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 stabil ity 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 elemen ts Ni (YNi2: defect superstructure of cubic Laves phase C15) and Cu (Y Cu2: orthorhombic CeCu2-type structure). As a test of the db initio ca lculations the cubic YAl2 Laves phase compound has been included in th e investigations. The ab initio calculations confirm the reversal of t he relative stabilities of the CeCu2 and C15 phases in YCu2 and YNi2 a nd give also the correct results for YNi2. Furthermore a pressure-indu ced structural transition is predicted for YCu2. The high-pressure x-r ay experiments show that there is very good agreement between the calc ulated and the measured pressure dependence of the lattice parameters of YCu2 up to about 10 GPa. Above 10 GPa the structure starts to becom e irreversibly amorphous. This instability may be a hint at the struct ural phase transition predicted by the calculations.