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 Vₘₒₗ of divalent Eu is rapidly diminished, equaling or falling below Vₘₒₗ(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.