Pressure-volume relations and polymorphism of europium and ytterbium to 30 GPa

Pressure-volume relations and high-pressure phase transitions of Eu and Yb have been studied by powder X-ray diffraction up to 30 GPa at room temperature. A structural phase transition from BCC to HCP occurs in Eu near 12.5 GPa with a relative volume change of 4%. The corresponding transition in Yb starts near 26 GPa with a volume discontinuity of 2.4%. A new close-packed phase is observed in Eu which is stable between 18 and at least 27 GPa. In the pressure range 0-20 GPa both Eu and Yb exhibit an anomalously large compression which is attributed to a pressure-induced quantum mixing between divalent and trivalent 4f configurations. Some PV data for Gd up to 20 GPa are also reported.