Since Kamerlingh Onnes discovered that mercury (Hg) suddenly starts carrying a current without resistance at an extremely low temperature in 1911 1,2, the achievement of room temperature superconductor is a dream for the superconductivity research. The theory that hydrogen can be metallized at high pressure was developed in 1935 and was proposed by Winger and Huntington 3. According to the theory of superconductivity proposed by Bardeen, Cooper and Schrieffer in 1957, the transition temperature of superconductivity is proportional to the Debye temperature. 4. Due to this theory, hydrogen, the lightest element, would be the prospect of room-temperature superconductor after metallization 5, but because of the difficulty of the hydrogen metallization 6,7, the theory about hydrogen pre-compression was proposed by Ashcroft that the hydrogen-rich compounds could be a great option for the high Tc superconductors 8,9. The theory of chemical pre-compression refers to the addition of other elements to the synthesized hydrogen-rich compounds at a lower pressure than synthesizing pure hydrogen 10. Based on this conclusion, many great hydrogen-rich compounds have been designed and predicted to be potential superconductors with high Tc 11–13. The first successful predictions were H3S and LaH10 with high Tc exceeding 200 K 14–16, and these predictions were successfully confirmed by experiment soon 17–20.
Over these years, with the efforts of our researchers, almost all binary hydrides were explored, people commence the study of ternary hydride formed by adding a new element into binary hydrides. In 2019, Li2MgH16 with the highest Tc to date (473 K at 250 GPa), designed by filling the anti-bonding orbital of the H2 molecular unit of MgH16 with the element Li 21. H-C-S compounds and Lu-N-H compounds have been widely studied for some time due to the claimed observation of room temperature superconductivity. However, there are still some controversial issues about the stoichiometry and the crystal structure 22–25. Recently, a new kind of fluorite-type clathrate ternary hydrides AXH8 (A = Ca, Sr, Y, La, X = B, Be, Al) in the main chain of hydrogen alloys has been predicted 26. The most prominent, LaBeH8, is dynamically stable down to 20 GPa and has a high Tc up to 185 K. The exciting thing is that the cubic clathrate superhydrides LaxY1−xH6,10 have been experimentally synthesized by laser heating of yttrium-lanthanum alloys, which exhibited a maximum critical temperature Tc of 253 K without increasing pressure 27. According this experiment, it is practicable to incorporate a metal element in the clathrate hydride to keep the compounds steadily.
It is a widespread attention about the prominent superconductivity of the clathrate hydrides. Clathrate hexahydrides Im-3m-XH6 (X = Mg, Ca, Sc, Y, La, Tm, Yb, Lu) are widespread in alkaline earth and rare earth metal superhydrides 16,28–32. In this structure, there is a body-centered cube (bcc) with center occupied by a metal atom, and there is a H24 cage of hydrogen atoms in the void of the bcc lattice. CaH6 and YH6 have been experimentally synthesized with high Tcs of 215 K at 172 GPa 33,34 and 227 K at 166 GPa, respectively 35. Theoretically predicted Tcs of MgH6, ScH6 and LaH6 are 260 K at 300 GPa, 147 K at 285 GPa and174 K at 100 GPa, respectively. YbH6 and LuH6 in full 4f-orbital shells are predicted to exhibit high Tc superconductivity at relatively low pressures (145 K, 70 GPa vs. 273 K, 100 GPa, respectively) 32. With unfilled 4f orbitals, TmH6 is stable at 50 GPa, but has a relatively low Tc at 25 K. There was a report that the structures of superhydrides at low pressure could keep stable by f electrons, such as lanthanide clathrate hydrides CeH9 36, PrH9 37 and NdH9 38. Although the filling of the metal atoms' f orbital could make the structure more stable at low pressure, there are some negative effects on on superconductivity for unfilled atoms.
The properties of TmH6, YbH6 and LuH6 indicated the magnificent potential of such structures for low-pressure stability. In alkaline earth and rare earth metals hydrides Im-3m-XH6 are common, such as CaH6 28, MgH6 29, YH6 15,16,30, ScH6 31, (Tm/Yb/Lu)H6 32. The structure can also be extended into the ternary structure Pm-3m-ABH12, such as (Y,Ca)H6 39–41, (Mg,Ca)H6 42, (Sc,Ca)H6 43, (La,Y)H6 44, (Ca/Sc/Y,Yb/Lu)H6 45. In recent years, based on this sodalite-like clathrate structure, we have designed a series of high-temperature superconductors that can be stable under moderate pressures by adding heavy rare earth elements Yb/Lu to sodalite-like clathrate hydrides 45. Among them, Y3LuH24 and YLu3H24 are the room-temperature superconductors with the lowest stabilizing pressure predicted by current theory (283 K, 120 GPa and 288 K, 110 GPa, respectively). This result shows that room-temperature superconductivity of hydrogen-based superconductors is possible at medium pressure.
In this work, we designed XTmH12 (X = Y, Yb, Lu, and La) to obtain higher Tc while maintaining low pressure stability. Most prominently, YbTmH12 can stable at 60 GPa. Compared with binary TmH6 hydride, its Tc was increased to 48 K. The results provide an effective method for the rational design of moderate pressure stabilized hydride superconductors.