Calcium and magnesium carbonates are believed to be the host compounds for most of the oxidized carbon in the Earth's mantle. Here, using evolutionary crystal structure prediction method USPEX, we systematically explore the MgO-CO 2 and CaO-CO 2 systems at pressures ranging from 0 to 160 GPa to search for thermodynamically stable magnesium and calcium carbonates. While MgCO 3 is the only stable magnesium carbonate, three calcium carbonates are stable under pressure: well-known CaCO 3 , and newly predicted Ca 3 CO 5 and CaC 2 O 5 . Ca 3 CO 5 polymorphs are found to contain isolated orthocarbonate CO 4 4tetrahedra, and are stable at relatively low pressures (>11 GPa), whereas CaC 2 O 5 is stable above 33 GPa and its polymorphs feature polymeric motifs made of CO 4 -tetrahedra. Detailed analysis of chemical stability of CaCO 3 , Ca 3 CO 5 and CaC 2 O 5 in the environment typical of the Earth's lower mantle reveals that none of these compounds can exist in the Earth's lower mantle. We conclude that MgCO 3 is the main host of oxidized carbon throughout the lower mantle.
We predict new tungsten borides, some of which are promising hard materials that are expected to be stable in a wide range of conditions, according to the computed composition-temperature phase diagram. New boron-rich compound WB is predicted to be superhard, with a Vickers hardness of 45 GPa, to possess high fracture toughness of ∼4 MPa·m, and to be thermodynamically stable in a wide range of temperatures at ambient pressure. Temperature dependences of the mechanical properties of the boron-richest WB and WB phases were studied using quasiharmonic and anharmonic approximations. Our results suggest that WB remains a high-performance material even at very high temperatures.
The standard paradigm in computational materials science is INPUT: Structure; OUTPUT: Properties, which has yielded many successes but is ill-suited for exploring large areas of chemical and configurational hyperspace.
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