https://doi.org/10.15255/KUI.2022.016
Published: Kem. Ind. 71 (11-12) (2022) 685–689
Paper reference number: KUI-16/2022
Paper type: Original scientific paper
Download paper:  PDF

Phase Separation in La0.5Sr0.5MnO3 System

A. Žužić and J. Macan

Abstract

Perovskites are investigated as potential working materials in new technologies, such as solid oxide fuel cells, thermochemical energy storage, and magnetic refrigerators. They are interesting due to their application properties, such as electrical conductivity, ferromagnetic properties, and reducibility. Properties of perovskites can be improved by doping and the formation of structural defects. The aim of this work was to investigate the possibility of preparing Sr-doped lanthanum manganite with the chemical formula La_0.5Sr_0.5MnO₃ by coprecipitation procedure, and to analyse the observed phase separation. The material was prepared by coprecipitation procedure in which sources of metal cations (La³⁺, Sr²⁺, Mn²⁺) were dissolved in 0.5 M HNO₃. The nitrate solution was added dropwise in 10 wt% NH₄HCO₃ solution, both solutions being heated to 65 °C. To achieve coprecipitation of all present metal cations, the pH value was maintained at 8 by the addition of aqueous ammonia. The obtained precipitate was filtered and dried to obtain the precursor powder. One part of the prepared powder was calcined at 1200 °C/2 h (sample CL_Sr0.5_p), while the other part was pressed into a tablet and sintered in the same conditions (sample CL_Sr0.5_tab). Even though FTIR and PXRD analyses of the sample CL_Sr0.5_p (Figs. 1A-C) showed the formation of the pure phase, light and dark spots were observed during the SEM analysis of the tablet (CL_Sr0.5_tab) fracture surface (Fig. 2). These spots were analysed by energy-dispersive spectroscopy (EDS). EDS analysis (Table 1) showed that the light spots were SrMnO₃ phase, while the dark spots were La_0.76Sr_0.24MnO₃ phase. The tablet was then ground and FTIR and PXRD analyses were conducted. The obtained FTIR results (Figs. 1A and 1B) showed some bands characteristic for the SrMnO₃ phase, and low-intensity maxima originating from the SrMnO₃ phase were observed on the diffractogram (Fig. 1C) of CL_Sr0.5_tab sample, confirming the existence of separated phase. This was additionally investigated by the Rietveld refinement analysis (Table 1), and it was determined that Sr concentration was 0.24 instead of the targeted 0.5. Therefore, the La_0.76Sr_0.24MnO₃ phase was formed, while the rest of added strontium during the synthesis crystallised in the SrMnO₃ phase. Since successful preparation of La_0.5Sr_0.5MnO₃ by hydrothermal synthesis was reported by multiple authors, it can be concluded that the coprecipitation procedure is not suitable for high concentration doping of LaMnO₃ due to the separation of Sr into the secondary phase, especially when in tablet form.

This work is licensed under a Creative Commons Attribution 4.0 International License

Keywords

doped lanthanum manganite, coprecipitation, phase separation