**2.2 Glycine-nitrate route**

496 Advances in Crystallization Processes

These compounds have been divided into three groups and their compositional details are

Fig. 1. Flowcharts for the: (a) ceramic, (b) glycine-nitrate, (c) sol-gel and (d) freeze-drying

The selection of each method and composition for each series of perovskites was based on

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

methods used to obtain the perovskite compounds shown in the present work.

the desired applications that are described later.

**2.1 Ceramic solid state reaction** 

obtained.

summarised in Table 1. Preparation procedures are detailed below.

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 shown in Figure 1b.

#### **2.3 Sol–gel**

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