Configuration interaction of Er3+with a charge transfer configuration in the elpasolite compound Cs2NaErCl6

The discrepancies of the crystal-field analysis in Cs2NaErCl6 are lowered by letting the 4f11 configuration interact with a charge transfer configuration enclosing a p electron. 75 levels with a total degeneracy equal to 130 were fitted with a mean deviation of 10.5 cm−1, compared with 21.4 cm−1 in the standard model. It is found that: (i) the p electrons involved in the process originate most probably from the six Cl− ligands; (ii) 14% of the 3p6 orbitals are projected onto the central ion coordinate system as an unique orbital with p angular character; (iii) this projected orbital can be coupled with 4fn configurations and handled by tensorial methods; and (iv) the most probable mechanism responsible for the improvement of the crystal-field analysis is a 4f11 3p6-4f123p5 interaction. The first configuration represents Er3+ surrounded by six Cl− ligands, whereas in the second one, the rare-earth ion and the ligands have gained and lost one electron respectively. The crystal-field analysis enables the estimation of the location of the charge transfer band.