Clathrate‐Like Alkali and Alkaline‐Earth Metal Borides: A New Family of Superconductors with Superior Hardness

Abstract

Conventional hard and superhard materials, such as diamond and cubic boron nitride, are attractive for fundamental material science and practical industrial application, but severely limited by their poor electrical conductivity. Therefore, it is desirable to design and fabricate novel materials for superior hardness and conductivity. Herein, a class of hard superconductors in alkali or alkaline‐earth metal (AM) borides, namely AMB7, constituted by a B23 cage with one centered metal atom (Li, Na, K, Mg, Ca, and Sr) is reported, which is the first stable clathrate structure in AMB systems. The theoretical calculations demonstrate that all these pressure‐stabilized clathrate structures can be quenched down to ambient conditions, which provides an essential prerequisite for experimental synthesis at moderate pressures. Among them, the highest hardness and maximum superconducting transition temperature (Tc) value are achieved in SrB7 (25.1 GPa) and MgB7 (29.3 K), respectively. Interestingly, the results show that KB7 simultaneously behaves high hardness (22.5 GPa) and superconducting transition temperature (Tc ≈26.2 K). This study opens up a new way to search and design novel superconductors with favorable mechanical properties under high pressure and high‐temperature conditions.

Discovery of superconductivity in technetium borides at moderate pressures

“…15 At present, the discovered bulk superconducting borides with the same stoichiometry as MgB 2 include CaB 2 ( T c ∼ 50 K 16 or 9.4–28.6 K 17 at ambient pressure, theory), NbB 2 ( T c ∼ 9.2 K at ambient pressure, experiment 18–20 ), OsB 2 ( T c = 2.1 K at ambient pressure, experiment 21 ), RuB 2 ( T c = 1.6 K at ambient pressure, experiment 21 ), ScB 2 ( T c = 1.5 K at ambient pressure, experiment 22 ), WB 2 (maximum T c = 15 K at 100 GPa, experiment 23 ), ZrB 2 ( T c = 5.5 K at ambient pressure, experiment 24 ), SiB 2 ( T c = 21 K at ambient pressure, theory 25 ) and MoB 2 ( T c = 32 K at 100 GPa, experiment 26 ). Superconducting borides with other stoichiometries include X 7 B 3 (X = Re and Ru with T c = 3.3 and 2.6 K, respectively, at ambient pressure, experiment 27,28 ), Re 3 B ( T c = 4.8 K at ambient pressure, experiment 28 ), X 2 B (X = Mo, Re, Ta and W with T c = 5.1, 2.8, 3.1 and 3.2 K, respectively, at ambient pressure, experiment 27 ), XB (X = Hf, Nb, Mo, Ta and Zr with T c = 3.1, 8.3, 0.5, 4.0 and 2.8–3.4 K, respectively, at ambient pressure, experiment 27 ), FeB 4 ( T c = 2.9 K at ambient pressure, theory and experiment 29,30 ), XB 5 (X = Na, K, Rb, Ca, Sr, Ba, Sc and Y with T c = 17.5, 14.7, 18.6, 6.6, 6.8, 16.3, 14.2 and 12.3 K, respectively, at ambient pressure, theory 31 ), BeB 6 ( T c = 24 K at 4 GPa, theory 32 ), CB 6 ( T c = 12.5 K at ambient pressure, theory 33 ), MgB 6 ( T c = 9.5 K at 32.6 GPa, theory 34 ), ScB 6 (in P 2 1 / m -, C 2/ m - and Cmcm -structure with T c = 5.8 K at ambient pressure, 2.2 K at 500 GPa, and 2.6 K at 800 GPa, respectively, theory 35 ), XB 6 (X = Nb, La, Th and Y with T c = 3.0, 5.7, 0.74 and 7.1 K, respectively, at ambient pressure, experiment 27 ), XB 7 (X = Li, Na, K, Mg, Ca and Sr with T c = 21.6, 18.3, 26.2, 29.3, 7.7 and 12.7 K, respectively, at ambient pressure, theory 36 ), RbB 6 and RbB 8 ( T c = 7.3–11.6 K and 4.8–7.5 K at ambient pressure, respectively, theory 37 ), YB 6 ( T c = 7.2 K at ambient pressure, experiment 38 ), LaB 8 ( T c = 14 K 39 or 20 K 40 at ambient pressure, theory), XB 12 (X = Nb, La, Th, Y and Zr with T c = 3.0, 5.7, 0.74, 7.1 and 5.8 K, respectively, at ambient pressure, experiment 27,41–43 ), ternary borides like SrB 3 C 3 ( T c = 22 K at 23 GPa, theory and experiment 44 ), or even quaternary borides RbYbB 6 C 6 and RbBaB 6 C 6 (both with T c ≈ 71 K at ambient pressure, theory 45 ).…”

Unveiling the influence of the boron clathrate lattice on superconductivity in a ternary Mg–La–B system

“…Together with the electronic property analysis, we conclude that the strong coupling between s electrons and the vibrational modes of the boron clathrate lattice plays a pivotal role in the superconductivity in MgLaB 10 . The T c of MgLaB 10 is higher than those of most boron clathrates under ambient pressure, such as LaB 8 (14.0-19.6 K), 30,31 CaB 7 (7.7 K) 32 and LiLaB 8 (9.5 K) 33 (Table S4, ESI †), attributed to its high boronderived density of DOSs at the Fermi level and the strong EPC effect. MgLaB 10 exhibts a lower superconductivity compared to MgB 2 23,70 and SrB 3 C 3 , 21,[71][72][73] primarily due to the lack of significant phonon softening at the G point associated with caged boron.…”

“…[26][27][28][29] Analogous to the hydrogen clathrate, the boron clathrate exhibits a strong EPC, thereby fostering superconductivity. Several boron clathrate-based superconductors have been proposed, including LaB 8 with a B 26 cage, 30,31 CaB 7 with a B 23 cage, 32 and LiLaB 8 with B 12 and B 24 cages, 33 exhibiting T c s in the range of 8-20 K under ambient pressure conditions. All of these indicate that boron clathrate compounds may hold great promise as potential superconductors.…”