Understanding internal mechanisms to obtain nanomanganites by hydrothermal synthesis: The particular case of 4H-SrMnO3

We have determined why hydrothermal synthesis is not an adequate method to obtain nanoparticles in the Sr-Mn-O system. At high KOH concentration, which is more appropriate for obtaining nanoparticles, Sr is partially incorporated inside an intermediate (K,Sr)xMnO2·nH2O layered phase that prevents the formation of 4H-nanoparticles. Numerous experiments have been performed, screening the parameters involved in such processes (precursors, pH, temperature, etc.) in order to understand their influence on the shape, size, and purity of the 4H-final product. On analyzing the experimental results obtained by XRD, TEM, TGA, and EDS, among others, some peculiarities of the synthetic procedure were detected, including the appearance of side phases, SrMn3O6–x nanoribbons or the above-mentioned birnessite-type (K,Sr)xMnO2·nH2O phase, and the insertion of K+ within the 4H-phase prepared at high KOH concentration. Magnetic features are also discussed