Abstract
Experimental discovery of the near-room-temperature superconductivity in highly compressed binary hydride H3S by Drozdov et al. (Nature 525, 73 (2015)) inaugurated a new era in superconductivity. To date, more than 580 superconducting phases of binary hydrides have been studied. The search for near-room-temperature superconductivity is currently extending to ternary and quaternary hydrides, where one of the findings is LaB2H8 phase with \({{T}_{{{\text{c,onset}}}}} = 105\,~{\text{K}}\) at \(P = 90~\,{\text{GPa}}\) (Song et al., J. Am. Chem. Soc. 146, 13797 (2024)). Here we analysed reported experimental data for LaB2H8 and determined the Debye \({{\Theta }_{{\text{D}}}}\left( P \right)\) and Einstein \({{\Theta }_{{\text{E}}}}\left( P \right)\) temperatures, the electron-phonon coupling constant \({{\lambda }_{{{\text{e}} - {\text{ph}}}}}\left( P \right)\) , and Fermi temperature, \({{T}_{{\text{F}}}}\left( P \right)\) in this phase. The obtained value of \({{\lambda }_{{{\text{e}} - {\text{ph}}}}}\left( P \right)\) is within its uncertainty limits and is consistent with the value calculated using first-principles calculations by Song et al. The derived ratio \({{{{T}_{{\text{c}}}}} \mathord{\left/ {\vphantom {{{{T}_{{\text{c}}}}} {{{T}_{{\text{F}}}}}}} \right. \kern-0em} {{{T}_{{\text{F}}}}}} = 0.008\) implies that lanthanum ternary borohydride LaB2H8 falls in the unconventional superconductor region of the Uemura plot. The obtained ratio \({{{{\Theta }_{{\text{D}}}}} \mathord{\left/ {\vphantom {{{{\Theta }_{{\text{D}}}}} {{{T}_{{\text{F}}}}}}} \right. \kern-0em} {{{T}_{{\text{F}}}}}} = 0.059\) implies a moderate level of non-adiabaticity in LaB2H8, which is similar to other hydrogen-rich superconductors such as H3S, LaH10, La4H23, LaBeH8, TaH3, as well as pnictides, cuprates, and MgB2.
