Here we present an ab initio investigation of the pressure-induced behavior of MgGeO3- and NaMgF3-perovskite (PV), traditional low-pressure analogs (LPAs) of MgSiO3-PV. The latter is an exceedingly important system in planetary sciences displaying new phases and phase reactions under pressure that are impractically high and still challenging to experiments. Specifically, we investigate the possibility of MgGeO3 and NaMgF3 to display at lower pressures the novel phases, I-42d-type A2BX4 [1] and P21/c-type AB2X5 [2] and dissociation/recombination transitions [3] displayed by MgSiO3 above ~500 GPa, alone or in the presence of its binary compounds, MgO and SiO2. We find that, although neither MgGeO3 nor NaMgF3 is perfect LPAs of MgSiO3, they are useful for several reasons: i) both display the post-PV transition observed in MgSiO3-PV; ii) starting at ~20 GPa the Na-Mg-F system produces a novel P21/c-type NaMg2F5 phase by dissociation of NaMgF3 or by its compression with MgF2. Instead, the Mg-Ge-O system produces an I-42d-type Mg2GeO4 by dissociation of MgGeO3 or by its compression with MgO starting at ~200 GPa. Such pressures are routinely accessible in laser-heated diamond-anvil cells today; iii) like MgSiO3, both systems ultimately dissociate into the binary AX and BX2 compounds, confirming this trend in ternary systems first predicted in MgSiO3. We also predict potential metastable phase transitions into a Gd2S3-type structure in MgGeO3 and into a U2S3-type structure in NaMgF3. Metastable polymorphic transitions may occur more easily than dissociation/recombination reactions under insufficiently heated compression.

Ab initio exploration of post-PPV transitions in low-pressure analogs of MgSiO3

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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