High-pressure structural study of UM2Al3 (M = Pd, Ni): evidence for a pressure-induced electronic transition in UPd2Al3

High-pressure x-ray diffraction studies on the magnetically ordering heavy- fermion superconductors UM2Al3 (M = Pd, Ni) have been performed. For UNi2Al 3 only a smooth reduction of the pressure dependence of the unit-cell volum e was observed. In the case of UPd2Al3 a structural phase transition takes place around p approximate to 25 GPa from the high-symmetry hexagonal PrNi2 Al3-type low-pressure structure into a low-symmetry orthorhombic high-press ure phase. The new phase is characterized by space group Pmmm and closely r elated to the PrNi2Al3 structure at ambient pressure. Its compressibility i s about twice that of the low-pressure phase. The remarkable high-pressure behaviour is discussed with respect to an electronic transition related to the structural high-pressure phase transition.