(a) Raman spectra for OH-stretching vibration of H₂O for MgCl₂ solutions, MgCl₂ · nH₂O, with n = 300, 200–80 in steps of 20, and 50–10 in steps of 2 (from bottom curve to top curve). (b) OH-stretching vibration of HDO in solution of MgCl₂ dissolved in D₂O mixed with dilute H₂O with a molar ratio of 4:1. Molar ratios of water to solute n are 200 (bottom curve), 150, 60, 50, 40, 30, 26, 16, 14, 12 (top curve). (c) Peak position for OH-stretching vibration as a function of n plotted on the basis of data from panels (a) (solid circle) and (b) (solid diamond). The data of MgCl₂ · nH₂O reported by Burikov et al.^[25] are also shown for comparison (empty square). Moreover, the peak position of the component around 3400 cm⁻¹ obtained by Gaussian fitting of the spectra in panel (a) is also presented (solid square). (d) Water content dependence of glass transition temperatures, T_g, of aqueous solution of MgCl₂. In dilute solutions with n > n c , glass transition occurs after spontaneous icing in cooling process, thus it involves only the glass transition of freeze-concentrated solution, which manifests an almost constant T_g, defined as T g ′ . The extension of T g ′ –n can intersect the fitting curve of T_g–n for concentrated solutions at a feature point, i.e., MgCl₂ · n_hH₂O, where n_h is the hydration number of solute. For more details, see Supplementary Information in Ref. [13].