  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/B2O3 flux at 700 {\deg}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.
