DISCUSSION

The main result of present study is the observation of superconductivity with maximum T_c ∼ 17.8 K in the high-pressure R-1 phase of ReO3 at 35 GPa, which sets the highest T_c record among the reported 5_d transition-metal oxide superconductors. As we know, the 3_d transition-metal oxides have strong electron correlations and give birth to the cuprates [33,34] and nickelates [35,36] high-T_c families. In contrast, 5_d transition-metal elements have more spatially extended orbitals, strong spin-orbital couplings and weak electron correlations, and often exhibit low T_c superconductivity in its oxides. ReO3 stands out with strong hybridization between Re-5_d and O-2_p_ states and exhibits a maximum T_c of ∼ 17.8 K. Our work indicates that the rotations of ReO6 octahedra under pressure strongly enhances the hybridizations between Re-5_d and O-2_p_ orbitals. The structural evolution gradually pushes the Re-O antibonding states to the Fermi level and simultaneously increases the N(_E_F). Thus, the emergence of high-T_c superconductivity in R-I phase of ReO3 represents a compelling example of phonon-mediated superconductivity driven by a synergistic effect between the heavy 5_d element and the light oxygen sublattice.

To further quantify the role of light element in boosting high-_T_c superconductivity, we make a comparison between ReO3 and representative covalent hydride superconductor H3S. For H3S, the high-T_c superconductivity is not merely driven by hydrogen; sulfur also plays a critical role for constructing strong covalent network and enhances the EPC [37]. While in ReO3, the extended Re-5_d orbitals provide necessary electronic framework, and the oxygen contributes ∼ 27% to N(_E_F) (∼ 42% for H in H3S) and ∼ 50% to λ at 30 GPa (∼ 67% for H in H3S) [38]. Therefore, ReO3 can be regarded as an oxide analogue of hydride superconductors in R-I phase, which share high similarity of pressure-optimized covalent hybridizations. However, the lattice dynamics of oxygen were intrinsically restricted the phonon frequencies compared to the lighter hydrogen and further set the upper limit for the maximum _T_c.

Inspired by the observation of high-T_c superconductivity in R-I phase of ReO3, our work highlights the promising routes to explore new superconductors in 5_d transition-metal-oxides via high-pressure or heterostructure engineering. It should be noted that the contribution of oxygen (∼ 50%) to the EPC is currently derived from DFT calculations and the direct experimental verifications would be provided with the oxygen isotope effect measurements, i.e. comparing the _T_c of Re16O3 and Re18O3 under high pressures.