INTRODUCTION

Since the discovery of superhydride superconductors with _T_c up to 260 K [1,2], much effort has been devoted to elucidate its origin and the results indicate that hydrogen plays a crucial role within the framework of electron-phonon coupling (EPC) mechanism [3], such as H3S [4]. Similar features have also been identified in other light-element superconductors such as MgB2 [5,6] and NaC8 [7], which exhibit layered-honeycomb-boron and layered-carbon frameworks, and show high _T_c reaching ∼ 39 K at ambient pressure (AP) and 28 K at 14 GPa, respectively. In addition, the discovery of mixed-valence main-group perovskite oxides bismuthates [8–10] and antimonates [11] superconductors have also received lots of attention due to suppression of competing charge orders and the important role of oxygen lattice in driving superconductivity. Experimental investigations and theoretical calculations on these systems have identified strong metal-oxygen covalency and enhanced EPC as the possible origin of high _T_c [12,13]. Recently, superconductivity has been discovered in simple binary oxides such as rock-salt LaO, which shows _T_c ∼ 5–6 K at AP and was further enhanced to 12.7 K under pressure. However, the density of states (DOS) at the Fermi level [N(E_F)] mainly comes from La-5_d electrons [14]. Realizing unconventional bonding features in metallic oxides remains challenging due to the strong electronegativity of oxygen, yet it continuously inspired researchers to discover favorable oxide superconducting systems.

For the exploration of oxide analogues of hydride superconductors, we turn to 5_d_ transition-metal-oxide ReO3, a high oxygen molar ratio and purple-colored metallic oxide with high conductivity comparable to that of copper and silver [15]. It adopts the A-site-vacant perovskite structure with corner-shared ReO6 octahedra [16]. Spectroscopy measurements confirm that the excellent conductivity originates from the bond covalency between Re-t_2g and O-p_π orbitals through the linear Re-O-Re bonds [15,17] and the hybridizations further raise the 2_p_ orbitals of oxygen to the Fermi level. On the other hand, ReO3 was predicted to be a superconductor at AP [18], but no superconductivity was observed down to 20 mK. Intriguingly, ReO3 undergoes a sequential pressure-driven structure phase transitions from cubic-I (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $Pm!!! \bar{,,,3}m$\end{document}) (C-I) to cubic-II (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $Im!!! \bar{,,,3}$\end{document}) (C-II) at ∼ 0.5 GPa, to monoclinic (_C_2/c) at ∼ 3 GPa, to rhombohedral-I (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $R!!! \bar{,,,3}c$\end{document}) (R-I) at ∼ 12 GPa, and to rhombohedral-II (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $R!!! \bar{,,,3}c$\end{document}) (R-II) at ∼ 39 GPa [18–23]. But no consensus has been reached on the accurate high-pressure phases and critical transition pressures, as summarized in Table S1 of the Supplemental Information (SI). Nevertheless, the presence of the R-I phase between 12 and 39 GPa has been well established, as shown in Fig. 1.

Figure 1.: Crystal structure evolution and lattice parameters of ReO3 under high pressure. (a) The horizontal arrow on the top shows the pressure-induced structure phase transitions of ReO3 from C-I to R-II phases at 0.5 GPa, 12 GPa, and 39 GPa. The bottom part displays the coupled rotation of oxygen layers from Kagome lattice to triangular lattice in the (111) plane. Bond lengths were obtained from DFT calculations, as shown in Table S2. The solid lines connecting the oxygen atoms serve as visual guides to illustrate the structural evolution and densification of the oxygen sublattice. They represent the nearest-neighbor O-O distances. (b) Pressure dependence of volume for different phases of ReO3. Solid lines represent the fitting by using Birch-Murnaghan (BM) equation of state. (c) Pressure dependence of the lattice parameters a and c of ReO3. (d) Pressure dependence of the coupled rotation angle of ReO6 octahedra in the R-I phase. The arrow indicates the critical pressure where the oxygen layers form the perfect hexagonal-close-packed lattice.

Inspired by realizing strong covalency between transition-metal and oxygen that would facilitate a higher T_c, we investigated the physical properties of 5_d transition-metal-oxide ReO3. By combining high-pressure structure characterizations and transport measurements, we created a high-T_c record with 17.8 K in the R-I phase of ReO3 at 35 GPa among 5_d transition-metal-oxides. The theoretical analyses further demonstrate that the superconductivity originates from a synergistic effect of pressure-enhanced Re-O hybridization and the substantial contributions to the EPC from close-packed oxygen stacking lattice. The R-I phase of ReO3 constitutes a promising oxide analogue of hydride superconductors. Thus, our work would stimulate more effort to further explore new high-T_c superconductors among 5_d transition-metal-oxides via high-pressure or heterostructure engineering.