2. Synthesis

#### 2.1 Solid-state reaction method

Inorganic compounds are mixed, following stoichiometric proportions; after that, the powders are thoroughly milled and then finally thermally treated to get the required product, been, perovskites synthesized using this method [15, 16].

#### 2.2 Sol-gel methodologies

These processes can be based on the hydrolysis condensation of metal alkoxides. In the important case of amorphous silica synthesis, silicic acid is first produced by

the hydrolysis of a silicon alkoxide, formally a silicic acid ether; then, the global reaction continues as a condensation polymerization to form high molecular weight polysilicates, which connect together to produce a gel [17].
