Structural stability of LaCu₂ and YCu₂ studied by high-pressure x-ray diffraction and ab initio total energy calculations

LaCu₂ is the only compound among the RCu₂ series (R from La to Lu, and Y) which crystallizes in the hexagonal AlB₂-type structure, whereas the other compounds show the orthorhombic CeCu₂-type structure. In agreement with ab initio calculations our high-pressure x-ray diffraction experiments show that LaCu₂ transforms at relatively low pressures to the CeCu₂-type structure, which can be regarded as a low-symmetry variant of the AlB₂-type structure. To investigate the stability of the CeCu₂-type structure we performed high-pressure x-ray diffraction experiments on YCu₂ using argon as the pressure transmitting medium. These experiments showed in contrast to an earlier study that the CeCu₂ structure is stable at least up to the highest attained pressure of 35 GPa. The tendency for amorphization at pressures above 10 GPa, as observed in the earlier study, was probably due to the non-hydrostatic conditions of the pressure transmitting medium, showing that the CeCu₂-type compounds are very sensitive to non-hydrostatic compression. A transition to the MgCu₂-type structure (cubic Laves phase) at about 28 GPa, as predicted by the ab initio calculations, has not been observed.