Stability and Superconductivity of Ternary Polyhydrides

ABSTRACT We review five years of experimental and theoretical attempts (2020–2025) to enhance the superconducting critical temperature ( T c ) of hydrogen‐rich compounds by alloying binary superhydrides with additional elements. Despite predictions of higher T c in ternary systems such as La–Y–H, La–Ce–H, and Ca–Mg–H, experiments consistently show that the maximum 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 T c ≈ 200 K near 100–110 GPa, while also strengthening vortex pinning and upper critical fields. Magnetic dopants suppress T c , whereas nonmagnetic additives leave it nearly unchanged, reminiscent of Anderson's theorem. These findings indicate that alloying is unlikely to raise T c , but can reduce the pressures required to stabilize high‐ 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 .