The effect of Non-Stochiometry on the Stability and Elasticity of Phase D

The distribution of volatile elements such as hydrogen can have a disproportionately large effect on many geophysically relevant parameters such as density, elastic properties, as well as transport properties. Thus, it is important to know the distribution and abundance of volatiles in the interior of the Earth and how they relate to the dynamics and evolution of our planet. Phase D, the most dense hydrous magnesium silicate (DHMS), is stable in colder regions of the uppermost lower mantle regions such as subduction zones. As Phase D decomposes, hydrogen/water is released and may ascent toward and into the transition zone where it can be incorporated into nominally anhydrous minerals. Previous theory has focused on stochiometric samples while experiments commonly observe the appearance silicon vacancies. In order to close this gap between experiment and theory we use Density-Functional-Theory to investigate the effects of hydrogen on the crystal chemistry in non-stochiometric PhaseD (MgSi1.75O6H3). Our preliminary results suggest that vacancy formation lowers the bulk-modulus by ~9% and increases the zero pressure volume by ~2.5%, consistent with available experimental work. Our preliminary results show that the decomposition of Phase D in the presence of vacancies occurs at P~35 GPa, that is a somewhat lower pressure than the stochiometric equivalent and that not all hydrogen bonds symmetrize before decomposition. This result indicates that previous studues may have overestimated the effect of hydrogen bond symmetrization and induced elasticity changes.