Stability and Superconductivity of Ternary Polyhydrides

Abstract

We review five years of experimental and theoretical attempts (2020–2025) to enhance the superconducting critical temperature (Tc) of hydrogen-rich compounds by alloying binary superhydrides with additional elements. Despite predictions of higher Tc in ternary systems such as La–Y–H, La–Ce–H, and Ca–Mg–H, experiments consistently show that the maximum Tc 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 Tc ≈ 200 K near 100–110 GPa, while also strengthening vortex pinning and upper critical fields. Magnetic dopants suppress Tc, whereas nonmagnetic additives leave it nearly unchanged, reminiscent of Anderson's theorem. These findings indicate that alloying is unlikely to raise Tc, but can reduce the pressures required to stabilize high-Tc 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 LaSc2H24.