**2.1 Ceramic method**

The well known as ceramic method consists of the mixing and grinding of the starting compounds in stoichiometric quantities, being afterwards heated in furnaces at relatively high temperatures for relatively long dwelling times. Eventually, the desired material will be obtained, with a smaller or higher grade of impurities.

The main disadvantage of this method is the low homogeneity of the obtained product, which demands repeated procedures of intermediate homogenization and thermal treatment at high temperature. Thus, the method requires the grinding (intermittently) of the material during the heating treatment in order to minimize the drawback created by the interface (Figure 2) resulting of the reaction between the starting reagents. Figure 2 shows how that interface would be formed by A and B chemically combined. That interface increases its size as a function of the time of reaction, giving rise to a slower and less efficient transfer of A and B in opposite direction to meet and react. Thus, by grinding the grain formed by A, B, and AB, one will be able to increase the contact surface between A and B and therefore to make more efficient the reaction. We should just take into account that the solid state reactions are extremely slow, and that is why the high temperature and high dwelling times are required, but besides, a high contact surface between the reagents is desired to increase the velocity of the reaction. As a result, high crystalline materials are obtained, which are not acceptable for catalytic applications.

Fig. 2. Reaction between grains corresponding to An+ and Bm+. The cross-lined area corresponds to the interface created between them.
