**1. Introduction**

106 Polycrystalline Materials – Theoretical and Practical Aspects

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> The perovskite is a calcium titanium oxide mineral species composed of calcium titanate, with the chemical formula CaTiO3. The mineral was discovered in the Ural mountains (Russia) by Gustav Rose in 1839 and is named after Russian mineralogist L. A. Perovski (1792-1856).

> The perovskite crystal structure was published in 1945 from X-ray diffraction data on barium titanate by the Irish crystallographer H. D. Megaw (1907-2002). It is a true engineering ceramic material with a plethora of applications spanning energy production (SOFC technology), environmental containment and communications.

> The general stoichiometry of the perovskite structure is *AB*X3, where *A* and *B* are cations and X is an anion. The *A* and *B* cations can have a variety of charges and in the original perovskite mineral (CaTiO3) the *A* cation is divalent and the *B* cation is tetravalent. CaTiO3 exhibits an orthorhombic structure with space group Pnma.

> The traditional view of the perovskite lattice is that it consists of small *B* cations within oxygen octahedra, and larger *A* cations which are XII fold coordinated by oxygen.

> Figure 1 shows a picture of the mineral perovskite which, as it was mentioned above, is composed by calcium titanate.

Fig. 1. Perovskite mineral species (CaTiO3).

Structural Characterization of New Perovskites 109

The sol-gel method is a wet-chemical process widely used in the field of materials science. The starting point is the colloidal solution (*sol*) that acts as the precursor for an integrated

Perovskites can be synthesized starting from the oxides or salts of the different metals wanted to be part of the final structure by dissolving them in a solution containing a complexing agent (citric acid, e.g.) and heating it up to form gel. Immediately after, a drastic heating leads to the decomposition of the organic part and therefore to the formation of a fine and intimate distribution of the different metals in the resulting precursor, meaning this a high contact surface between them that allows a more efficient reaction at a relatively high

The freeze-drying method, also known as lyophilisation, is a process consisting of the freezing of the solution of the starting reagents and the subsequent reducing of the surrounding pressure to allow the frozen solvent to sublime directly from the solid phase to

In the lab, the process is carried out by placing the solution in a decantation funnel and dropping it slowly in a container with liquid nitrogen in order to freeze it in small rounded "grains". Right after the dropping ends, the frozen 'grains' of solution are placed in an Erlenmeyer flask which is immediately connected to a freeze-dryer (Figure 3). Now the

Fig. 3. Freeze-dryer with several Erlenmeyer flasks connected containing different frozen

network (*gel*) of either discrete particles or network polymers.

temperature and a prudent dwelling time.

**2.3 Freeze-drying method** 

the gas phase.

solutions.

**2.2 Sol-gel method** 
