Ultra-high-pressure behavior of Mg₂SiO₄ and Mg₂GeO₄: A combined x-ray diffraction and density functional approach.

Changes in Silicon coordination in minerals influence their physical properties such as density, viscosity, and elemental affinities. However, there is no experimental evidence for silicon coordination greater than 6 in any high-pressure crystalline silicate. Higher coordinated phases can be important in large rocky exoplanets (mantle pressures up to ~1 TPa).

We have explored the high-pressure behavior of magnesium germanate to 275 GPa and over 2000 K using laser-heated diamond anvil cell experiments and density functional theory computations [1]. Our X-ray diffraction patterns are consistent with a disordered cubic Th₃P₄-type structure at pressures > 190 GPa, in which germanium adopts eight-fold coordination with oxygen. Simulations using the special quasirandom structure (SQS) method are consistent with partially ordered tetragonal  structure, indistinguishable from  Th₃P₄ in our experiments. Although, experiments are not feasible for the pressure, temperature ranges necessary for the phases to be stable in the silicate system, we are exploring the stability of these phases in Mg₂SiO₄ computationally.  

[1]       R. Dutta, S. J. Tracy, R. E. Cohen, F. Miozzi, K. Luo, J. Yang, P. C. Burnley, D. Smith, Y. Meng, S. Chariton, V. B. Prakapenka, and T. S. Duffy, ArXiv:2101.00347 [Cond-Mat, Physics: Physics] (2021).