ENDOR OF TRIGONAL SITES OF YB3+ IN CEO2 AND THO2

Trigonal sites of Yb3+ in CeO2 and ThO2, where an F- ion substitutes for an O2- ligand, have been characterised by 19F ENDOR. Transferred hyperfine interaction (THFI) parameters have been measured for the 19F ligand, and also in CeO2 for the 17O ligand on the C3 axis. A model which accounts for the magnitude and anisotropy of these interactions has been constructed based upon a molecular orbital theory: this requires large contributions to the THFI parameters from the electrostatic polarisation of both the ligand and the paramagnetic ion.