In the discovery and understanding of new materials, crystal growth plays an essential role. A single crystal provides the ability to study the properties of a new chemical compound free of grain boundaries and ideally with few defects. In addition, the magnetic, electronic, optical, thermal and elastic properties of a crystal depend on the crystal orientation, and much of this information is lost if only polycrystalline materials are studied. The additional insight gained through examining the properties of single crystals provides the baseline information necessary for accurate theoretical modeling and the ultimate understanding of the intrinsic properties of a new material. For example, measurements on a single crystal of a new permanent magnet can provide not only insights into why the material is magnetic, but can also provide limits as to how well the magnet will perform in a electric car motor.
The required size for a single crystal depends on the characterization technique. The crystal structure and composition can be determined with x-rays using crystals as small as 10 m, while inelastic neutron scattering experiments need a crystal with a volume of about 1 cm3. Most other characterization techniques require crystals with sizes that fall within these two limits.
There are several different methods for preparing single crystals and almost all of the methods are used and available at ORNL. The methods can range from a simple aqueous solution growth (e.g. NaCl in water), to analogous high-temperature flux growths (e.g. BaFe2As2 in molten FeAs, or K2V3O8 in molten KVO3). The techniques often involve a crucible that contains the molten material (e.g.n SiO2, Al2O3, graphite or Pt) but for some materials that melt at very high temperatures this is not possible because the crucibles react with the melt. Many of those materials can be grown without a crucible in an optical floating zone furnace, or by using a skull melting technique where the material acts as it own crucible. At ORNL, the vast majority of crystal growth efforts are driven by the desire to understand the properties of new materials.
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