Pressure-induced structural transitions in europium to 92 GPa

Synchrotron x-ray diffraction experiments have been carried out on europium metal at ambient temperature to pressures as high as 92 GPa (0.92 Mbar). Following the well-known bcc-to-hcp transition at 12 GPa, a mixed-phase region is observed from 18 to 66 GPa until finally a single orthorhombic (Pnma) phase persists from 66 to 92 GPa. These results are compared to predictions from density functional theory calculations. Under pressure the relatively large molar volume V-mol of divalent Eu is rapidly diminished, equaling or falling below V-mol(P) for neighboring trivalent lanthanides above 15 GPa. The present results suggest that above 15 GPa Eu is neither divalent nor fully trivalent to pressures as high as 92 GPa.