The exceptional behaviour of Pr, Ce and Tb in high-Tc superconductors

X-ray absorption and diffraction, elastic and inelastic neutron scattering and magnetization measurements are used to study the, electronic, structural and magnetic properties of two series of high-T-c superconductors containing Ce, Tb and Pr. These results are interpreted with regard to the well known valence instabilities often shown by these rare earth ions. Ce is found to be non-magnetic, in contrast to Tb, which exhibits complex 2D magnetic properties. Although the measurements provide detailed understanding of the behaviour of Ce and Tb, there is still no consensus why Pr suppresses superconductivity in particular cuprates.