Volume Contraction During Melting; Emphasis on Lanthanide and Actinide Metals

A review of the volume change during melting indicates that the majority of metals exhibits an average volume increase of 3.8% during this phase transformation. The volume increase is unusually small for La, Pr, Nd, and Np, and is negative for the semimetals plus cerium and plutonium. Based on a free‐volume relationship for viscous flow, a close‐packed structure for the liquid $V_l^0$ was calculated which was nearly the same as the volume of the solid (V_s) at the melting point for a number of close‐packed metals, the rare gas solids Ar, Kr, and Xe, the alkali metals, Hg, and the lanthanide metals La and Yb. The value of $V_l^0$ was significantly less than V_s for Bi, Ga, Ce, Pr, U, and Pu. Examination of the phase diagrams of Bi, Ga, and Pu revealed that $V_l^0$ corresponded to the volume of the phase formed under pressure at which point the slope of the melting curve became zero. The mechanisms postulated for the formation of $V_l^0$ smaller than V_s were closer‐packing for the semimetals and slight valence increases with corresponding reductions in metallic radii for Ce, Pr, U, and Pu. Additionally, the use of liquid compressibility values correctly predicted the pressures at which the slopes of the melting curves became zero. The Lindemann melting law and extensions derived from it were found to be invalid for all metals which contract during melting.