**3.4 Electric properties**

In **Figure 8**, we show the electrical conductivity of our polycrystalline pellets pressed and sintered at 1173 K. Greater conductivity differences are noticed to depend on the temperature. The tendency of LaCoO3 electrical conductivity is to increase when temperature is rising. This behavior of electrical conductivity in relation to the temperature indicates the process is thermally activated, which is characteristic for semiconductors. For our LaCoO3 sample, semiconducting behavior appears at temperatures above 353 K, whereas metallic behavior was observed at temperatures below 353 K. This is in accordance with the previous statements that LaCoO3 is ionic at elevated temperatures. The electrical conductivity is p-type for both ambient and elevated temperatures. With semiconductors, there are insufficient mobile carriers at low temperatures and resistance is high; but, as we heat the material, more and more of the lightly bound carriers escape and become free to conduct. Electrons are excited over the band gap and occupy energy levels in conductivity band whereas holes are created in valence band. The band gap in the metal is small and the electrons can easily jump to conduction band. For the metallic behavior with increasing temperature above absolute zero, the flowing electrons

**101**

**Figure 8.**

*temperature in (b).*

*Perovskite-Type Lanthanum Cobaltite LaCoO3: Aspects of Processing Route…*

will run into the atoms in the lattice. This will cause the atoms to move slightly out of their lattice sites and to interfere with electrons that travel freely. Effectively, they start to block electrons on their path, causing electrons to scatter. The change in slope of the conductivity-temperature plot is assumed to be the onset of ionic conductivity. LaCoO3 is a mixed conductor with contributions from ionic conduc-

*LaCoO3 total conductivity σ depending on absolute temperature in (a) and lnσ depending on absolute* 

*DOI: http://dx.doi.org/10.5772/intechopen.86260*

tivity and electronic conductivity.

**Figure 7.** *SEM micrograph of LaCoO3 powder.*

*Perovskite-Type Lanthanum Cobaltite LaCoO3: Aspects of Processing Route… DOI: http://dx.doi.org/10.5772/intechopen.86260*

*Cobalt Compounds and Applications*

Scanning electron micrograph of LaCoO3 powders obtained by solid state synthesis at 1273 K is displayed in **Figure 7**. The perovskite LaCoO3 powders obtained consist of pre-sintered conglomerates of grains. The agglomeration tendency is due to the high calcination temperature employed to obtain the single phase. The reaction product has well-defined shape and less uniform grain size distribution. The high-temperature calcination results in severe agglomeration. The sintering of many crystallites during high thermal treatment resulted in the larger grains, which are polycrystals. The grain limits and triple junction points

In **Figure 8**, we show the electrical conductivity of our polycrystalline pellets pressed and sintered at 1173 K. Greater conductivity differences are noticed to depend on the temperature. The tendency of LaCoO3 electrical conductivity is to increase when temperature is rising. This behavior of electrical conductivity in relation to the temperature indicates the process is thermally activated, which is characteristic for semiconductors. For our LaCoO3 sample, semiconducting behavior appears at temperatures above 353 K, whereas metallic behavior was observed at temperatures below 353 K. This is in accordance with the previous statements that LaCoO3 is ionic at elevated temperatures. The electrical conductivity is p-type for both ambient and elevated temperatures. With semiconductors, there are insufficient mobile carriers at low temperatures and resistance is high; but, as we heat the material, more and more of the lightly bound carriers escape and become free to conduct. Electrons are excited over the band gap and occupy energy levels in conductivity band whereas holes are created in valence band. The band gap in the metal is small and the electrons can easily jump to conduction band. For the metallic behavior with increasing temperature above absolute zero, the flowing electrons

**3.3 Microscopy analysis**

are well defined.

**3.4 Electric properties**

**100**

**Figure 7.**

*SEM micrograph of LaCoO3 powder.*

will run into the atoms in the lattice. This will cause the atoms to move slightly out of their lattice sites and to interfere with electrons that travel freely. Effectively, they start to block electrons on their path, causing electrons to scatter. The change in slope of the conductivity-temperature plot is assumed to be the onset of ionic conductivity. LaCoO3 is a mixed conductor with contributions from ionic conductivity and electronic conductivity.

**Figure 8.** *LaCoO3 total conductivity σ depending on absolute temperature in (a) and lnσ depending on absolute temperature in (b).*

Variation of the electrical conductivity with temperature obeys Arrhenius formula, Eq. (9), and is calculated based on the following:

$$
\sigma = \sigma\_0 \exp\left(\frac{-E\_a}{\text{KT}}\right) \tag{9}
$$

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

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

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 can explain the increase of partial pressure of oxygens:

$$2\,\mathrm{Co}\_{\mathrm{Co}}^{\bullet} + \mathrm{O}\_{\mathrm{O}}^{\mathrm{x}} = \mathrm{V}\_{\mathrm{O}}^{\bullet \bullet} + 2\,\mathrm{Co}\_{\mathrm{Co}}^{\mathrm{x}} + \frac{1}{2}\mathrm{O}\_{2} \text{ (\(\mathbb{R}\)}\tag{10}$$

where CoCo <sup>x</sup> = Co3+ , CoCo ● = Co4+ and OO <sup>x</sup> = O2−.

Eq. (11) expresses the charge disproportion defect as follows:

$$2\,\text{Co}^{3\*} \leftrightarrow \text{Co}^{2\*} \star \text{Co}^{4\*} \tag{11}$$

**103**

*Perovskite-Type Lanthanum Cobaltite LaCoO3: Aspects of Processing Route…*

compounds. Ionic defects have their most pronounced effect on transport properties, which are ionic conductivity and diffusion. For LaCoO3 samples, the oxygen

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 tempera-

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 supercon-

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

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

We conclude the present work stating that using this conventional method as solid state reaction allows obtaining highly single-phase LaCoO3 powders for

This work was supported by the National Authority for Scientific Research

and Innovation/Romanian Ministry of Education and Research, project "RESTORE"—117/16.09.2016 ID/Cod My SMIS: P\_37\_595/104958.

tures (above 353 K) and are mediated by presence of oxygen vacancy.

the gaseous by-products or the waste to collect it and dispose it.

applications and uses of advanced functional materials.

*DOI: http://dx.doi.org/10.5772/intechopen.86260*

vacancies are the mobile ionic defects.

ductivity to superconductors.

presented in this chapter.

**Acknowledgements**

**4. Conclusions**

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

**Figure 9.** *Variation of dc conductivity (σ) with inverse of absolute temperature for LaCoO3.*

compounds. Ionic defects have their most pronounced effect on transport properties, which are ionic conductivity and diffusion. For LaCoO3 samples, the oxygen vacancies are the mobile ionic defects.
