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Struktur und Dynamik Poster: Mi., 14:00–16:30 M-P155
Structures of the Metal Oxyhalides MOX (M = Ti, V and X = Cl, Br) at
Low Temperature
Andreas Schönleber 1 , Lukáˇs Palatinus 2 , Sander van Smaalen 1
1 Lehrstuhl für Kristallographie, Universität Bayreuth, BGI Gebäude, Bayreuth, Germany
– 2 Laboratoire de Cristallographie, EPF Lausanne, BSP-Dorigny, Lausanne,
Switzerland
The metal(III) oxyhalide MOX structures with M = Ti, V and X = Cl, Br [1] are
isostructural with FeOCl at room temperature: they are built by slabs consisting of a
M2O2 bilayer enclosed by layers of X atoms. Their symmetry is orthorhombic, space
group P mmn. Two phase transitions have been observed in TiOCl and TiOBr upon
cooling, suggesting the presence of a spin-Peierls state [2,3]. In VOCl only one phase
transition is observed [4].
We have performed temperature dependent single crystal X-ray diffraction experiments
down to T = 10 K at beam line D3, HASYLAB-DESY, Germany. The aim of the
experiments was to explore the phase transitions and to develop precise and detailed
structural models for the low temperature and intermediate temperature phases of
these compounds.
The low temperature phases of TiOCl and TiOBr are twofold superstructures of the
room temperature phases [5,6]. They show a dimerization of the Ti atoms along the
[010] direction in accordance with spin-Peierls magnetic ordering. The intermediate
phases are incommensurately modulated with temperature dependent modulation wave
vectors [7,8].
The analysis of the variations of the structural parameters will be presented as function
of temperature. The results are compared and set into relation to the various degrees
of magnetic order in these compounds, which are responsible for the phase transitions.
Therefore the knowledge of the superstructures is important, because it sheds further
light on the nature of the phase transitions in the MOX compounds.
[1] H. Schäfer et al., Z. Anorg. Allg. Chemie 295 (1958) 268. [2] A. Seidel et
al., Phys. Rev. B 67 (2003) 020405. [3] T. Sasaki et al., arXiv:cond-mat 0501691
(2005) unpublished. [4] A. Wiedenmann et al., J. Phys. C: Solid State Phys. 16 (1983)
5339. [5] M. Shaz et al., Phys. Rev. B 71 (2005) 100405(R). [6] L. Palatinus et al.,
Acta Crystallogr. C 61 (2005) i47. [7] S. van Smaalen et al., Phys. Rev. B 72 (2005)
020105(R). [8] A. Schönleber et al., Phys. Rev. B (2006) in press.