## 3 Results and Discussion  
### Materials Characterization  
The XRD patterns of all samples are showed in Figure 1. Basal reflections at 12.5 and 25.1\({}^{\circ}\), characteristic of the birnessite layered structure are observed for all samples. The reflections at higher 20-values exhibit some differences, indicative of structural dissimilarity between the samples. Thus, the wide maxima at 20-36.8\({}^{\circ}\) (2.44 A) and 20-66.3\({}^{\circ}\) (1.41 A) for KMn4 indicate a birnessite with turbostractiven. In contrast, all reflections of the KMn8 could indexed to a hexagonal unit cell with a = 2.864(3) and c = 14.274(2) A. On the other hand, HKMn maxima are coherent with a rhombohedral symmetry and cell parameters a = 2.846(9), 21.751(6) A. Table 1 gathers relevant crystallographic data obtained from Rietveld refinements of XRD data for KMn8 and HKMn. The corresponding profiles are shown in Figure 2. The structural models for both samples are also included (insets), evidencing the differences between them. Whereas the unit cell of KMn8 can be described as two-layer hexagonal (2H), the HKMn unit cell is three-layer hexagonal (3H). Thus, the close-packing notation for the 2H model is: AbC (water)-CbA (water)-AbC(water), and for the 3H model is: AbC(water)-CaB(water)-BcA(water), where A, B, C represent the oxygen positions and a, b, c refer to Mn positions [10,11]. The potassium ions and water molecules share the interlayer space as shown in Figure 2.  
Figure 1: XRD patterns of (a) KMn4, (b) KMn8 and (c) HKMn. Vertical bars indicate Bragg permitted reflections. Insets show corresponding structural models: layers of MnO\({}_{6}\) octahedra are represented in pink; water molecules and potassium cations are situated between layers. EDX analysis indicated the presence of potassium in all the samples but in different concentration. Thus, examination of a representative number of crystals in each case led to mean different formulae. For KMn4 and KMn8, 0.3 K per formula was obtained. In the case of the HKMn sample, mean composition K\({}_{0.03}\)MnO\({}_{2}\) was obtained, though in some crystals the obtained ratio was 0.3:1. Therefore, the samples prepared by different methods seem to retain different amounts of K in the interlayer space. In addition, the thermogravimetric analysis in the 50-600 \({}^{\circ}\)C temperature range (Figure S1) displays three regions where decomposition takes place. The initial weight loss up to 120 \({}^{\circ}\)C can be associated with evaporation of surface-adsorbed water, whereas \begin{table}
\begin{tabular}{c c c} \hline \hline
**Phase** & **KMn8** & **HKMn** \\ \hline Space group & \(P\)\(6_{3}/mmc\) & \(R\) -3\(m\) \\ a (Å) & 2.864 (5) & 2.846 (9) \\ c (Å) & 14.274 (2) & 21.751 (6) \\ R\({}_{\rm P}\) & 19.3 & 15.3 \\ R\({}_{\rm WP}\) & 18.2 & 16.1 \\ R\({}_{\rm B}\) & 9.1 & 4.7 \\ \hline \end{tabular}
\end{table} 
Table 1: Crystallographic data of KMn8 and HKMn. Figure 2: Calculated (line), experimental (dot) and difference (bottom line) Rietveld profiles for (**a**) KMn8 and (**b**) HKMn. the second region, up to 200 \({}^{\circ}\)C, can be attributed to the dehydration of the interlayer water. Finally, the weight loss obtained when the material is heated above 250 \({}^{\circ}\)C, is usually associated with oxygen loss [12]. Furthermore, when the material is heated up to 600 \({}^{\circ}\)C, the HKMn is transformed into a 2 \(\times\) 2 channel structure, as previously suggested [13]. Such changes are shown in the XRD diagram of the residue (Figure S1b,c). However, the XRD profiles of the KMn4 and KMn8 residues indicated a mixture of phases in which birmissite and a structure with 2 \(\times\) 2 channel are present. Therefore, the amount of K between octahedra layers seems to be key in the thermal stability of birmissite structure. Therefore, in principle, hydrothermal method led to phases where the K amount is smaller than the obtained by thermal treatment. In order to know the Mn oxidation state, XPS experiments were carried out.