Traveling-Solvent Floating-Zone (TSFZ) method
The expertise of our crystal growth lab
is to synthesize large single crystals for neutron scattering experiments. The technique we are using is the so called Traveling-Solvent Floating-Zone (TSFZ) method. The major advantage of this technique is that crystals can be grown without getting into touch with other materials, such as crucibles, which eliminates a major source for contaminations. Currently our lab is operating two image furnaces. With our latest model crystals with high melting point as well as a large length can be grown. The latter is particularly important in the case of incongruent melting systems, where large deviations between the nominal composition and that of the grown crystal are observed in the beginning of the growth experiment.
Both image furnaces are equipped with two elliptic mirrors (see figure). As a heat source halogen lamps are installed in the mirrors outer focus. Their light is reflected into the inner focus, which for both mirrors coincides with the location of the melt. The melt is situated between the polycrystalline feed rod (upper shaft) and the grown material (lower shaft), and hold in place by surface tension, only. Feed rod and grown crystal spin in opposite directions to guarantee a homogenous melt as well as a homogenous temperature distribution in the melt. Crystal growth is initiated by a slow downward translation of the feed rod and the lower shaft, which effectively corresponds to a melt traveling along the feed rod. The optimum/maximum growth speed depends on the material and varies between a few tenth of a millimeter and several centimeters per hour. The diameter of the crystal is controlled by the ratio of the translation velocity for the feed rod and the lower shaft. Feed rod, melt and crystal are located within a sealed quartz tube, which allows performing crystal growth in different atmospheres.
Watch a movie of a crystal growth. (.wmv format ~1Mb
) or (.mpg format ~5Mb
Orienting, cutting and polishing of single crystals
After growing a crystal it is important to characterize its properties. In contrast to neutron scattering, many characterization techniques, such as magnetization and resistivity measurements as well as single crystal X-ray diffraction, require singlecrystalline pieces with a particular shape, crystallographic orientation and polished surfaces. Corresponding equipment in our crystal growth lab includes a Laue camera to determine the crystal orientation, diamond blade and wire saws, different types of polishing jigs, a polishing machine, and a polarization microscope. The Laue camera is mounted on a rotating anode X-ray generator (Rigaku). Different types of goniometers are available, depending on the needs. All goniometers fit on our saws and one of them can be mounted on a polishing jig, which allows the preparation of polished surfaces with an error less than 1 degree in the orientation. Our large angle goniometer is useful for orienting new compounds with irregular shape, since Laue pictures from different crystal sides can to be taken without remounting the crystal. Our large polishing machine is capable of polishing several crystals at a time. Furthermore, it is equipped with a special sample holder which allows to polish the small face sides of several inch long single crystals.
On the one hand characterization measurements are useful to check the quality of the crystals before expensive neutron diffraction experiments are performed. On the other hand, these measurements often reveal important results which go far beyond a simple quality inspection. Magnetization and electronic transport measurements are performed with our Multiple Property Measurement System (MPMS) from Quantum Design with a maximum magnetic field of 7 Tesla and temperatures between 2K and 400K. For the resistivity measurements the MPMS is equipped with a resistance bridge from Linear Research Inc. Single crystal X-ray diffraction experiments are performed at the NSLS
, the APS
and at HASYLAB