Structural stability of YM₂ compounds (M = Al, Ni, Cu) studied by ab initio total-energy calculations and high-pressure x-ray diffraction

A combination of ab initio total-energy calculations and high-pressure x-ray diffraction experiments has been used to study the phase stability of Y-based 1:2 compounds. The motivation for these investigations was to clarify the origin of the change of the structure among the YT₂ compounds (T = 3d transition element) between the neighbouring elements Ni (YNi₂: defect superstructure of cubic Laves phase C15) and Cu (YCu₂: orthorhombic CeCu₂-type structure). As a test of the ab initio calculations the cubic YAl₂ Laves phase compound has been included in the investigations. The ab initio calculations confirm the reversal of the relative stabilities of the CeCu₂ and C15 phases in YCu₂ and YNi₂ and give also the correct results for YAl₂. Furthermore a pressure-induced structural transition is predicted for YCu₂. The high-pressure x-ray experiments show that there is very good agreement between the calculated and the measured pressure dependence of the lattice parameters of YCu₂ up to about 10 GPa. Above 10 GPa the structure starts to become irreversibly amorphous. This instability may be a hint at the structural phase transition predicted by the calculations.