Hydrothermal synthesis of the perovskite manganites Pr0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 and alkali-earth manganese oxides CaMn2O4, 4H-SrMnO3, and 2H-BaMnO3

We report the synthesis of the perovskite manganites Pr0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 using mild hydrothermal conditions. Both are formed as polycrystalline powders from solutions of metal salts in aqueous potassium hydroxide at 240 degrees C, and crystallise as a tetragonal polymorph (space group I4/mcm). Scanning electron microscopy shows both materials to contain cuboid-shaped crystallites several microns in dimension, and the average particle size is verified by light scattering measurements. We also report the first hydrothermal synthesis of 2H-BaMnO3 and 4H-SrMnO3, and the first subcritical hydrothermal synthesis of CaMn2O4 (marokite). Despite the formation of these alkali-earth manganese oxides at 240 degrees C, we have been unable to isolate rare-earth manganese oxides LnMnO(3) using similar conditions. We discuss the formation of perovskite manganites in hydrothermal reactions by relating our new results to those manganites already reported to form under hydrothermal conditions, and rationalise the trends seen by considering tolerance factor of the perovskite and the variance of the A-site metal radius. 2005 Elsevier Inc. All rights reserved.