On the Bond Length Change upon 4f 1 → 5d 1 Excitations in Eightfold Coordination: CaF2:Ce3+ Cubic Defects

Abstract

The bond lengths of 4f1 and 5d1 electronic states of cubic (CeF8)5− defects in fluorite have been calculated using quantum mechanical embedding, spin-free relativistic Hamiltonian, and dynamic electron correlation through second-order perturbation theory. The results predict the bond length between Ce and the surrounding eight F to shorten upon 4f1 → 5d1 excitation. This result coincides with previous findings for lanthanide and actinide ions in sixfold octahedral complexes where the ligand field splitting of the 5d (6d) orbitals is inverted with respect to the eightfold cubic field splitting. Altogether, the results of sixfold and eightfold coordination indicate that the bond shrinkage experienced upon 4fn to 4fn-15d1 excitations (5fn to 5fn-16d1 in the actinides) seems to be a general result of f-element complexes. These theoretical results contradict a widespread assumption according to which the bond length increases upon f→ d excitation and, therefore, experimental measurements of the sign of the bond length distortion that either validate or refute the quantum chemical predictions are most desirable.