Mg-carbonate is one of the main carbonates subducting from the Earth’s surface into the mantle and possible reactions of this compound with the main mantle minerals determine the stability of carbonates into the Earth’s mantle. Recently, we have theoretically shown that at pressures and temperatures of the Earth’s lower mantle MgCO3 should react with MgO, producing Mg2CO4. Here, using diamond anvil cell technique, we have performed in situ investigation of the product of MgCO3 + MgO reaction at pressures around 50 GPa and temperatures above 2000 K. Investigation of Raman and X-ray diffraction patterns unambiguously indicates that reaction has been realized and as the reaction product we suggest the modification similar to M-g2CO4-$$P{{2}_{1}}{\text{/}}c$$. Using the transmission electron microscopy technique, we show the presence of the new Mg-orthocarbonate/carbonate phase in the sample extracted from high-pressure environment.

Recent research on superconductivity of high‐pressure hydrides generated many phase stability calculations with a lack of their experimental verification; a typical example is Pt–H system. The stability of eight PtH structures was predicted, while the experiments revealed the existence of only hexagonal close‐packed (hcp) and trigonal PtH. Face‐centered cubic (fcc) PtH was predicted to be nearly isoentalpic to the hcp PtH and stable near 100 GPa, but never observed experimentally. Here we report the first synthesis of the fcc PtH using laser‐heated diamond anvil cell. It was found to occupy a high‐temperature area of the phase diagram in a wide pressure range of 20–100 GPa, being metastable at room temperature. Our results look promising for uncovering weak approximations in current high‐pressure hydrides stability ab initio calculations.

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