Intermetallic clathrates are at the forefront of research in novel thermoelectrics. They are inclusion compounds in which atoms of one type (guest) are enclosed in oversized cavities formed by the framework atoms of another type (host). This structural feature leads to extremely low and even ''glass-like'' thermal conductivity whereas the electrical conductivity remains high and "crystal-like". This concept which is usually referred to as ''phonon glass - electron crystal'' (PGEC) makes clathrates a promising class of thermoelectric materials.
Synthesis of single phase clathrates is often problematic even when X-ray powder diffraction of polycrystalline samples shows no foreign phases. The foreign phases appear as thin intergrain layers, which can have a great impact on transport properties. Because these phases are very air sensitive the polycrystalline samples are chemically and mechanically unstable. Therefore investigations of intrinsic properties of some clathrate phases should be performed on single crystals.
Here we present our study of crystal growth of type-I transition metal clathrates Ba₈TM_xSi_46-x and Ba₈TM_xGe_46-x (TM = Cu, Au) in two different regimes. Bulk single crystals were grown in a floating zone furnace. We studied segregation effects of main constituting elements and of low doping impurities by investigation the composition profiles along the growth direction. Because of absence of air sensitive intergrain inclusions the grown single crystals, contrary to polycrystalline samples, are chemically and mechanically very stable.
The structural feature of clathrates and their extremely low thermal conductivity imply their specific growth behaviour. It manifests itself most pronounced in a rapid crystallization process. Ultra rapid crystallization in our study was realized using the melt spinning technique. This technique which provides the cooling rate of about 10⁵-10⁶ K/s was originally developed for the production of amorphous metallic materials and is used also for preparation of nanograined materials. Our numerous experiments on melt spinning of various clathrates have revealed surprisingly large grains of at least 1 μm [1-4]. Because of the anomalously high growth rate of the clathrate phase the formation of impurity phases is kinetically suppressed, and the melt spun samples appear to be air stable, too. We present our transmission electron microscopy (TEM) investigation of melt spun samples and discuss thermodynamic and kinetic aspects of the unusual clathrate crystallization: the interplay of nucleation and bulk growth as well as the effect of composition on the grain size.


[1] A. Prokofiev, S. Paschen, H. Sassik, S. Laumann, P. Pongratz, Utility patent AT: 10749 U1 2009-09-05, DE: 20 2008 006 946.7, patent applications US: 12/231,183, JP: 135994/2008.
[2] S. Paschen C. Gspan, W. Grogger, M. Diensleder, S. Laumann, P. Pongratz, H. Sassik, J. Wernisch, and A. Prokofiev. J. Cryst. Growth, 310, 1853 (2008)
[3] S. Laumann, M. Ikeda, H. Sassik, A. Prokofiev, and S. Paschen. J. Mater. Res. 26, 1861 (2011)
[4] S. Laumann, M. Ikeda, H. Sassik, A. Prokofiev, and S. Paschen. ZAAC 201100353 (2011)

We acknowledge financial support from the Austrian Science Fund (FWF project TRP 176-N22)

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