Ever since Heike Kamerlingh Onnes made the astonishing discovery1 that the electrical resistance of mercury disappeared as temperature dropped to a few Kelvins via liquid-helium cooling, this exotic quantum phenomenon, known as superconductivity, has attracted tremendous interest in fundamental scientific inquiry and spurred intense efforts for applications ranging from detection of faint magnetic signals to generation of intensive magnetic fields. The long-sought overarching goal is to find materials with sufficiently high superconducting critical temperature (Tc) to facilitate experimental exploration and practical implementation, ultimately allowing ambient-environment applications. Over the years, the search for room-temperature superconductors has been among the most luring and challenging topics in condensed matter physics; notable progress includes materials with Tc in liquid-hydrogen and liquid-nitrogen temperature ranges achieved at ambient pressure2-3, making it more easily manageable to utilize superconductors in advanced devices. More recently, superconductivity with Tc approaching or even exceeding the room temperature has been predicted in a class of hydrogen-rich compounds, termed superhydrides, under extreme compression. Theoretical prediction followed by experimental synthesis led to the discovery of superconductors with Tc of 203 K in covalent SH34-6 and 250-260 K in ionic LaH107-10, obtained at very high pressures between 150-200 GPa. A Tc of 288 K was reportedly achieved at extremely high pressures around 270 GPa in a C-S-H system with unknown composition and structure11, but it has caused intense debate12-14 and still awaits confirmation.
Meanwhile, inspired by the prediction of high-temperature superconductive CaH615, extensive ensuing studies have established a large family of clathrate superhydrides, including CaH616,17, YH618, YH919,20, CeH9, CeH1021-23, and LaH107-10, which comprise an array of distinct hydrogen cages anchored at the center by a variety of metal atoms. The diverse structural and compositional forms of these binary compounds offer a platform for exploring material parameters to optimize the energetic and superconducting states. Systematic studies indicate that atomic radius, electronegativity, and valence electron of metal atom play important roles in tuning the most concerned properties of superhydrides, i.e., superconductivity and stability24. Among these ionic superhydrides, LaH10 faces an inescapable problem of its harsh synthesis pressure (>150 GPa), despite its very high Tc9,10; whereas CeH9 or CeH10 with relatively low Tc of 95-115 K can be synthesized around megabar pressure20. Improving superconducting properties of these superhydrides with moderate synthesis and stability pressures presents pressing challenges in this active research field.
Binary superhydrides are highly restricted in their configurational space with limited structural and compositional variations. To expand the material horizon, related research recently shifted focus to ternary systems with much higher degrees of freedom, which offer a larger number and richer variety of structural prototypes for superconducting superhydride screening25. The potential of this approach is demonstrated in a theoretical work26 showing that Li doping introduces an extra electron into the molecular-like hydrogen in MgH16 to generate atomic-like hydrogen, and the formed ternary Li2MgH16 has a Tc of 473 K at 250 GPa. In addition, theoretical studies have also designed a series of ternary superhydrides25,27, such as XYH828-30, Ca-Y-H31, and Ca-Mg-H32 that are expected to possess desirable high-Tc features or prospects. A sticking point, however, is to find ternary superhydrides that host high-temperature superconductivity at moderate, near or below megabar, pressures.
Rare-earth (RE) metals have similar electronegativities, electronic configurations, and atomic radii, so their disordered solid solution alloys are easy to form33,34. This offers a viable avenue to use suitably selected binary compounds as the base template to construct ternary alloy superhydrides that share the same crystal structure. It has been shown that LaH6 and YH10 units, which are experimentally unreachable in the binary systems, do appear in La-Y alloy hydrides at pressures of 170–196 GPa35, although superconductivity is not improved. It is of great interest to explore the cubic La-H9,10,36 and hexagonal close packed (hcp) Ce-H21-23 systems to find a desirable ternary platform for experimental realization of high-temperature superconductivity in materials that can be stabilized at relatively low pressures.
In this work, we experimentally investigated the crystal structure, superconductivity, and stability pressure range of a La-Ce alloy superhydride. Substitutional (La,Ce)H9 with essentially equal metal-atom occupancy was obtained using the starting materials of equiatomic La-Ce alloy and ammonia borane (NH3BH3) under the synthesis conditions of about 110 GPa and 2,100 K. The synthesized ternary alloy was maintained to at least 90 GPa during decompression. Compared with binary CeH9, the Tc is dramatically increased by up to 80 K in ternary (La,Ce)H9, which contains an unprecedented LaH9 unit that is unstable in pure compound form but stabilized in the base binary CeH9 structural framework. This atypical compositional structure unit has a major impact on the giant enhancement of Tc of the ternary (La,Ce)H9 alloy. The present findings demonstrate that substitutional alloying is effective in tuning and improving high-temperature superconductivity in superhydrides by stabilizing new and unusual structural and compositional configurations that are conducive to greatly enhanced Tc, and this approach may be extended to constructing additional ternary and higher multinary alloys to usher in more breakthrough discoveries in the quest of finding optimal clathrate superhydrides through a thoughtful choice of substitutional alloying metal-element combinations.