Peculiarities of hydrothermal synthesis

Hydrothermal synthesis, developed in the second half of the XIX century by French mineralogists for modeling natural processes, is now widely used to prepare many compounds of practical importance [19], 

This method is based on the ability of water to dissolve the substances at increased temperature and pressure. The use of water as a solvent is based on self-ionization 2H2O - H3O + OH.  

Ionization degree increases with temperature rise. When some limiting amount of a substance dissolved in solution is exceeded (limiting supersaturation), its spontaneous crystallization starts. This process can be governed by changing of the temperature (and pressure, correspondingly) as it is carried out in a closed vessel. 

Substantial differences between solid-phase reactions and hydrothermal synthesis reactions have been stated in numerous investigations. In solid-phase processes, the sequence of intermediate products formation does not depend on reagents ratio in the initial mixture, and the excess product appears to be a compound with the highest crystallization temperature. On contrary, for the formation of a definite product by hydrothermal synthesis, the initial mixture should contain reagents at an exact stoichiometric ratio [19,20]. In solid-phase reactions, the interaction rate is determined by the rate of diffusion processes, while in hydrothermal processes the determining factor is the rate of dissolution of the initial products in the water. Water simplifies diffusion transport of particles in the system the formation of nuclei and crystal growth occur faster than in solids. 

Hydrothermal synthesis reactions are accelerated substantially in the solutions of NaCl, KCl, NH4CI, etc. The catalytic action of salts is connected with the increase of dissolution rate of initial compounds in the presence of salts. The treatment of mixtures of co-precipitated substances and gels under hydrothermal conditions provides a substantial acceleration of their crystallization. 

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