Controllable adjustment of the crystal symmetry of K-MnO₂ and its influence on the frequency of microwave absorption

Microwave (MW) is becoming an important polluter. Because of its wide frequency range in electromagnetic radiation, finding or developing materials with high efficiencies to absorb and attenuate MW is one of the most urgent research areas in material science. In developing this type of material, the ability to efficiently absorb MW in a wide frequency range is critical. In this study, Fe(iii) was doped in nanosized cryptomelane (K-MnO₂) to achieve MW absorption in an adjustable frequency range. Rietveld refinement of powder X-ray diffraction patterns and X-ray photoelectron spectroscopic results confirmed the presence of Fe(iii) as partial substitute for Mn(iii) and Mn(ii) in the [MnO₆] octahedra of K-MnO₂. Electron microscopic observations showed a progressive change of morphology from nanofibrous into micrometer-sized prismatic crystals as Fe(iii) doping increased. The lattice images of high-resolution transmission electron microscopy (HRTEM) showed a d-spacing of 0.71 nm for (110) of tetragonal Fe-K-MnO₂, while a d-spacing of 0.56 nm corresponded to (200) of monoclinic Fe-K-MnO₂. The frequency band of MW attenuation expanded and displayed a blue shift as the Fe(iii) doping increased. Such a tunable property suggests that K-MnO₂ and possibly other manganese dioxides could be customized to reduce potential hazards from a wider range of MW sources.