But zone melting can also be a useful technique to grow single crystals for x-ray structure determination. Especially compounds with low melting points are suitable here, that means mainly compunds which are liquid at room temperature. The single crystals can then be grown directly on the x-ray diffractometer by a method introduced by D. Brodalla, D. Mootz, R. Boese and W. Osswald, J. Appl. Cryst. (1985) 18, 316-319
The compound of interest is filled into a glass capillary of 0.2-0.3 mm diameter. The capillary should be sealed on both ends. With the help of the low temperature equipment of the diffractometer the substance is cooled down until it solidifies to a microcrystalline powder. The temperature is then raised again until approximately 15 K below the melting point of the compound. At this temperature the zone melting is performed.
The heat for the zone melting is generated by an ordinary lamp (for example from a slide projector). The light of the lamp is focused onto the capillary by the use of a concave mirror. This mirror is mounted movable, so that the focus can be moved up and down the capillary. The movement can be done manually or driven by a computer.
In a first step there must be a crystal seed in the capillary, which is generated from the microcrystalline powder. This crystal seed can afterwards be grown by zone melting to a suitable crystal for x-ray diffraction.
Further developments by the research group of Prof. Boese apply a focused
infrared laser beam as a heat source.
Structural Chemistry Group,
University of Essen