High pressure investigations on hydrous magnesium silicate-phase A using first principles calculations: H---H repulsion and changes in hydrogen bond geometry with compression

We have carried out first principles structural relaxation calculations on the hydrous magnesium silicate phase A (Mg7Si2O8(OH)6) under high pressures. Our results show that phase A does not undergo any phase transition up to ∼45 GPa. We find that non-bonded H---H distance reaches a limiting value of 1.85 Å at about 45 GPa. The hydrogen bond bending mechanism for countering the H---H repulsive strain that was proposed by Hofmeister et al. for phase B is not the only one operating in phase A. It also has contributions based on the reduction of one of the O–H bond distances and opening up of the H---O---H angle with compression. The contraction of the O–H distance with pressure, perhaps the first one by density function theory calculations, will have implications for the spectroscopic measurements.