Structural stability of LaCu₂and YCu₂studied by high-pressure x-ray diffraction and<i>ab initio</i>total energy calculations

LaCu2is the only compound among the RCu2series (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 initiocalculations our high-pressure x-ray diffraction experiments show that LaCu2transforms at relatively low pressures to the CeCu2 -type structure, which can be regarded as a low-symmetry variant of the AlB2 -type structure.