High-Temperature Polymorphism in Rare Earth Elements

摘要

Most rare earth (RE) elements exhibit structural polymorphic transitions from low-temperature (I) to high-temperature (II) modifications in the vicinity of the corresponding melting temperatures. Among the fourteen 4/elements (in which the 4/electron orbital is occupied), there are apparently only two (europium and thulium) that are not subject to such high-temperature structural transformations. We have considered the crystal structures, evaluated the temperature intervals of the existence of the equilibrium RE polymorphs I and II, and determined the corresponding characteristic (Debye) temperatures 6, and 6M. In addition, we have established relationships between the 0, and 6n values, on the one hand, and the temperatures Tk of the corresponding polymorphic transitions and the melting temperatures Tm, on the other hand. The enthalpies AHk of the polymorphic transitions determined using the 0, and 6II values are similar to the available experimental data.