Ca-doped YTiO3 system exhibits a structural first-order phase transition accompanied
by cossover phenomena in structural and magnetic properties. In contrast to most
transtion metal oxides, we find the metallic phase to be stabilized at low temperature.
For Ca-concentrations around 35-50 % we find first evidence for static charge ordering
in the insulating phases. The vanadate family of oxides AV2O5 shows a variety of lowdimensional
phenomena originating from their peculiar crystal structures. These transition
metal oxides are quasi two dimensional (2D) materials with layers formed by VO5
square pyramids while the A ions are situated between the layers as intercalants and
act as electron donors. For A=Li the vanadium ions are nominally in a mixed-valence
state with an average valency of +4.5. Thus, vanadium 4+ zigzag chains are separated
by non-magnetic 5+ zigzag chains turning this system quasi onedimensional (1D). We
were able to confirm the persistence of charge ordering of LiV2O5 down to 2 K by single
crystal neutron diffraction. The hollandite K2V8O16 room-temperature structure is
tetragonal and consists of octahedral VO6 double-chains running along the c-direction.
The potassium ions align in the free space between these vanadium oxide double-chains
and form chains in c-direction parallel to the VO6 double-chains. The vanadium ions
in this compound possess an average valency of 3.75+. K2V8O16 exhibits a metalinsulator
(MI) transition at 175 K. Concomitantly with the MI-transition there is a
phase transition from the tetragonal to a monoclinic structure. We performed single
crystal X-ray diffraction measurements and were able to solve the complex, distorted
structure below 175 K for the first time. It turned out that there is a dimerization of
the vanadium ions in one of the two vanadium chains forming the VO6 double-chain
and a zig-zag-chain formation in the neighbouring vanadium chain. Furthermore, the
analyses of the bond distances in terms of bond-valence sums point to charge ordering
in this complex distorted structure.