High-pressure solid solutions of molecular hydrogen in amorphous magnesium silicates

According to the thermal desorption analysis, the hydrogen content X of the quenched MgySiO2+y-XH2 samples nonlinearly decreased with increasing concentration y of the magnesium ions Mg2+ from X = 0.600(3) at y = 0 to X = 0.259(3) at y = 0.88. The samples with y ≥ 0.49 evolved a significant portion of the dissolved hydrogen on heating in vacuum above 0 °C therefore showing a higher thermal stability than the hydrogenated silica and silicates with low magnesium concentrations. Raman spectroscopy demonstrated that hydrogen was dissolved in all samples in the form of H2 molecules, and the width of the H2 stretching line narrowed approximately four-fold with increasing magnesium concentration. Both this effect and the changes in the H2 desorption kinetics presumably resulted from the decreasing dispersion in the size of silicate cavities in the amorphous matrix, which changes from the silica glass structure at y ≤ 0.32-0.49 to the close-packed enstatite glass structure at higher magnesium concentrations.