Superconductivity in barium hydrides via incorporation of light elements

TL;DR

This work investigates superconductivity in Ba-H systems under high pressure by introducing light elements to expand the structural landscape. It combines high-throughput crystal-structure screening with first-principles electronic/phonon calculations and a fast networking-value $T_c$ estimator to identify promising candidates, followed by detailed DFPT analysis. The study predicts BeBaH$_8$ as a near-term metallic hydride with $T_c$ ≈ 49 K at 100 GPa (rising to ≈107 K at 200 GPa) and BeBaH$_6$ as thermodynamically stable at 100 GPa, with BeBaH$_4$ and BeBaH$_8$ near the stability hull; it also notes persistence of H$_2$ units in many low-enthalpy structures. Overall, the results demonstrate a viable path to raise $T_c$ and access broader hydride chemistry by incorporating light elements, with potential experimental realization under high-pressure synthesis and opportunities to approach ambient-pressure behavior via anharmonic/quantum stabilization.

Abstract

Barium hydrides are of interest for their potential in both ionic conductivity and superconductivity. Recently, a superconducting hydride BaH$_{12}$ containing H$_2$ and H${_3}^{-1}$ molecular units was experimentally reported with a critical temperature $T_\text{c}$ of 20 K at 140 GPa [Nat Commun 12, 273 (2021)]. Herein, we combine ab initio methods with a rapid calculator of $T_\text{c}$ based on the networking value model to predict that the introduction of light elements, such as Be, can effectively expand the structure diversity and structure space of barium hydrides. Although molecular hydrogen units are still widely present in thermodynamically stable and metastable crystal structures, we find that a metastable phase of BeBaH$_8$ shows a high $T_\text{c}$ of 49 K at 100 GPa, which is only 38 meV/atom above the thermodynamic stability energy. This BeBaH$_8$ remains dynamically stable at 15 GPa. Furthermore, our study shows that increasing pressure can further elevate $T_\text{c}$ beyond 100 K by enhancing the electron-phonon coupling constant. Our study proposes a feasible method for broadening the structural landscape in the exploration of superconducting phases of barium hydrides.

Figures (5)

• Figure 1: The lowest-lying states at 200 GPa of (a) Li$_2$BaH$_{12}$ with space group $Cc$, (b) Be$_2$BaH$_{12}$ with space group $Cmmm$, (c) B$_2$BaH$_{12}$ with space group $P4bm$, and (d) C$_2$BaH$_{12}$ with $P1$. The H-H bond is plotted if the distances are less than 1.0 Å, $A$-$A$ and $A$-H bonds are plotted if the distances are less than 1.4 Å.
• Figure 2: The phase diagram at 100 GPa along with the crystal structures of thermodynamically stable BeBaH$_6$, and two metastable phases including BeBaH4 and BeBaH8. The structural parameters and atomic coordinates of BeBaH4, BeBaH6, and BeBaH8 are listed in Supplementary Table V of Supplementary Material.
• Figure 3: The electronic and phonon properties of BeBaH4 at 100 and 50 GPa. (a) Electronic band structure, electronic density of states (DOS), phonon band structure and phonon density of states (PDOS) at 100 GPa. (b) Electronic band structure, DOS, phonon band structure and PDOS at 50 GPa.
• Figure 4: The electronic and phonon properties of BeBaH8 at 100 and 15 GPa. (a) Electronic band structure, electronic density of states (DOS), phonon band structure and phonon density of states (PDOS) at 100 GPa. (b) Electronic band structure, DOS, phonon band structure and PDOS at 15 GPa.
• Figure 5: Eliashberg spectral function $\alpha^2F(\omega)$ (solid blue lines) and the cumulative frequency-dependent electron-phonon coupling function $\lambda$($\omega$) (dashed red lines) of BeBaH8 at 100, 150 and 200 GPa.