Flux method

Crystallization
Fundamentals
Crystal Crystal structure Nucleation
Concepts
Crystallization Crystal growth Recrystallization Seed crystal Protocrystalline Single crystal
Methods and technology
Boules Bridgman–Stockbarger method Van Arkel–de Boer process Czochralski method Epitaxy Flux method Fractional crystallization Fractional freezing Hydrothermal synthesis Kyropoulos method Laser-heated pedestal growth Lely method Micro-pulling-down Shaping processes in crystal growth Skull crucible Verneuil method Zone melting

The flux method of crystal growth is a method for recrystallization. The solvent is a flux, typically a solid at room temperature, which dissolves the sample when molten. Crystals of the desired compound often precipitate as well-formed crystals from the cooled flux. The method is particularly suitable for crystals needing to be free of thermal strain.

Crucibles

The flux is typically molten in a crucible made of highly stable, non-reactive material. For production of oxide crystals, metals such as platinum, tantalum, and niobium are common. Production of metallic crystals generally uses crucibles made from ceramics such as alumina, zirconia, and boron nitride. For air sensitive growths, contents are often isolated from the air, either by sealing them in a quartz ampoule or by using atmosphere controlled furnace.

Ideally the flux and crucible should be chosen to avoid contamination through unwanted side reactions. However, there are certain instances where allowing such reactions may be beneficial. Notably, for flux growth of La₅Pb₃O and La_0.4Na_0.6Fe₂As₂, positive effects of using an Al₂O₃ crucible with reactive flux were observed. In the case of La₅Pb₃O, the Al₂O₃ crucible helped oxidize La, forming the final crystalline product. Similarly, for La_0.4Na_0.6Fe₂As₂, reaction between the flux and crucible helped form an oxygen free environment facilitating the formation of the desired product. This controlled interaction between flux and crucible demonstrates the strategic considerations involved in optimizing crystal growth conditions.

Fluxes

An ideal flux should have multiple properties. Most importantly, it must be capable of dissolving the solute, with large changes in solubility with varying temperature. It should also not form a stable compound with the solute, although this may be difficult, due to the flux and solute typically having similar chemical properties. A good flux should also have low volatility at final temperatures, low viscosity at growth temperatures, be non-toxic and be easily available in a cheap and pure form. Often metals and salts are used as fluxes.

Implementation

The starting materials, flux and crucible are heated to form a complete solution, then cooled to a temperature where the solution is fully saturated. Further cooling allows the desired material to precipitate, decreasing the concentration of starting materials in solution. Evaporation of flux is an alternative method to achieving supersaturation rather than cooling. Crystal formation can begin by spontaneous nucleation or may be encouraged by the use of a seed. As material precipitates out of the solution, the amount of solute in the flux decreases and the temperature at which the solution is saturated lowers. This process repeats itself as the furnace continues to cool until the solution reaches its melting point or the reaction is stopped artificially. In flux method synthesis, divergent crystal growth kinetics may emerge, with a small number of crystallites growing at the expense of neighbouring ones, resulting in abnormal grain growth.

One advantage of this method is that the crystals grown often display natural facets, which can simplify preparing samples for measurement. A disadvantage is that most flux method syntheses produce relatively small crystals. However, some materials such as the "115" heavy fermion superconductors (CeXIn₅, X=Co,Ir,Rh) may grow up to a few centimeters. Relative to a pure melt, flux growth minimizes incongruent melting. This allows flux growth to be done on both incongruently and congruently melting crystals.

Flux separation

Following crystallization, a solid crystal-flux matrix may develop, which may cause defects in the crystals due to thermal stresses caused by thermal expansivity differences. A solvent can be used to dissolve the matrix around the crystals, but it may be difficult to find a suitable solvent capable of only dissolving the matrix, especially if the matrix and crystals possess similar chemical properties. One alternative to separate the flux is to do so in the liquid form through the use of either a centrifuge or by attaching a seed to a pulling rod. This method is best used when thermal stresses are an issue. In the centrifuge technique, silica wool acts as a sieve, separating the mixture. In the seed method, such as the Czochralski method, a seed crystal is grown in the liquid solution, allowed to grow and cool, and pulled out, leaving only the crystal. Flux separation can also be done at room temperature through removal of the solid matrix through a drill or evaporation.

See also

• Chemical vapor deposition
• Crystal growth
• Crystallography
• Czochralski process
• Epitaxy
• Hydrothermal synthesis
• Micro-pulling-down
• Verneuil process

External links

• Flux Method for Preparing Crystals
• Growth of single crystals from metallic fluxes
• Flux Technique