So far, we have covered the high-temperature and high-pressure ranges for preparing inorganic compounds, and have shown how syntheses by self-propagating combustion or mechanical grinding can give rise to less energy-intensive processes for preparing powders with varied properties. However, some technological applications require the use of inorganic crystals of various sizes and qualities. We can't stress enough that it was the synthesis of high-purity Si (silicon) rods in the 1960s that enabled microelectronics and computing to take off to the extent they have today. This lecture therefore deals with the growth of single crystals via methods involving reactivity in the liquid rather than the solid state. Numerous single-crystal synthesis routes, such as the Czochralski method, the Bridgman and Verneuil process, as well as floating-zone processes, and even growth by vapor-phase transport or molten salts, will be described using concrete examples. Special attention will be paid to molten salts, pioneered by chemist Henri Moissan. Molten salt synthesis, the art of which is based on the choice of a flux (the salt) with the lowest melting temperature and chemical compatibility with the reactants, enables the preparation of a multitude of crystals, and even new crystalline and nanocrystalline materials, as well as powders (oxides, chalcogenides and others).
Finally, in the context of sustainable development, it is essential to carry out these reactions in molten salts at more moderate temperatures - the arrival of ionic liquids as fluxes enabling synthesis to be carried out at low temperatures. As a result, inorganic synthesis in ionic liquids is booming. It enables the preparation of new metastable compounds at temperatures below 300°C.
