Oxide fluxes are often combined to reduce volatility, viscosity, and reactivity towards the crucibles. Metallic fluxes aren't typically combined, as they do not suffer from the same volatility, viscosity, and reactivity issues. An ideal flux should have the following properties:
* Good solubility for desired compound at growth temperatures.
* Low melting point.
* Large gap between melting and boiling point.
* Easily removed from crystals.
* Unreactive with crucible and starting materials at growth temperatures.