The hidden impact of structural water – how interlayer water largely controls the Raman spectroscopic response of birnessite-type manganese oxide

Birnessite-type manganese oxides, consisting of stacked MnOx sheets, separated by charge-balancing metal ions and structural water are potential candidates for electrochemical applications. Due to their structural complexity, Raman spectroscopy is one of the most widely used techniques for studying this class of materials. However, the interpretation of the Raman spectra is still debated. In fact, the observed Raman bands are often intuitively assigned to either Mn–O or M–O vibrations, where M corresponds to charge balancing metal ions such as Na, Ca, and K-ions which are present in natural forms of birnessite. Here, we report a combined experimental and computational study that, opposite to the commonly accepted assignments, strongly suggests that many of the characteristic Raman bands can be attributed to vibrations related to the interlayer water. Our computational findings are compared with detailed ex situ and in situ Raman spectroscopy/X-ray diffraction, and cyclovoltammetry results for K–birnessite, allowing for a full explanation of potential-dependent changes in the Raman spectra. Furthermore, the excellent correlation between the intensity of a band positively assigned to water vibrations and the d-spacing, give support for the important influence of interlayer water on the Raman spectra. This study points to the crucial role of arrangement and content of structural water and deepens the current understanding of hydrated birnessites.