Water-rich amorphous state from drying mixed-metal sulfate solutions

Amorphous and glassy materials are important for many advanced applications, from flexible solar cells to drug delivery systems. To this end, new glasses are in high demand, but precise chemical design of amorphous materials remains challenging. By studying the crystallization of mixed salt solutions, we have discovered an entirely new type of amorphous material: water-rich amorphous mixed sulfates. Specifically, we show that drying of sulfate salt mixtures of both mono- and higher valency cations almost exclusively yields a glassy or amorphous state, where the stability of the amorphous state depends on the cations present and ranges from seconds to months. Furthermore, we show that the glassy state is viscoelastic, behaves like a soft solid (G' 10^5 - 10^6 Pa), retains a large amount of water (30 to 40 weight percent), and is X-ray amorphous. Additionally, confocal Raman microspectroscopy reveals disordered sulfate orientations and Fourier-transform infrared spectroscopy highlights increased hydrogen bonding during drying, which together with strong cation hydration is hypothesized to prevent crystallization. These results provide insights for the production of a new class of amorphous materials, and help to elucidate the mystery of the high abundance of such amorphous salts found on Mars.