Dresden 2003 – wissenschaftliches Programm

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HL: Halbleiterphysik

HL 49: Poster II

HL 49.57: Poster

Donnerstag, 27. März 2003, 16:30–19:00, HSZ/P2

NMR as a local probe in tin clathrates of the alkali metals M₈Sn₄₄₂ with M = K, Rb, Cs — •Frank Haarmann, Michael Baitinger, and Yuri Grin — Max-Planck-Institut für Chemische Physik fester Stoffe

The elements of group 14 (E14 = Si, Ge, Sn) form binary compounds with alkali metals, having the clathrate I structure. The 3D network is built by E14 atoms, the cavities accommodate alkali metals. Tin clathrates of the alkali metals K, Rb, and Cs show defects located at one of the Sn positions. These were detected by an extended X-ray diffraction study. No long range order of the vacancies was observed. A probable change of the physical properties introduced by these defects which were not observed in the structure type I clathrates of silicon is of interest. Thermoelectric measurements of Cs₈Sn₄₄₂ [1] showed that the concept of rattle-type phonon scattering is not applicable due to vacancies in the network of Sn atoms. Analysis of the dislocations around vacant positions indicates that defects induce additional interaction between the cage atoms and the framework. Wide line and MAS NMR experiments on ¹¹⁷Sn, ¹¹⁹Sn and ⁸⁷Rb using samples of M₈Sn₄₄₂ with M = K, Rb, Cs were done. The influence of the defects on the crystal structure can be seen in a broad distribution of chemical shifts or Knight Shifts, respectively. An analysis of the line shape, spin-lattice, and spin-spin relaxation will be presented.

[1] G.S. Nolas, B.C, Chakoumakos, B. Mahieu, G. J. Long, T.J.R. Weakley, Chem. Mater. 12, 1947 (2000).