Synthesis and High-Pressure Regulation of Hexagonal ReO₃

ReO₃ with A-site-vacant perovskite structure undergoes sequential pressure-driven structural transformations. Recently, we found that its high-pressure rhombohedral R-Ⅰ phase (space group R$\overline{3}$c) is superconducting with an optimal superconducting transition temperature (T_c) of 17 K via high-pressure resistance measurements. To explore new superconductors among Re oxides, in this work we prepared a metastable hexagonal phase of ReO₃ (space group P6₃22) by treating the ReO₃ precursor under 10 GPa and 600 ℃, and characterized its crystal structure, magnetic and electrical transport properties. The results show that P6₃22 phase is not a superconductor down to 2 K at ambient pressure, but displays an anomaly around 250 K in resistivity. High-pressure resistance measurements show that the anomaly at about 250 K in ambient pressure disappears quickly upon compression, and P6₃22 phase shows typical metallic behavior in the whole temperature range without showing any signature of superconductivity down to 1.5 K under pressures up to 62 GPa. In the future, comparative theoretical studies of the hexagonal P6₃22 phase and the R-Ⅰ phase of ReO₃ will help to understand the mechanism of superconductivity in this system.