**2.3 Freeze-drying method**

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 the gas phase.

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 solutions.

Structural Characterization of New Perovskites 111

also allows uniform (homogeneous) doping of trace amounts of rare-earth impurity ions in a

Microwave heating allows a rapid heating rate, however, the final yield decreases compared with the conventional methods. It leads to the synthesis of materials by consumption of less

The sonochemical method is that in which the molecules undergo a chemical reaction due to

Ultrasonic irradiation differs from traditional energy sources (such as heat, light, or ionizing

Many transition metal oxides show the very versatile perovskite structure. The rich variety of physical properties such as high-temperature superconductivity and colossal magnetoresistance observed in these compounds makes them very attractive from both

The general chemical formula for perovskite compounds is *AB*X3, where *A* and *B* are two cations of very different sizes, and X is an anion that bonds to both. The *A* atoms are larger than the *B* atoms, and besides, its ionic radii close to that on the anion X, thus they can form together a cubic close packing. The smaller *B* cation is usually a 3d-transition metal ion which occupies the octahedral interstices of the close packing. X is oxygen or a halide ion. The ideal cubic-symmetry structure has the *B* cation in 6-fold coordination, surrounded by an octahedron of anions, and the *A* cation in 12-fold cuboctahedral coordination. In short, in can be also described as a network of edge sharing octahedra *B*X6. The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of *A* cations, *B* cations or both are reduced. The orthorhombic and

For the stoichiometric oxide perovskites, the sum of the oxidation states of *A* and *B* cations should be equal to six. The occupancy of *A* and *B* positions of different ions with appropriate ionic radii as well as for mixed occupancy of both cation positions leaded to the preparation of numerous compounds with wide spectrum of physical and chemical properties. Among the most famous representatives of perovskite class are the dielectric BaTiO3, high-temperature superconductor YBa2Cu3O7-x, materials exhibiting colossal magnetoresistance *R*1-x*A*xMnO3, where R = La3+, Pr3+ or other rare earth ion, A = Ca2+, Sr2+,

The structure of an ideal cubic perovskite is shown in Figure 4, where the *A* cations are shown at the corners of the cube, and the *B* cation in the centre with oxygen ions in the facecentred positions. The space group for cubic perovskites is Pm3m (221); the equivalent

the application of powerful ultrasound radiation (20 kHz–10 MHz).

radiation) in duration, pressure, and energy per molecule.

tetragonal phases are the most common non-cubic variants.

single step.

energy.

**2.6.2 Microwave assisted method** 

**2.6.3 Sonochemical method** 

**3. Perovskite-type structures** 

fundamental and applied perspectives.

Ba2+, multiferroic materials, etc.

positions of the atoms are detailed in Table 1.

freeze-dryer will carry out its function: to provoke the sublimation of the solvent. For that, the frozen solution undergoes an important vacuum created by a pump, allowing the solvent to sublimate and being the latter frozen once again into a big drum at a very low temperature inside the device.

After one/two day(s) the dried solution – called now *precursor* – is disconnected from the freeze-dryer and treated at certain temperature in order to allow the reaction between the different reagents which are mixed in extremely fine and close way. This method allows much lower both dwelling times and heating temperatures, at the time that makes easier the way to get pure phases.
