High Pressure Superconducting transition in Dihydride BiH$_2$ with Bismuth Open-Channel Framework

Abstract

Metal hydrides MHx with low hydrogen content are not expected to show high-Tc superconductivity owing to the low hydrogen-derived electronic density of states at Fermi level and the limited hydrogen contribution to electron-phonon coupling strength. In this work, we report on the successful synthesis of a novel bismuth dihydride superconductor, Cmcm-BiH$_2$, at approximately 150 GPa, and the discovery of superconductivity with Tc about 62 K at 163 GPa, marking the first instance of superconductor among the MH$_2$-type metal dihydrides. Cmcm-BiH$_2$ adopts a unique host-guest type structure, in which the Bi atoms via weak Bi-Bi covalent bonds form a three-dimensional open-channel framework that encapsulates H$_2$-like molecules as guests, thereby broadening the structural diversity of hydrides under high pressures. The occurrence of superconductivity is evidenced by a sharp drop of resistivity to zero and the characteristic downward shift of Tc under applied magnetic fields. Notably, Cmcm-BiH$_2$ remains stable down to at least 97 GPa during decompression, with the calculated lowest pressure for dynamic stability of 10 GPa. In-depth analysis reveals that the covalent bismuth open-channel structure forms metallic conduction channels, dominates the electronic states near the Fermi level, and contributes approximately 51% of the total $lambda$ in Cmcm-BiH$_2$, distinguishing it from known high-pressure hydride superconductors. These findings highlight the critical role of non-hydrogen elements in producing superconductivity and open new avenues for the design and optimization of high-Tc hydride superconductors.

Figures (4)

• Figure 1: (Color online) XRD patterns and the result of Rietveld refinement for BiH$_{2}$ of s1 at 163 GPa in the Cmcm structure. Black circles: experimental data; red line: simulated XRD based on the structural model; purple and yellow vertical lines: Bragg diffraction positions of Cmcm-BiH$_{2}$ and Im-3m-Bi, respectively; blue line: the difference between the simulated and the original XRD. The upper panel shows the XRD image of corresponding powder XRD pattern with the incident X-ray wavelength of 0.6199 Å. (b) Representative integrated XRD patterns for BiH$_{2}$ of s1 upon decompression from 163 to 97 GPa. (c) Experimentally obtained volume per formula unit for BiH$_{2}$ plotted as a function of pressure. Red solid circles, green solid squares and purple triangle represent experimental data for BiH$_{2}$. Theoretical EOS of BiH$_{2}$ is plotted as blue dashed line. Brown and yellow dashed line represents ideal mixtures of Bi + H and Bi + 2H.
• Figure 2: (Color online) (a) Crystal structure of Cmcm-BiH$_{2}$. Blue and pink spheres represent bismuth and hydrogen atoms, respectively. (b) The ELF projected on the plane (0 0 1) (left panel) for H-H contacts and the nearest Bi-H contacts, plane (0 2 1) for the nearest Bi-Bi contacts (up to the right panel), and plane (0 11 -1) for the sub-closest Bi-H contacts (down to the right panel) of BiH$_{2}$.
• Figure 3: (Color online) (a) Temperature-dependent resistance of s2 at 152 GPa. The left inset shows the configuration of sample in chamber and the permutation of electrodes. (b) Temperature-dependent resistance of s2 under selected pressures. (c) Pressure dependent $T_\mathrm{c}$ of Cmcm-BiH$_{2}$ in different runs. (d) Temperature dependences of resistance under various magnetic fields for s2 at 152 GPa. (e) Temperature dependence of $\mu$$_{0}$H$_\mathrm{c2}$(T) fitted by the WHH two-band model.
• Figure 4: (Color online) (a) Calculated phonon dispersion, projected phonon density of states (PhDOS), Eliashberg phonon spectral function $\upalpha$$^{2}$F($\omega$), and integrated electron-phonon coupling $\lambda$($\omega$) for Cmcm-BiH$_{2}$ at 150 GPa. (b) The calculated electronic band structure and projected DOS of Cmcm-BiH$_{2}$ at 150 GPa. (c) The calculated electron-phonon coupling constant $\lambda$, logarithmic average frequency $\omega_\mathrm{log}$ and $T_\mathrm{c}$ for $\mu$$^{*}$ = 0.1, 0.13 and 0.16 at the pressure range of 10 to 170 GPa.