**2. Powders prepared by chemical methods**

#### **2.1 Solid-state method**

It is the simplest and most widely used synthesis method in solid-state chemistry. It consists of making the various solid reagents react directly at high temperature [4]. The reagents used for this method are simple oxides: ZnO, Cr2O3, Fe2O3, NiO, MgO, fluorides: BaF2, CaF2 [5], carbonates: NaCO3, K2CO3 [6], nitrates: CuNO3 [7] and oxalates: Fe(COO)2. There are reagents, which are difficult to handle, due to their hygroscopy and high sensitivity to water vapor. For example, lanthanum oxide (La2O3), reacts quickly to the air to form carbonates and hydroxides, so it should be calcined first at 900 °C to remove all traces of impurities. The powders are weighed according to the stoichiometric quantities provided by the reaction equation and should grounded in an agate mortar to obtain the desired phase. It is done in the absence or in the presence of a volatile liquid (acetone) to ensure good dispersion of the grains and good homogeneity of the powder [8]. Then, the materials are subjected to a controlled heat treatment, would to form well-defined crystalline structures. The powder obtained could be annealed as it is or shaped into pellets pressed uniaxially. During calcination, there is release of carbon dioxide or oxygen dioxide and possibly a little water vapor. Successive anneals after intermediate grinding are generally necessary for the reaction to be complete. Synthesis temperatures are generally of the order of 900 to 1000 °C.

Although this method is considered the simplest and most economical compared to other methods, it has drawbacks. In fact, the reagents should be of high purity, because some impurities could have marked effects and do not give the desired phase and therefore, it would influence the properties of materials. In addition, this method leads to the formation of powders with large particles and with low homogeneity. Finally, the synthesis of ceramics by this method requires reactions for hours or even days at relatively high temperatures.

The materials synthesized by this method are numerous, among them, the cuprates A2BO4: La2CuO4, Pr2CuO4, Sr2CuO4) [9], the perovskites ABO3: (CaTiO3, LaTiO3, BaTiO3 [10], the pyrochlore A2B2O7: Dy2Ti2O7, La2Zr2O7, Cd2Re2O7 [11], the brownmillerite A2B2O5: Ca2(Al,Fe)2O5 [12].
