With the motivation of searching for new superconductors in the Mg–B system, we performed ab initio evolutionary searches for all the stable compounds in this binary system in the pressure range of 0–200 GPa. We found previously unknown, yet thermodynamically stable, compositions MgB 3 and Mg 3 B 10 . Experimentally known MgB 2 is stable in the entire pressure range 0–200 GPa, while MgB 7 and MgB 12 are stable at pressures below 90 GPa and 35 GPa, respectively. We predict a reentrant behavior for MgB 4 , which becomes unstable against decomposition into MgB 2 and MgB 7 at 4 GPa and then becomes stable above 61 GPa. We find ubiquity of phases with boron sandwich structures analogous to the AlB 2 -type structure. However, with the exception of MgB 2 , all other magnesium borides have low electron–phonon coupling constants λ of 0.32–0.39 and are predicted to have T c below 3 K.
A First-Principles Exploration of NaxSy Binary Phases at 1 atm and Under Pressure
Interest in Na-S compounds stems from their use in battery materials at 1 atm, as well as the potential for superconductivity under pressure. Evolutionary structure searches coupled with Density Functional Theory calculations were employed to predict stable and low-lying metastable phases of sodium poor and sodium rich sulfides at 1 atm and within 100–200 GPa. At ambient pressures, four new stable or metastable phases with unbranched sulfur motifs were predicted: Na2S3 with C 2 / c and Imm2 symmetry, C 2 -Na2S5 and C 2 -Na2S8. Van der Waals interactions were shown to affect the energy ordering of various polymorphs. At high pressure, several novel phases that contained a wide variety of zero-, one-, and two-dimensional sulfur motifs were predicted, and their electronic structures and bonding were analyzed. At 200 GPa, P 4 / m m m -Na2S8 was predicted to become superconducting below 15.5 K, which is close to results previously obtained for the β -Po phase of elemental sulfur. The structures of the most stable M3S and M4S, M = Na, phases differed from those previously reported for compounds with M = H, Li, K.
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