Stability and Superconductivity of Ternary Polyhydrides

We review five years of experimental and theoretical attempts (2020-2025) to enhance the superconducting critical temperature (\textit{T_c}) of hydrogen-rich compounds by alloying binary superhydrides with additional elements. Despite predictions of higher \textit{T_c} in ternary systems such as La-Y-H, La-Ce-H, and Ca-Mg-H, experiments consistently show that the maximum \textit{T_c} in disordered ternary superhydrides does not exceed that of the best binary parent hydrides within experimental uncertainty. Instead, alloying primarily stabilizes high-symmetry polyhydride phases at lower pressures, enabling \textit{T_c} = 200 K near 110-120 GPa, while also improving vortex pinning and upper critical fields. Magnetic dopants suppress \textit{T_c}, whereas nonmagnetic additives leave it nearly unchanged, reminiscent of Anderson's theorem. These findings indicate that alloying is unlikely to raise \textit{T_c}, but can reduce the pressures required to stabilize high-\textit{T_c} phases. We propose that fully ordered ternary hydrides, synthesized via controlled hydrogenation of intermetallic precursors, offer a promising route toward this goal. One of the most promising compounds of this kind is the recently discovered LaSc$_2$H$_{24}$.