**4. Conclusions**

*Cobalt Compounds and Applications*

Variation of the electrical conductivity with temperature obeys Arrhenius

where σ0 is the pre-exponential factor, Ea is the activation energy, K is the Boltzmann constant, and T is the absolute temperature. The activation energy of

by the oxygen elimination with increase in the temperature (**Figure 9**).

For the undoped LaCoO3 oxide, the oxygen non-stoichiometry is considered to determine the carriers for the conduction phenomena. This is believed to be caused

●● + 2CoCo

2Co3+ ↔ Co2+ + Co4+ (11)

When a voltage is applied to our sintered LaCoO3 sample, a current flow is observed. At a given temperature, the type of charge carrier species is not defined. Because of this, it is useful to depict the conductivity dependence on temperature. The existence of ions is assured by definition, mixed conductors being ionic

<sup>x</sup> + \_1

The Kröger-Vink set of conventions are used to describe electric charge and lattice position for point defect species in crystals. The following ions and vacancies are used: La3+, Co2+, Co3+, Co4+, O2<sup>−</sup>, oxygen vacancies. Making use of these in Eq. (10), the defect equilibrium between the charge species and oxygen partial pressure

\_\_\_ −Ea

KT ) (9)

<sup>2</sup> O2 (g) (10)

formula, Eq. (9), and is calculated based on the following:

the electrical conductivity was found to be Ea = 74 kJ/mol.

can explain the increase of partial pressure of oxygens:

● + OO

and OO

*Variation of dc conductivity (σ) with inverse of absolute temperature for LaCoO3.*

<sup>x</sup> = VO

<sup>x</sup> = O2−. Eq. (11) expresses the charge disproportion defect as follows:

2CoCo

, CoCo ● = Co4+

<sup>x</sup> = Co3+

where CoCo

σ = σ<sup>0</sup> exp(

**102**

**Figure 9.**

In this study, we successfully used solid state synthesis as a dry method for preparation of rhombohedral lanthanum cobaltite LaCoO3 perovskite-type oxides.

The solid state synthesis process is thermally activated and requires high temperatures. The X-ray diffraction analysis suggests that LaCoO3 phase nucleates readily at 973 K. The formation of LaCoO3 occurs gradually with increasing temperature, at the expense of precursors' consumption. At 1273 K, the reaction is essentially complete. Thermal analysis suggests that the reaction is accompanied by reversible transformation of La(OH)3 into La2O3, which is a native surface phase of the oxide phase. The reaction kinetics is strongly mediated by Co3O4 transformation. Co3O4 starts to transform before its decomposition temperature. The overall electronic transport properties exhibited by the prepared powders are typically ionic at elevated temperatures (above 353 K) and are mediated by presence of oxygen vacancy.

The interrelationship between structure and properties is a key element of materials science and engineering. Thinking of the variety of oxide perovskite materials, each particular application is based on a certain functional property of the material. Most macroscopic properties observed in bulk (e.g., crystalline quality, ionic conductivity, density, etc.) are the consequence of the constituent particles' properties of the material. For applications, it is essential to know the relation between material microstructure and the macroscopic properties in order to adjust its performance to more specific requirements and also to keep the fabrication costs low. Properties like ionic conductivity, for example, make the perovskite material promising to be used as a solid electrolyte for solid oxide fuel cells (SOFC). Similarly, ferroelectric and piezoelectric behaviors lead to potential applications for thermistor, actuator, and piezoelectric transducers, and the property of superconductivity to superconductors.

We consider this method to be beneficial for both environmental protection and industrial applications because it does not have by-products or waste at the end of the reaction and the precursors are not expensive. In addition to this, an important advantage of this method is the sustainability to scale up the process. We recommend this for batch production which allows flexibility to adjust the process as required for a specific application. There is no need for additional facilities to collect the gaseous by-products or the waste to collect it and dispose it.

With appropriate optimization, this method we applied to LaCoO3 can be applied to other perovskites; the synthesizing parameters like reaction time and temperature should be obtained by exploration as performed with the recipe presented in this chapter.

We conclude the present work stating that using this conventional method as solid state reaction allows obtaining highly single-phase LaCoO3 powders for applications and uses of advanced functional materials.
