Advances in the Low Temperature Preparation and Structural Characterization of Lanthanum Strontium Manganite Powder

Abstract

Perovskite oxides of formula Lal-xSrMnO 3 have been obtained by the thermal decomposition of precursor powders. Two different kinds of precursors, carbonates and citrates have been prepared by low temperature, i.e., "chimie douce" technique. The careful control of the chemical and the hydrodynamic parameters during the synthesis process allows obtaining nice homogeneous and small size particles (80 nm for the ex-carbonates and 30 nm for ex-citrates). Pure perovskite phase is observed after a low temperature thermal treatment, from 550 'C. The structure of these oxides is either rhombohedral or cubic and depends on the strontium content, the temperature and the partial pressure of oxygen during the thermal treatment. The Mn-O distances and the Mn-O-Mn angles are directly related to the amount of Mn 4 + content. INTRODUCTIONIn recent years, the doped perovskite manganites such as Lal-,SrMnO 3 have attracted growing attention due to the colossal magnetoresistance (CMR) properties. Several studies have shown that the microstructure, such as grain size, plays a very significant role in the intrinsic properties [ 1,2]. The synthesis methods are very important to obtain polycrystalline materials with specific microstructure.Traditional way of processing is usually the mixing of oxides, hydroxides or carbonates, followed by high temperature (T > 1000 'C) processing. Consequently, the materials obtained with these methods are constituted of large particles with low surface area. "Soft chemistry" techniques have been developed in order to obtain -at lower temperature -the same materials as the one observed at high temperature with the ability to control the particle size, the surface area and the stoichiometry of the powders [3]. The advantages of using powders of reduced grain size to prepare ceramics or thick films is the increase of the reactivity and the possibility of lowering the sintering temperature. The homogeneity is a very important parameter for having reproducible properties. Our previous studies performed on spinel manganites have evidenced the influence of the powder quality on the final material properties [4,5].The electronic properties are related to the mixed valence state Mn3+/Mn4+ that leads to mobile charge carriers. Low temperature synthesis allows moreover a larger oxidation state [6]. It was also observed that high oxygen ionic conductivity correlates with a cubic or an orthorhombic structure [7,8]. The coefficient of non stoichiometry 8 depends on the temperature of thermal treatment and on the oxygen partial pressure [9]. The highest values of 8 have been obtained for low temperature synthesis technique [10]. Many recent studies indicate that the LaMnO 3 + 8 compounds exhibit different structural types as orthorhombic Pnma, rhombohedral R3c,

Internal friction investigation of phase transformation in nearly stoichiometric LaMnO3+δ

“…5 Furthermore, they are attractive for environmental issues as cathode materials in solid oxide fuel cells. 6,7 Various methods have been tested to synthesize the lanthanum manganite LaMnO 3+δ including the conventional solid-state reaction, 8 co-precipitation, 9,10 sol-gel 11 and molten salts reactions. 12 The literature is scarce on hydrothermal synthesis.…”

Microwave‐Assisted Hydrothermal Synthesis, Crystal Structure, and Thermal Decomposition of Strontium Citrate Monohydrate Sr3(C6H5O7)2·H2O

“…[ 12 ] Moreover, Sr 3 (C 6 H 5 O 7 ) 2 · 3.5H 2 O can be found in literature with the water content being determined by thermogravimetry, only. [ 8 ]…”