Hydrothermal synthesis includes the various techniques of synthesizing substances from high-temperature aqueous solutions at high pressures; also termed "hydrothermal method". The term "hydrothermal" is of geologic origin. Geochemists and mineralogists have studied hydrothermal phase equilibria since the beginning of the twentieth century. George W. Morey at the Carnegie Institution and later, Percy W. Bridgman at Harvard University did much of the work to lay the foundations necessary to containment of reactive media in the temperature and pressure range where most of the hydrothermal work is conducted. In the broadest definition, a process is considered hydrothermal if it involves water temperatures above 100 °C (212 °F) and pressures above 1 atm.
In the context of material science, hydrothermal synthesis focuses on the production of single crystal. Under high temperature > (300 °C) and pressure (> 100 atm), ordinarily insoluble minerals become soluble in water. The crystal growth is performed in an apparatus consisting of a steel pressure vessel called an autoclave, in which the reactant ("nutrient") is supplied along with water. A temperature gradient is maintained between the opposite ends of the growth chamber. At the hotter end the nutrient solute dissolves, while at the cooler end it is deposited on a seed crystal, growing the desired crystal.
