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

Abstract

New amorphous solutions MgySiO2+y-XH2 with the magnesium concentration varying from y = 0 to y = 0.88 were synthesized at a hydrogen pressure of 75 kbar and a temperature of 250 °C, followed by quenching to the temperature of liquid nitrogen. The quenched samples were studied by thermal desorption, X-ray diffraction and Raman spectroscopy. The X-ray diffraction study showed that all hydrogenated samples preserved the amorphous state and had no crystalline inclusions. At the same time, the positions of the first sharp diffraction peak (FSDP) of the samples with y = 0.32 and 0.6 were shifted by ΔQ = 0.14 Å−1 after the hydrogenation thus signaling on changes in the amorphous network and even its small depolymerization.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.