**Abstract**

Lanthanum cobaltite (LaCoO3) perovskite-type oxide is an important conductive ceramic material finding a broad range of technical applications. Physical and chemical properties of the final lanthanum cobalt oxide powder material obtained are strongly dependent on the method of preparation. Taking in account these considerations, we focus our investigation on the solid state reaction process. The characterization of prepared lanthanum cobalt oxide material was studied by using X-ray diffractometry (XRD), scanning electron microscopy (SEM), thermogravimetry-differential scanning calorimetry (TG-DSC), and conduction properties. Following the experimental results, it can be concluded that with proper improvement, the solid state reaction process may also provide an efficient preparation method for perovskite-type LaCoO3 powder. Important to mention is that we looked into the aspects to produce again same which showed consistently reproducibility of batch to batch powder properties. This is a key factor to overcome a successful commercialization of new material synthesis development.

**Keywords:** perovskite, LaCoO3, oxide powders synthesis, solid state reaction

### **1. Introduction**

The continuous interaction between structure and properties allows several intrinsic properties of perovskite materials to advance a very broad range of practical applications. These oxides are being increasingly applied to electronic and magnetic materials [1–3], automotive exhaust and water splitting catalysts [4, 5], and electrode materials for fuel cells and batteries [6, 7]. Among these perovskites, the cobalt-based type LaCoO3 perovskite ignited interest in the research since the early 1960s [8, 9] and continues to be the material of the moment. LaCoO3 perovskite has been shown to have promising catalytic activity for oxygen evolution reaction (OER) [10, 11]. Lanthanum, La, is a relatively large cation and gives structural stability to the catalyst. Rare-earth oxides with full or partially filled inner shells of lanthanide ions would involve the 4f electrons. The 4f electrons contribute to the density of states around the Fermi level, and degenerate strongly, the bandwidth is

narrow and steep. The presence of La3+ with no 4f electrons is beneficial because the electrical conductivity increases and the effective mass decreases [12].

Cobalt, Co, cation, a transition metal with smaller size than La cation, is responsible for catalytic activity. In its compounds, cobalt nearly always exhibits a +2 or +3 oxidation state, although states +4, +1, 0, and −1 are also known [13]. The outer electrons of the element are either in the 3d or 4s subshell. Cobalt chemistry is dominated by the behavior of the 3d electrons. Oxidation state of Co helps the LaCoO3 catalytic activity for OER which can be associated with Co3+ oxidation state. Co3+ active sites by absorption of HO<sup>−</sup> may act as reactants for OER [14]. Electrocatalysis at room temperature, in correlation to the fundamental electronic structure, is still not fully clarified. For Co3O4 with the band gap of 1.9 eV and LaCoO3 with about 0.8 eV, the resistivity is 104 and 10 Ωcm, respectively [15, 16].

On the other hand, crystallographic structure is important for the functional properties of the oxides. For perovskite, the ideal cubic structure goes through different structural distortions due to the ionic radii differences.

The equation determined by Goldschmidt correlates crystal structures geometrically in terms of the ionic packing using the Goldschmidt's tolerance factor t. For a stable perovskite structure, the tolerance factor should have values between 0.75 and 1; otherwise, the ideal cubic structure is a distorted structure. Mathematical expression involving the unit cell length ratio, here rLa, rCo, and rO, is the ionic radii for La, Co, and O respectively; t is given as:

<sup>t</sup> <sup>=</sup> (rLa <sup>+</sup> rO) \_\_\_\_\_\_\_\_\_\_\_\_\_ √ \_\_ 2 (rCo + rO) (1)

**93**

production.

**Figure 1.**

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

Despite these various methods, some technological applications are still limited for LaCoO3 compounds. Therefore, knowledge of the preparation conditions and

The advantages like low temperatures used during processing of wet preparation routes lead to oxide perovskite powders with high-purity nanoparticles. However, these techniques cannot achieve full industrial potential. For instance, major drawbacks of these methods are the high costs of raw materials, the large quantity of gaseous by-products and waste products such as nitrates, acetates, and additional chelating agents used as precursors. The use of such materials requires elaborate systems for gas collection and storage since the by-products are poisonous. Their significant toxicity is not environmentally friendly, making it not sustainable for large-scale applications. In addition to that, these methods involve many stages during the synthesis process. One way to circumvent that is to seek solutions for adapting clean processing routes to specific needs of the products without altering the physical and chemical properties of the resulting powders. For instance, if particle-size distribution is not especially required for certain applications, solidstate synthesis route has superior advantage over wet processing routes owing to its simplicity, less equipment requirements, and ease of scaling up for industrial

The elimination of wet processing for perovskite powder preparation means that

products will cost less both ways, to produce as well to buy, and have an important economic advantage. In addition, by mixing dry powdered precursors in the

optimization of the technical parameters are key challenges.

*The crystal structure of rhombohedral LaCoO3.*

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

Taking in consideration the Shannon crystal ionic radii for all ions along with their coordination numbers [17], the calculated tolerance factor of LaCoO3 is 0.97 for Co3+ and 0.905 for Co2+.

Experimentally, by neutron diffraction technique, the change of lattice parameter with temperature has been observed when the lattice length and angle become longer and smaller, respectively, when temperature increases [18].

The crystal structure of LaCoO3, as shown in **Figure 1**—La atoms, in blue, at corners; O atoms, in red, on face centers; and Co atoms, in green, at the center of the lattice—is a rhombohedron having R3c symmetry at room temperature. This is considered as the most stable: LaCoO3 structure up to around 1698 K when the crystal is cubic [19].

Not only the temperature but also the oxygen partial pressure of environments, p(O2) leads the LaCoO3 to a wide range of oxygen deficiency represented by the formula LaCoO3−δ with δ being the oxygen deficiency. It is observed that moderate oxygen deficiency in LaCoO3-δ causes a slight distortion of the ideal cubic structure which is rombohedral [20].

Since the structure-properties relations are strongly dependent on the preparation method, a lot of interest is focused on their synthesis which ultimately determines its potential applications.

The aim of this research is to explore the possibility to improve the solid state preparation method for obtaining LaCoO3 micro- and nanocrystals, which may contribute to the development of a large-scale production route of LaCoO3 with controlled properties.

Numerous routes to prepare perovskite powders have been proposed. These address various problems involved in the preparation of polycrystalline perovskite powder with single phase, resulting in various microstructures and properties.

Among the adopted techniques, solid state reaction [21], mechanochemical processing [22], Pechini method [23], combustion synthesis [24], sol-gel method [25], and microwave route [26] are widely used.

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

*Cobalt Compounds and Applications*

0.8 eV, the resistivity is 104

for Co3+ and 0.905 for Co2+.

which is rombohedral [20].

controlled properties.

mines its potential applications.

[25], and microwave route [26] are widely used.

narrow and steep. The presence of La3+ with no 4f electrons is beneficial because

Cobalt, Co, cation, a transition metal with smaller size than La cation, is responsible for catalytic activity. In its compounds, cobalt nearly always exhibits a +2 or +3 oxidation state, although states +4, +1, 0, and −1 are also known [13]. The outer electrons of the element are either in the 3d or 4s subshell. Cobalt chemistry is dominated by the behavior of the 3d electrons. Oxidation state of Co helps the LaCoO3 catalytic activity for OER which can be associated with Co3+ oxidation state. Co3+ active sites by absorption of HO<sup>−</sup> may act as reactants for OER [14]. Electrocatalysis at room temperature, in correlation to the fundamental electronic structure, is still not fully clarified. For Co3O4 with the band gap of 1.9 eV and LaCoO3 with about

 and 10 Ωcm, respectively [15, 16]. On the other hand, crystallographic structure is important for the functional properties of the oxides. For perovskite, the ideal cubic structure goes through dif-

The equation determined by Goldschmidt correlates crystal structures geometrically in terms of the ionic packing using the Goldschmidt's tolerance factor t. For a stable perovskite structure, the tolerance factor should have values between 0.75 and 1; otherwise, the ideal cubic structure is a distorted structure. Mathematical expression involving the unit cell length ratio, here rLa, rCo, and rO, is the ionic radii

(1)

√ \_\_ 2 (rCo + rO)

longer and smaller, respectively, when temperature increases [18].

Taking in consideration the Shannon crystal ionic radii for all ions along with their coordination numbers [17], the calculated tolerance factor of LaCoO3 is 0.97

Experimentally, by neutron diffraction technique, the change of lattice parameter with temperature has been observed when the lattice length and angle become

The crystal structure of LaCoO3, as shown in **Figure 1**—La atoms, in blue, at corners; O atoms, in red, on face centers; and Co atoms, in green, at the center of the lattice—is a rhombohedron having R3c symmetry at room temperature. This is considered as the most stable: LaCoO3 structure up to around 1698 K when the crystal is cubic [19].

Not only the temperature but also the oxygen partial pressure of environments, p(O2) leads the LaCoO3 to a wide range of oxygen deficiency represented by the formula LaCoO3−δ with δ being the oxygen deficiency. It is observed that moderate oxygen deficiency in LaCoO3-δ causes a slight distortion of the ideal cubic structure

Since the structure-properties relations are strongly dependent on the preparation method, a lot of interest is focused on their synthesis which ultimately deter-

The aim of this research is to explore the possibility to improve the solid state preparation method for obtaining LaCoO3 micro- and nanocrystals, which may contribute to the development of a large-scale production route of LaCoO3 with

Numerous routes to prepare perovskite powders have been proposed. These address various problems involved in the preparation of polycrystalline perovskite powder with single phase, resulting in various microstructures and properties. Among the adopted techniques, solid state reaction [21], mechanochemical processing [22], Pechini method [23], combustion synthesis [24], sol-gel method

the electrical conductivity increases and the effective mass decreases [12].

ferent structural distortions due to the ionic radii differences.

for La, Co, and O respectively; t is given as:

<sup>t</sup> <sup>=</sup> (rLa <sup>+</sup> rO) \_\_\_\_\_\_\_\_\_\_\_\_\_

**92**

Despite these various methods, some technological applications are still limited for LaCoO3 compounds. Therefore, knowledge of the preparation conditions and optimization of the technical parameters are key challenges.

The advantages like low temperatures used during processing of wet preparation routes lead to oxide perovskite powders with high-purity nanoparticles. However, these techniques cannot achieve full industrial potential. For instance, major drawbacks of these methods are the high costs of raw materials, the large quantity of gaseous by-products and waste products such as nitrates, acetates, and additional chelating agents used as precursors. The use of such materials requires elaborate systems for gas collection and storage since the by-products are poisonous. Their significant toxicity is not environmentally friendly, making it not sustainable for large-scale applications. In addition to that, these methods involve many stages during the synthesis process. One way to circumvent that is to seek solutions for adapting clean processing routes to specific needs of the products without altering the physical and chemical properties of the resulting powders. For instance, if particle-size distribution is not especially required for certain applications, solidstate synthesis route has superior advantage over wet processing routes owing to its simplicity, less equipment requirements, and ease of scaling up for industrial production.

The elimination of wet processing for perovskite powder preparation means that products will cost less both ways, to produce as well to buy, and have an important economic advantage. In addition, by mixing dry powdered precursors in the

stoichiometric ratios followed by heating to obtain the desired reaction product, at the end of the reaction, there is no waste to dispose. In this way, the method is also environmentally friendly.

The overall price of oxide perovskite powders is strongly influenced by the industrial production route. Solid state reaction technology is resource- and energy-saving. The processing route allows one to obtain the reaction products readily in one step by using less expensive precursors, without waste products. The upscale of solid-state synthesis route resumes to batch production. This is different from flow processing which is a continuous production technique that has inflexibility to adjust if the case. Additionally, the absence of waste and by-products is another major positive aspect regarding the environmental impact of the entire production process. This is why we consider this method useful for both environmental protection and industrial use.

In solid state reaction, rates are typically diffusion-limited; consequently, decreasing the diffusion distances through intimate mixing of reactants possibly contributes to overcome the diffusion barrier in addition to high temperature during the thermal treatment. The characteristics of the starting materials impact the area of contact between the reacting particles with influence of the rate of reaction.

During the heating regime, a non-isothermal transformation takes place, and during the dwell regime, an isothermal transformation takes place. A significant kinetic parameter for studying the phase transformation of precursors is the activation energy Ea, representing the energy barrier for atoms and molecules to move and rearrange. For a given differential scanning calorimetry curve with the heating rate β, one observes the maximum reaction rate at the peak temperature T.

In this study, the kinetic parameters of solid state transformations are determined from the maximum reaction rate at the peak temperature T. For a set of differential scanning calorimetry curves under a constant heating rate, the kinetic transformation is described with the mathematical expression of Kissinger Eq. 2 [27]:

$$\frac{\mathbf{d}\left(\frac{\ln\beta}{T^2}\right)}{\mathbf{d}\left(\frac{1}{T}\right)} = -\frac{\mathbf{E}}{\mathbf{R}}\tag{2}$$

where β is the heating rate, Ea is the activation energy, T is the absolute temperature corresponding to maximum process rate, and R is the gas constant.

For the processing the choice of dwell temperature was taking in account Tammann's temperature described as the temperature above which its constitutive cations become mobile so that their bulk diffusion is possible [28, 29].

The electrical conduction phenomenon in perovskite materials is very important since many properties depend on it. There is a strong correlation between the electrical conductivity of the materials, temperature, and the nature of the sample analyzed.

In bulk materials, two types of conductivity phenomena occur: long-range conductivity and localized transport oxygen vacancies. The conduction mechanism can be ionic, electronic, or both. The proportion of ionic to electronic conduction in the materials varies upon temperature and the purity of material. The variation of electrical conductivity with temperature is explained by Eq. (3):

$$\sigma = \mathbf{A} \exp^{\frac{-\mathbf{E}\_b}{kT}} + \mathbf{B} \exp^{\frac{-\mathbf{E}\_{db}}{kT}} \tag{3}$$

Eq. (4) describes the phenomena at higher temperatures where the intrinsic conduction process dominates.

$$
\sigma = \mathbf{A} \exp^{\frac{-E\_b}{kT}} \tag{4}
$$

**95**

**3. Results**

**3.1 Thermal behavior**

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

performed to find out the proper sintering thermal treatment.

conjunction with a continuous nitrogen flow cryostat.

For both equations, Ea and Eb are the activation energy for the intrinsic and extrinsic conduction processes, respectively, A and B are constants, T is the absolute

The formation of the perovskite phase via solid state reaction was done under controlled rates for heating and cooling as well as for the dwell of thermal treatment. A 2°C/min rate of heating and cooling was selected. Preparatory trials were carried out in order to find the suitable temperature for dwell covering 600–1000°C.

Equimolar quantities of La2O3 and Co3O4 powders from Aldrich, with >99.8% purity, were mixed and ground thoroughly in agate mortar. The mixing was done with isopropanol from Chimopar SA, purity >96%. For the thermal treatment, the powder in alumina crucibles was placed in chamber furnace. Once the powder was synthesized, compacted ceramics were prepared by uniaxial pressing technique. After this, they were submitted for sintering. Similarly, preparatory trials were

The crystalline structures of the prepared powders were characterized by heating in chamber furnace, environmental air atmosphere, at 50°C increments, making X-ray diffraction measurements. A MiniFlex 600 Rigaku analyzer was used. 2θ scans were recorded between 5 and 90° and a speed of 1°/min, with resolution of 0.1°/step. The as-obtained reflexion patterns are indexed by using the Inorganic Crystal Structure Database (ICSD). Lattice constants and quantitative values for the identified phases are obtained from the fit to the corresponding X-ray diffraction spectra by using the PDXL powder diffraction analysis package from Rigaku. The microstructure and morphology of the as-prepared powders were examined using a scanning electron microscope VP CARL ZEISS (Field Emission Scanning Electron Microscope— FESEM) with LaB6 cathode enabling 2-nm resolution. Specimens of powders were prepared by depositing it on a conductive carbon-based double-faced adhesive tape. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements were carried out on precursors and their equimolar mixture using STA 449 F5 Jupiter® from NETZSCH-Gerätebau GmbH. The instrument is equipped with Proteus® software to carry out the measurement and evaluation of the resulting data. The DSC/TG curves are recorded up to 1273 K, with a heating rate of 10 K/min. The analyses of electrical properties of the sample were carried out from 133 to 513 K using Alpha-A Novocontrol Technologies Novocontrol GmbH analyzer in

Thermal analysis results of precursors, **Figure 2**, indicate that Co3O4 powder, in red color, is stable below 1023 K while La2O3 powder, in black color, exhibits several steps of mass loss during the heating regime. La2O3 precursor features three weight-loss regions. The first, a gradual weight loss not shown here, is related to

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

**2. Experiment**

**2.1 Material preparations**

**2.2 Materials characterization**

temperature, and K is the Boltzmann constant.

For both equations, Ea and Eb are the activation energy for the intrinsic and extrinsic conduction processes, respectively, A and B are constants, T is the absolute temperature, and K is the Boltzmann constant.
