**1. Introduction**

Solid state chemistry thrives on a rich variety of solids that can be synthesized using a wide range of techniques. It is well known that the preparative route plays a critical role on the physical and chemical properties of the reaction products, controlling the structure, morphology, grain size and surface area of the obtained materials (Cheetham & Day, 1987; Rao & Gopalakrishnan, 1997). This is particularly important in the area of ABO3 perovskite compounds given that they have for long been at the heart of important applications. From the first uses of perovskites as a white pigments, PbTiO3 in the 1930's (Robertson, 1936) to the MLC capacitors (mostly based in substituted PbTi1-xZrxO3 or BaTiO3 materials) in which today's computers rely on to operate, synthetic methods have been a key factor in the optimization of their final properties (Pithan et al., 2005).

Traditionally, most of these ceramic materials have been prepared from the mixture of their constituent oxides in the so called solid state reaction, "shake and bake" or ceramic method, a preparative route for which high temperature is a must in order to accelerate the slow solid–solid diffusion (Fukuoka et al., 1997; Inaguma et al., 1993; Safari et al., 1996). Despite its extended use in practically all fields in which perovskite-structured materials are needed, not all applications are better off with this method since the low kinetics and high temperature also yield samples with low homogeneity, with the presence of secondary phases and with uncontrolled (and typically large) particle size of low surface area which are undesired for some applications such as in gas sensors or in catalysis where small particles and high surface area are needed (Bell et al., 2000). This conventional route, however, is widely employed due to its simplicity and low manufacturing cost. Nevertheless, with appropriate optimisation, as when softmechanochemical processing is used prior to calcinations at high temperature (Senna, 2005), the method results in high quality single phase perovskites that can be used in electroceramic applications.

Synthetic Methods for Perovskite Materials – Structure and Morphology 495

For some applications, such as in multiferroic materials based devices, it is the crystal symmetry of the multiferroic what matters. In these materials the presence or not of a centre of symmetry is crucial for the observation of ferroelectricity. With this regard, there are cases, as in some AMnO3 perovskites (A=Y or Dy), in which the synthetic route determines whether an orthorhombic compound with a centre of symmetry (i.e. non ferroelectric) or a hexagonal phase without the centre (i.e. ferroelectric) is formed (Carp et al., 2003; Dho et al., 2004). Consequently, preparative conditions have to be carefully selected in order to obtain crystal phases with the adequate structure. The use of more than one synthesis method is

In this work three different groups of perovskite compounds have been prepared and their crystal structure and microstructure have been studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Each group of samples had its own structural characteristics so, prior to choosing one synthetic approach, trials were carried out using different methods. In all cases, the final method chosen was the one that maximised phase purity and resulted in better properties. Here we demonstrate that phase pure samples susceptible to be compared depending on the desired characteristics can be obtained using

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),

x in A

<r

x in B

Solid state reaction 1.25 LSSB125-ss Glycine-nitrate route 1.25 LSSB125-gn

Freeze-drying 0.1 NSMC10-fd

Sol-gel 0.2 NSMC20-sg Freeze-drying 0.2 NSMC20-fd

Sol-gel 0.3 NSMC30-sg Freeze-drying 0.3 NSMC30-fd

A>(Å)

0.2 LPS20

1.34 LB134

0.1 NSMC10-sg

**Compound Synthetic method Variable Label** 

La0.40Nd0.30Sr0.23Ca0.07FeO3- Solid state reaction 0.3 LNSC30 La0.20Pr0.40Sr0.26Ca0.14FeO3- Solid state reaction 0.4 LPSC40 La0.19Pr0.31Sr0.26Ca0.24FeO3- Solid state reaction 0.5 LPSC50 La0.19Pr0.21Sr0.26Ca0.34FeO3- Solid state reaction 0.6 LPSC60 La0.18Pr0.12Sr0.26Ca0.44FeO3- Solid state reaction 0.7 LPSC70 La0.20Sr0.25Ca0.55FeO3- Solid state reaction 0.8 LSC80

La0.34Nd0.16Sr0.12Ba0.38FeO3- Glycine-nitrate route 1.31 LNSB131 La0.04Nd0.46Sr0.24Ba0.26FeO3- Glycine-nitrate route 1.28 LNSB128

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

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

Sol-gel

which have the potential for their use in different applications.

La0.50Pr0.30Sr0.20FeO3- Solid state reaction

La0.50Ba0.50FeO3- Glycine-nitrate route

thus worth trying in all cases.

different synthetic methods.

La0.05Sm0.45Sr0.32Ba0.18FeO3-

Nd0.8 Sr0.2 Mn0.9Co0.1O3

Nd0.8 Sr0.2 Mn0.8Co0.2O3

Nd0.8 Sr0.2 Mn0.7Co0.3O3

in the Ln0.5A0.5FeO3.

**2. Experimental** 

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 controlled particle size, morphology and improvement in surface area.

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 superconducting oxides at relatively lower temperatures than before.

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 characteristics for this type of application.

For some applications, such as in multiferroic materials based devices, it is the crystal symmetry of the multiferroic what matters. In these materials the presence or not of a centre of symmetry is crucial for the observation of ferroelectricity. With this regard, there are cases, as in some AMnO3 perovskites (A=Y or Dy), in which the synthetic route determines whether an orthorhombic compound with a centre of symmetry (i.e. non ferroelectric) or a hexagonal phase without the centre (i.e. ferroelectric) is formed (Carp et al., 2003; Dho et al., 2004). Consequently, preparative conditions have to be carefully selected in order to obtain crystal phases with the adequate structure. The use of more than one synthesis method is thus worth trying in all cases.

In this work three different groups of perovskite compounds have been prepared and their crystal structure and microstructure have been studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Each group of samples had its own structural characteristics so, prior to choosing one synthetic approach, trials were carried out using different methods. In all cases, the final method chosen was the one that maximised phase purity and resulted in better properties. Here we demonstrate that phase pure samples susceptible to be compared depending on the desired characteristics can be obtained using different synthetic methods.
