Fusion curve of europium, fusion, and fcc-bcc transformation in ytterbium at high pressures

The melting behavior of Eu to 70 kbar and melting and fcc-bcc transformation in Yb to 40 kbar have been studied. Europium has a maximum in the fusion curve at about 35 kbar and 995°C and retains the bcc structure in the pressure and temperature range investigated. The fusion curve of Yb has an initial slope of 19°/kbar and rapidly flattens in the 35-kbar region. There appears to be only one fcc-bcc phase boundary and not two as proposed in an earlier study. This boundary intersects the temperature axis close to the known fcc-bcc transformation temperature at atmospheric pressure, and an initial slope of -16°/kbar is suggested. This transformation in Yb behaves exactly like the analogous transformation in Sr, exhibiting pressure and temperature hysteresis and also reversing the sign of the resistance discontinuity above 24 kbar. The striking parallelism in high-pressure behavior between Eu and Ba and Yb and Sr is discussed. The greater density of the bcc phase compared to fcc or hcp noted in several instances is rationalized. The melting behavior of Eu is discussed from the standpoint of coordination in the liquid state