Structural and electronic changes in europium and ytterbium under pressures of 200 kbar and above

— Lattice parameters of Eu and Yb under pressures of 200 kbar and above have been determined by energy dispersive X-ray diffraction on polycrystalline samples in diamond anvil high-pressure cells. Eu shows a strong compression at low pressures, a phase transition from bcc to hep structure with about 5 % volume decrease at 145 kbar and normal compression at higher pressures. The compressibility of Yb decreases strongly up to about 200 kbar. A new phase transition from bcc to hep structure with a volume change of less than 1 % is observed in Yb at about 300 kbar. A comparison of these high pressure phases with the compression and phase diagrams of the regular trivalent rare earth metals indicates that Eu above 200 kbar and Yb above 300 kbar behave almost but not completely like regular trivalent rare earth metals. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19795141