Pressure-induced superconductivity in europium metal

Of the 52 known elemental superconductors among the 92 naturally occurring elements in the periodic table, fully 22 only become superconducting under sufficiently high pressure. In the rare-earth metals, the strong local magnetic moments originating from the 4f shell suppress superconductivity. For Eu, however, Johansson and Rosengren have suggested that sufficiently high pressures should promote one of its 4f electrons into the conduction band, changing Eu from a strongly magnetic (J=7/2) 4f7-state into a weak Van Vleck paramagnetic (J=0) 4f6-state, thus opening the door for superconductivity, as in Am (5f6). We report that Eu becomes superconducting above 1.8 K for pressures exceeding 80 GPa, Tc increasing linearly with pressure to 142 GPa at the rate +15 mK/GPa. Eu thus becomes the 53rd elemental superconductor in the periodic table. Synchrotron x-ray diffraction studies to 92 GPa at ambient temperature reveal four structural phase transitions.