We report on the growth of single-crystal potassium birnessite (K0.31MnO2·0.41H2O) 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/B2O3flux at 700 °C. Single-crystal X-ray diffraction revealed an average orthorhombic symmetry, with space groupCmcm. 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. MnO6octahedra and the K/H2O planes were clearly imagedviaintegrated 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 thec-axis. These charge-ordered domains are clearly correlated with a reduction in thec-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.
Kulish, L. (Contributor), Nukala, P. (Contributor), Scholtens, R. (Contributor), Uiterwijk, M. (Contributor), Hamming-Green, R. (Contributor) & Blake, G. (Contributor), University of Groningen, 2-Nov-2020