**2. Experimental**

494 Advances in Crystallization Processes

Alternative routes to the solid-state reaction method are wet chemical synthetic methods such as co-precipitation (with oxalates, carbonates, cyanides or any other salt precursors), combustion (including all variants from low to high temperature), sol–gel (and all of its modifications with different chelating ligands), spray-pyrolysis, metathesis reactions, etcetera (Patil et al., 2002; Qi et al., 2003; Royer et al., 2005; Segal, 1989; Sfeir et al., 2005). In all cases the idea is to accelerate the pure phase formation, a goal that is achieved due to the liquid media which permits the mixing of the elements at the atomic level resulting in lower firing temperatures. Other advantages of these methods are the possibility of having

In most cases, the final microstructure of the sample is the key issue in choosing the synthetic method, but phase purity is also a must, and is sometimes overlooked when authors praise their particular synthetic method (Kakihana & Yoshimura, 1999). This was pointed out by Polini et al. (Polini et al., 2004) in the case of the preparation of substituted LaGaO3 phases for SOFC cathodes: methods that supposedly have been developed to improve the scalability and uniformity of the samples, such as the Pechini method (a particular case of the sol-gel method), do not always result in a single phase of the crystalline sample required. Similar cases are common in the literature, as in the case of La1-xSrxMnO3-δ phases (Conceição et al., 2009). This clearly indicates that there is not such a thing as the ideal synthetic method: every method has its advantages and disadvantages.

Ideally, as many as possible synthetic methods should be tried and optimised for each compound of interest in order to obtain better crystals with the proper microstructure. But this is obviously time consuming and very costly. Consequently, researchers usually choose to follow the general trends that have been observed to work in a particular area of interest. As a result, each field has its preferences. For example, the ceramic method, widely used at the beginning of the first years of the high-Tc superconductors was soon replaced because it almost always resulted in non-stoichiometric products with some undesired phases that complicated the interpretation of the superconducting properties. These materials got so much attention in the late 1980's and early 1990's that completely new synthesis methods were introduced including many modifications of sol-gel methods with the ample use of alcoxides as precursors (Petrikin and Kakyhana, 2001). In this case, the synthetic route consisted on the preparation of mixed coordination compounds with alcoxy ligands in aqueous media which ensured good distribution of all metals involved and yielded purer

On the other hand, combustion methods (glycine-nitrate, urea based, and other modifications) have been proposed as one of the most promising methods to prepare perovskite oxide powders to be used as cathode materials in Solid Oxide Fuel Cell technology. (Bansal & Zhong, 2006; Berger et al., 2007; Dutta et al., 2009; Liu & Zhang, 2008). This method consist on a highly exothermal self-combustion reaction between the fuel (usually glycine, urea or alanine) and the oxidant (metal nitrates), that produces enough heat to obtain the ceramic powders. Compared to the ceramic method this synthetic route has much faster reaction times and lower calcination temperatures leading to powders with large compositional homogeneity and nanometric particle sizes, which are desired

controlled particle size, morphology and improvement in surface area.

superconducting oxides at relatively lower temperatures than before.

characteristics for this type of application.

Up to four different synthetic methods (solid state reaction, glycine-nitrate route, sol-gel and freeze-drying) have been used to synthesize a group of 14 perovskite compounds (Figure 1), which have the potential for their use in different applications.


x: doping level in Ln1−xAxFeO3−δ and Nd0.8Sr0.2(Mn1-xCox)O3 series; <rA>: average ionic radius of A-cation in the Ln0.5A0.5FeO3.

Table 1. Nominal compositions, synthetic methods, and labels of the studied perovskites.

Synthetic Methods for Perovskite Materials – Structure and Morphology 497

The preparations of the perovskite compounds with the general composition Ln0.5A0.5FeO3 by glycine-nitrate route involved the use of some nitrates instead of the oxides as starting materials: Ba(NO3)2 (99.99%), Sr(NO3)2 (99.9%) and Fe(NO3)3 (99.98%). The oxides used were La2O3 (99.99%), Sm2O3 (99.999%), Gd2O3 (99.99%). The oxides were dissolved in diluted nitric acid to obtain the corresponding nitrates and the metal nitrates dissolved in distilled water. The solutions were mixed in a 1 litre glass beaker, under constant stirring and placed on a hot plate to evaporate the water excess. After a significant reduction of the solution volume the glycine was added. The amount of glycine used was calculated in order to obtain a glycine/nitrate molar ratio of 2:1. This amino acid acts as complexing agent for metal cations and as the fuel for the combustion reaction. The resulting viscous liquid was auto-ignited by putting the glass beaker directly in a preheated plate (at ~400ºC). The obtained powders were pelletized and fired at 800ºC for 2 hours to remove the carbon residues. The heatings at temperatures above 800ºC were repeated until pure phases were obtained. A flowchart with more details and heat treatments involved in this synthesis is

The sol-gel method was used for the oxides of general composition Nd0.8Sr0.2(Mn1-xCox)O3 (x = 0.1, 0.2 and 0.3). Initially, the oxide Nd2O3 (99.9%) was dissolved in aqueous nitric acid followed by the addition of Sr(NO3)2 (99%), Co(NO3)2.6H2O (99%) and Mn(C2H3O2)2.4H2O (99%). Citric acid was then used as the quelating agent and ethylene glycol as the sol forming product. The solution was slowly evaporated in a sand bath for 24 h and the gel obtained was subjected to successive heat treatments at the temperature of 850ºC (with intermediate grindings). Each firing was of 10 h and was carried out under nitrogen

In the freeze drying method, standardized nitrate solutions were mixed according to the stoichiometry of the final products: Nd0.8Sr0.2(Mn1-xCox)O3 (x = 0.1, 0.2 and 0.3). The starting materials were Nd2O3 (99.9%) which had to be dissolved in diluted nitric acid before the addition of the other compound; Sr(NO3)2 (99%); Co(NO3)2.6H2O (99%) and

The mixture for freeze-drying method was frozen drop-by-drop under liquid nitrogen and subjected to freeze drying at P = 5.10-2 mbar. Thermal decomposition was achieved by slow heating in air up to 600ºC. The pure phases were obtained after repeated heatings at 850ºC (with intermediate grindings), each of 10 h, under nitrogen atmosphere. A flow chart for this

Room temperature X-ray powder diffraction data were collected in the 18≤2≤110° range with an integration time of 10 s/0.02° step. A Bruker D8 Advance diffractometer equipped with a Cu tube, a Ge (111) incident beam monochromator (λ = 1.5406 Å) and a Sol-X energy

atmosphere. The flowchart for this synthesis is shown in Figure 1c.

**2.2 Glycine-nitrate route** 

shown in Figure 1b.

**2.4 Freeze-drying technique** 

Mn(C2H3O2)2.4H2O (99 %).

method is shown in Figure 1d.

**2.5 Characterization** 

**2.3 Sol–gel** 

These compounds have been divided into three groups and their compositional details are summarised in Table 1. Preparation procedures are detailed below.

Fig. 1. Flowcharts for the: (a) ceramic, (b) glycine-nitrate, (c) sol-gel and (d) freeze-drying methods used to obtain the perovskite compounds shown in the present work.

The selection of each method and composition for each series of perovskites was based on the desired applications that are described later.

## **2.1 Ceramic solid state reaction**

The compounds prepared via the ceramic route were obtained from mixing stoichiometric amounts of the raw oxides with 2-propanol in an agate mortar. Starting materials were always oxides of high purity such as La2O3 (99.99%), Sm2O3 (99.999%), Pr2O3 (99.9%), Gd2O3 (99.99%), BaO (99.99%), SrO (99.9%), CaO (99.9%) and Fe2O3 (99.98%). Afterwards, these mixtures were shaped into pellets and were fired in air at 950ºC for 2h. The products obtained were ground, pelletized again and fired at higher temperatures. The flowchart shown in Figure 1a details the heat treatments required in each case until pure samples were obtained.
