A study of the manganites La0.5M0.5MnO3(M = Ca, Sr, Ba) prepared by hydrothermal synthesis

We present results of the first systematic study of the hydrothermal synthesis of the manganites La0.5M0.5MnO3 (M = Ca, Sr, Ba), and a detailed characterisation of the products using a number of experimental methods. All three materials can be produced as phase-pure, polycrystalline powders in one step from solutions of metal salts in aqueous potassium hydroxide solution at temperatures of 240 °C for M = Sr or Ba and 270 °C for M = Ca. Iodometric titration and Mn K-edge X-ray absorption near edge spectroscopy (XANES) combined with elemental analysis for metals confirms the average manganese oxidation state as 3.5 in each of the materials. Scanning electron microscopy shows that the materials are made up of cube-shaped particles 1 µm in dimension for La0.5Ba0.5MnO3, 10 µm on average for La0.5Sr0.5MnO3 and 20 µm for La0.5Ca0.5MnO3. La0.5Ba0.5MnO3 is shown to adopt an A-site ordered perovskite structure (tetragonal, P4/mmm, a = 3.9160(1) A, c = 7.8054(2) A); this is determined using powder neutron diffraction, thermogravimetric analysis and magnetisation measurements. High-resolution powder neutron diffraction data from La0.5Sr0.5MnO3 show that the material adopts a distorted perovskite structure (tetragonal, I4/mcm, a = 5.44778(40) A, c = 7.7353(10) A) similar to that previously reported for materials of the same composition prepared by a solid-state route. Hydrothermal La0.5Ca0.5MnO3 adopts an orthorhombic perovskite structure (Pnma, a = 5.410(1) A, b = 7.604(1) A, c = 5.443(2) A). Finally, we examine poorly ordered precursors of La0.5Ba0.5MnO3, formed prior to the crystallisation of the perovskite, using Mn K-edge X-ray absorption fine structure spectroscopy. This shows that at the earliest stage of the reaction the manganese is found in an oxidation state of 3.5 in edge-shared octahedral sites, consistent with the formation of a K0.5MnO2·nH2O, birnessite-like phase before the formation of the perovskite.