Ultra-high-pressure behavior of Mg₂GeO₄: Analogue for phases of deep exoplanet interiors

Silicon coordination changes in minerals influence their physical properties such as density, viscosity, and elemental affinities. Coordination changes in mantle minerals with pressure are expected to influence the structure and dynamics of planetary interiors. In the Earth, 6-coordinated silicates are believed to be stable throughout the lower mantle. However, there is no experimental evidence for silicon coordination greater than 6 in any high-pressure crystalline silicate.
Germanates are known to be effective analogues for silicates as they undergo similar sequences of phase transitions but at lower pressures. A recent theoretical study on MgSiO₃ at exoplanetary interior pressures suggest a 3-stage breakdown starting with a partial dissociation of MgSiO₃ post-perovskite into Mg₂SiO₄(I4-2d-type)+ MgSi₂O₅ at 750 GPa. For the Mg₂GeO₄ system, these calculations predict that MgGeO₃ and MgO will combine to form an eight-coordinated phase of Mg₂GeO₄ (I-42d-type) at 175 GPa. While, the pressure range for the silicate is experimentally inaccessible, the germanate can be studied with the laser-heated diamond anvil cell. In this work, we show that Mg₂GeO₄ transforms to a disordered 8-coordianted cubic phase on heating at 160 GPa.