Raman spectroscopy study of K-birnessite single crystals

Raman studies of manganese dioxide (MnO₂), a crucial material for sustainable and innovative solutions in energy storage and environmental remediation, have predominantly been conducted on fine-grained aggregates, leaving the identification of Raman peaks open to debate. To address this, in this study the Raman spectra of potassium (K)-birnessite single crystals with varying crystal thickness, temperature, and polarization configuration are measured. An acoustic phonon mode of birnessite is identified, which is found to be sensitive to both crystal thickness and interlayer spacing, with its frequency increasing by approximately 35% when the c-axis lattice parameter is reduced from 0.70 to 0.65 nm by the removal of interlayer water. In contrast, the dependence of the optical phonon modes on crystal thickness and interlayer spacing is not particularly noticeable. It is demonstrated that the characteristic Raman peak of K-birnessite, observed at approximately 559 cm⁻¹, originates from a two-dimensional hexagonal configuration of cations and water molecules within the interlayer space, rather than from the MnO₆ octahedra. Additionally, the doubly degenerate vibrational mode of MnO₆ octahedra, corresponding to the motion of oxygen atoms in the basal plane, splits into two, confirming that the MnO₆ octahedra are distorted by the Jahn-Teller effect.