Regensburg 2010 – wissenschaftliches Programm
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DF: Fachverband Dielektrische Festkörper
DF 8: Electrical and mechanical properties
DF 8.1: Vortrag
Dienstag, 23. März 2010, 14:00–14:20, H9
High-pressure single-crystal structure investigations of sillenites — •Leonore Wiehl, Alexandra Friedrich, Eiken Haussühl, Wolfgang Morgenroth, and Björn Winkler — Institut für Geowissenschaften, Goethe-Universität, Frankfurt/Main, Germany
Sillenites, Bi12MO20 (M = Si, Ge, Ti), crystallize in the cubic non-centrosymmetric space group I23. They are piezoelectric and optical active, show an electro-optic effect and a high photoconductivity, leading to applications using the photorefractive effect. It is assumed, that some characteristics of these properties are correlated with the stereochemical activity of the 6s2 lone electron pair of Bi3+.
At ambient conditions the Bi3+ lone pairs are oriented towards large cavities in the crystal structure of sillenites. Thus the stereochemical activity is expected to decrease, or even vanish, if the cavities are compressed under high external pressure. The crystal structures of Bi12SiO20, Bi12GeO20 and Bi12TiO20 were investigated at high pressure in diamond anvil cells. Single crystal X-ray intensity data were collected at ambient conditions in house and at pressures up to 16.8(3) GPa with synchrotron radiation at HASYLAB (D3).
We found the largest compression of interatomic distances along the lone pair directions. A complete collapse of the cavities, however, seems to be prevented by the high symmetry of the crystals. Changes of the Bi[5+2]O7 polyhedra will be discussed.
Financial support from the DFG (HA 5137/3-1) and from HASYLAB is gratefully acknowledged. We thank HASYLAB for synchrotron beamtime and Martin Tolkiehn for assistance at D3.