Solid-Liquid and Solid-Solid Transformations in the Rare-Earth Metals at High Pressures

The fusion behavior and solid-solid transformations in the rare-earth metals have been investigated at pressures in the range 6-65 kbar. The phase diagrams are presented for La, Pr, Nd, Sm, Gd, and Tb. The initial melting slopes of Dy to Lu have been obtained. Since most of the rare-earth (R.E.) metals melt from a bcc structure, they offer a series for comparison. When the initial melting slope is plotted against atomic number they fall into groups according to their valency; the typically divalent metals Eu and Yb in one, and the typically trivalent metals La, Gd, and Lu in another. Samarium and the metals from Tb to Tm have melting slopes which are intermediate between those of the typically divalent and trivalent metals. It is suggested that these metals assume a partial divalent character in the liquid state and possibly also in the bcc phase. This suggestion derives support from the fact that the gaseous neutral atoms of the R.E. elements, with the exception of La, Gd, and Lu, have no $5d$ electron and are therefore divalent. Cerium exhibits a unique fusion behavior and its negative melting slope has been attributed to the tetravalency ($4f\to 5d$ electron promotion) of the atoms in the liquid. For the very small melting slope of Pr, it is suggested that the atoms in the liquid have partial tetravalent character due to $4f\to 5d$ electron promotion. Comparisons with alkali metals are made and it is deduced that the R.E. metals are quite compressible at high temperatures. Pressure induced transformations in the rare-earth metals are in the sequence hcp→Sm-type→double hcp→fcc. A plausible mechanism based on the variation of $\frac{c}{a}$ ratio with pressure is discussed for hcp→Sm-type transformation.