## 2 Materials and Methods  
The following chemicals were used for the experiment: MgBr\({}_{2}\)-6H\({}_{2}\)O (\(>\)99%, PENTA, Prague, Czech Republic) and MgO (\(>\)98%, PENTA, Prague, Czech Republic). Deionized water (16.8 M\(\Omega\)) was used for all syntheses. Both precursors were analyzed using X-ray fluorescence, confirming their high purity. The weight percentages of the elements (for MgO given on the basis of oxides) can be seen in Table 1.  
The ratio 5 MgO to MgBr\({}_{2}\)-6H\({}_{2}\)O was used for synthesis of the stoichiometric phase of MOB with composition 5 Mg(OH)\({}_{2}\)-MgBr\({}_{2}\)-8H\({}_{2}\)O. To prepare the sample, 9.4 g of MgBr\({}_{2}\)-6H\({}_{2}\)O was dissolved in 4.1 g of deionized water in a plastic beaker. Next, 6.5 g of magnesium oxide was added, and the suspension was intensively stirred for 5 min. The following equation (Equation (1)) summarizes the formation of Phase 5:  
\[5\text{MgO}+\text{MgBr}_{2}\text{-6H}_{2}\text{O}+3\text{H}_{2}\text{O}\to 5 \text{Mg(OH)}_{2}\text{-MgBr}_{2}\text{-8H}_{2}\text{O} \tag{1}\]  
Similarly, the stoichiometric phase 3 Mg(OH)\({}_{2}\)-MgBr\({}_{2}\)-8H\({}_{2}\)O was prepared using the ratio 3 MgO to MgCl\({}_{2}\)-6H\({}_{2}\)O to 5 H\({}_{2}\)O. We dissolved 11.6 g of MgBr\({}_{2}\)-6H\({}_{2}\)O in 3.6 g of deionized water in a plastic beaker. In the next step, 4.8 g of magnesium oxide was added, and the suspension was intensively stirred for 5 min. The formation is summarized in the following equation (Equation (2)):  
\[3\text{MgO}+\text{MgBr}_{2}\text{-6H}_{2}\text{O}+5\text{H}_{2}\text{O}\to 3 \text{Mg(OH)}_{2}\text{-MgBr}_{2}\text{-8H}_{2}\text{O} \tag{2}\] \begin{table}
\begin{tabular}{c c c} \hline \hline
**Element** & **Content in MgBr\({}_{2}\)-6H\({}_{2}\)O (wt\%)** & **Content in MgO (wt\%)** \\ \hline Mg & 11.44 & 99.19 \\ P & 0.01 & - \\ Cl & 0.10 & 0.38 \\ Br & 88.45 & 0.04 \\ Ca & - & 0.16 \\ Al & - & 0.13 \\ S & - & 0.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Table of element percentages in the precursors obtained by XRF. Figure 1: Crystal structures of magnesium oxybromide (MOB) phases MOB-318 (**left**) and MOB-518 (**right**). Mg atoms are located in the centers of the distorted octahedra formed by oxygen atoms. Br atoms are brown; O atoms are blue. Brown-blue spheres in the MOB-518 structure correspond to a mixed site occupied by bromide anions and water molecules (hydrogen atoms are not shown). The phases magnesium bromide-trihydroxide tetrahydrate (stoichiometry 3 MgO-MgBr\({}_{2}\)8H\({}_{2}\)O or Mg\({}_{2}\)(OH)\({}_{3}\)Br-4H\({}_{2}\)O) and magnesium bromide-pentahydroide tetrahydrate (stoichiometry 5 MgO-MgBr\({}_{2}\)-8H\({}_{2}\)O or Mg\({}_{3}\)(OH)\({}_{5}\)Br-4H\({}_{2}\)O) are termed MOB-318 and MOB-518, respectively. Both prepared samples are shown in Figure 2.  
X-ray fluorescence analysis (XRF) was performed using an Axios sequential WD-XRF spectrometer (PANalytical, Almelo, Netherlands) equipped with an Rh anode end-window X-ray tube fitted with a 50 \(\upmu\)m beryllium window. The resulting data were collected by SuperQ software. The analyzed material was pressed, without any binding agent, onto H\({}_{3}\)BO\({}_{3}\) pellets with a total thickness of approximately 5 mm and a diameter of 40 mm. X-ray powder diffraction (XRD) was carried out by Bruker D2 Phaser, the powder diffractometer with Bragg-Brentano geometry, applying CuK\({}_{\alpha}\) radiation (\(\lambda\) = 0.15418 nm, U = 30 kV, I = 10 mA) and sample rotation (5 rounds per minute). The step size was set to 0.02025\({}^{\circ}\) (29), and the overall data were collected from the angular range of 5-80\({}^{\circ}\). Measurements were performed after 5 weeks of curing. Scanning electron microscopy (SEM) performed on a Tescan MAIA 3 was used for the study of surface morphology. The elemental composition and mapping were characterized using an energy dispersive spectroscopy (EDS) analyzer (X-Max150) with a 20 mm\({}^{2}\) SDD detector (Oxford Instruments, High Wycombe, UK) and AZtecEnergy software.