Notably, there is negligible X-ray contrast between Gd 3+ and other RE 3+ ions; neutron diffraction is inhibited by the presence of 157 Gd; and NMR techniques are inhibited by paramagnetism. However, Xray absorption spectroscopy allows some combinations of Gd 3+ and RE 3+ to be studied without interference between their spectra: this is the case for RE = Tm 3+ or Yb 3+ , whose L3-edges are sufficiently far in energy from that of Gd 3+ to allow them to be resolved by EXAFS.