High-pressure Mg-Sc-H phase diagram and its superconductivity from first-principles calculations

Abstract: In this work, global search for crystal structures of ternary Mg-Sc-H hydrides (Mg$x$Sc$_y$H$_z$) under high pressure ($100 \le P \le 200$ GPa) were performed using the evolutionary algorithm and first-principles calculations. Based on them, we computed the thermodynamic convex hull and pressure-dependent phase diagram of Mg$_x$Sc$_y$H$_z$ for $z/(x+y) < 4$. We have identified the stable crystal structures of four thermodynamically stable compounds with the higher hydrogen content, i.e., $R\bar{3}m$-MgScH${6}$, $C2/m$-Mg${2}$ScH${10}$, $Immm$-MgSc${2}$H${9}$ and $Pm\bar{3}m$-Mg(ScH${4}$)${3}$. Their superconducting transition temperatures were computationally predicted by the McMillan-Allen-Dynes formula combined with first-principles phonon calculations. They were found to exhibit superconductivity; among them, $R\bar{3}m$-MgScH${6}$ was predicted to have the highest $T{c}$ (i.e. 23.34 K) at 200 GPa.