COMPOUNDS OF THE RARE-EARTH METALS WITH ALUMINUM AND THEIR CRYSTALLINE STRUCTURES.

The data of the investigation and of the existing literature allow certain conclusions to be made about the compositions of compounds of the rare-earth metals with aluminum. While the compounds RAl2 of the MgCu2 type exist in all systems, the compositions of the compounds richest in aluminum are not identical their composition depends upon the atomic radii of the rare-earth metals. As the atomic radius decreases the RAl4 compounds R La, Ce, Pr, Nd of the BaAl4 type give way to the RAl3 compounds R Sm, Ge, Er with structures of the Mg3Cd or AuCu3 types, i.e., superlattices of the densest packing. Europium, whose atomic radius is larger than the radii of the neighboring elements, samarium and gadolinium, forms the compound EuAl4, which is isostructural to compounds of La, Ce, Pr and Nd. With respect to the values of the grid constant the compound EuAl4 is closer to the compound BaAl4 a 4.530 A, c 11.14 A, ca 2.460 than to LaAl4 and its analogs for which ca 2.31 - 2.32, which is connected with the similar atomic radii of Ba and Eu. In the systems of the rare-earth metals with shorter atomic radii the compounds RAl3 are close in composition and type of structure to the compounds HfAl3 and ScAl3.