Structural modulation in potassium birnessite single crystals

TL;DR

This study reports the growth and detailed structural analysis of potassium birnessite single crystals, revealing local charge ordering and structural features that influence its magnetic properties.

Contribution

It provides the first detailed structural characterization of single-crystal potassium birnessite, including local charge ordering and layered structure insights.

Findings

Layered structure with potassium and manganese oxide planes confirmed.

Local charge ordering of Mn3+ and Mn4+ observed.

Charge ordering correlates with lattice parameter reduction.

Abstract

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.