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(3 + 1)-dimensional structure refinement of the fresnoite framework-structure type compound Ba(2)TiGe(2)O(8)
Thomas Höche1, Saeid Esmaeilzadeh, Reinhard Uecker
1AG Kristallographie, Institut für Physik, Humboldt-Universität zu Berlin, Berlin, Germany. hoeche@uni-leipzig.de
Acta Crystallographica. Section B, Structural Science
|March 27, 2003
Summary
The complex modulated structure of barium titanate germanate (Ba(2)TiGe(2)O(8)) was solved using superspace methods. Positional shifts, particularly of oxygen atoms, drive the incommensurate modulation, linked to underbonded barium ions.
Area of Science:
- Crystallography
- Solid-state chemistry
- Materials science
Background:
- Fresnoite framework-structure type compounds exhibit complex structural behaviors.
- Incommensurate modulation in materials can significantly alter their properties.
- Understanding these modulations is key to materials design.
Purpose of the Study:
- To elucidate the incommensurately modulated structure of Ba(2)TiGe(2)O(8).
- To investigate the origin and nature of the structural modulation.
- To correlate structural features with material properties.
Main Methods:
- Utilized a (3+1)-dimensional superspace approach for structure solution.
- Employed neutron powder diffraction on a single crystal.
- Performed bond-valence sum calculations to analyze atomic environments.
Main Results:
- The structure is orthorhombic with superspace group Cmm2(0,beta,1/2)s00 and beta ≈ 0.635.
- Modulation primarily involves positional displacements of oxygen atoms.
- Underbonded barium positions were identified as critical to the incommensurate modulation.
Conclusions:
- Frozen-in rigid-unit modes likely cause the observed modulation.
- Positional modulation significantly improves the bond-valence sum for barium ions.
- The findings highlight the role of barium ion coordination in driving incommensurate structures.