Quantum chemical analysis of the bond lengths in fn and fn−1d1 states of Ce3+, Pr3+, Pa4+, and U4+ defects in chloride hosts

Abstract

It is widely believed that impurity–ligand bond distances in lanthanide (Ln) and actinide (An) doped crystals, are larger in the fn−1d1 energy levels than in the fn ones. This idea, which was not justified and is probably based on the fact that Ln 5d (An 6d) orbitals have a radial extent much larger than Ln 4f (An 5f ) orbitals, has been neither confirmed nor rejected experimentally in spite of the fact that a very large number of absorption/emission spectroscopic studies on f-element doped hosts exist, because the band shapes depend on the square of the bond length offsets between initial and final electronic states. Recent quantum chemical calculations on Ln and An impurities in fluoride and chloride cubic hosts, which considered host embedding, dynamic electron correlation, and relativistic spin–free and spin–orbit coupling effects, have shown that impurity–ligand bond distances are classified in three sets according to their configuration, with the following trend: Re[fn−1d(t2g)1]<Re[fn]<Re[fn−1d(eg)1], in contradiction with the assumed expectations. In this paper we give an interpretation of this, on the basis of a constrained space orbital variation analysis of the chemical bond in states of the fn, fn−1d(t2g)1, and fn−1d(eg)1 configurations of four model systems: Cs2NaYCl6:Ce3+, Cs2NaYCl6:Pr3+, Cs2ZrCl6:Pa4+, and Cs2ZrCl6:U4+. The analysis shows that the basic difference between fn and fn−1d1 configurations regarding bond effects which are responsible for the bond distance is that, in the former, all the open-shell electrons are shielded from the ligands by the 5p (6p) filled shell and the bond length is determined by closed-shell interactions between the outermost Ln 5p6 (An 6p6) shell and the ligands, whereas in the latter one electron has crossed the 5p (6p) barrier and is much more exposed to bonding interactions with the ligands, at the same time that an internal 4f (5f ) hole has been created which induces ligand to Ln (An) charge transfer, all of it resulting in the shown trends.