High-Pressure Mg-Sc-H Phase Diagram and Its Superconductivity from First-Principles Calculations

In this work, a global search for crystal structures of ternary MgScH hydrides (MgxScyHz) under high pressure (100 <= P <= 200 GPa) was performed using the evolutionary algorithm and first-principles calculations. On their basis, we computed the thermodynamic convex hull and pressure-dependent phase diagram of MgxScyHz for a wide range of compositions (x + y = 2, 3, 4 and z = 212, 14, 16, 18). Our crystal structure search and convex hull analysis revealed no thermodynamically stable compounds in the hydrogen-rich range (z/(x + y) >= 4). On the other hand, we identified the crystal structures of four thermodynamically stable compounds in the hydrogen-middle range (3 <= z/(x + y) < 4), that is, R<(3)over bar>m-MgScH6, C2/m-Mg2ScH10, Immm-MgSc2H9, and Pm (3) over barm-Mg(ScH4)(3). Their superconducting transition temperatures were computationally predicted by the McMillan formula combined with first-principles phonon calculations. They were found to exhibit superconductivity; among them, R (3) over barm-MgScH6 was predicted to have the highest Tc (i.e., 41 K) at 100 GPa.