Structural modulation in potassium birnessite single crystals

Abstract

We report on the growth of single-crystal potassium birnessite (K_0.31MnO₂·0.41H₂O) and present both the average and local structural characterization of this frustrated magnetic system. Single crystals were obtained employing a flux growth method with a KNO₃/B₂O₃ flux at 700 °C. Single-crystal X-ray diffraction revealed an average orthorhombic symmetry, with space group Cmcm. A combination of high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with atomic resolution energy dispersive X-ray spectroscopy (EDS) demonstrated the layered structure of potassium birnessite with manganese-containing planes well separated by layers of potassium atoms. MnO₆ octahedra and the K/H₂O planes were clearly imaged via integrated differential phase contrast (iDPC) STEM. Furthermore, iDPC-STEM also revealed the existence of local domains with alternating contrast of the manganese oxide planes, most likely originating from charge ordering of Mn³⁺ and Mn⁴⁺ along the c-axis. These charge-ordered domains are clearly correlated with a reduction in the c-lattice parameter compared to the rest of the matrix. The insight gained from this work allows for a better understanding of the correlation between structure and magnetic properties.