Metallization and Superconductivity in the Hydrogen-Rich Ionic Salt BaReH₉

BaReH₉ is an exceedingly high-hydrogen-content metal hydride that is predicted to exhibit interesting behavior under pressure. The high-pressure electronic properties of this material were investigated using diamond-anvil-cell electrical conductivity techniques to megabar (100 GPa) pressures. The measurements show that BeReH₉ transforms into a metal and then a superconductor above 100 GPa with a maximum transition temperature (T_c) near 7 K. The occurrence of superconductivity was confirmed by the suppression of the resistance drop upon application of magnetic fields. The transition to the metallic phase is sluggish, but it is accelerated by laser irradiation. Raman scattering and X-ray diffraction measurements, used to supplement the electrical measurements, indicate that the Ba-Re sublattice is largely preserved upon compression under the conditions explored, but there is a possibility that hydrogen atoms are gradually disordered under pressure. This is suggested by the sharpening of the peaks in Raman spectroscopy and X-ray diffraction upon heat treatment, as well as the temperature dependence of the resistance under pressure. The data suggest that the transition to the superconducting state is first-order. The possibility that the transition is associated with the breakdown of BeReH₉ is discussed.