Why Lanthanide ErIII SIMs cannot Possess Huge Energy Barriers: A Theoretical Investigation

Complete active space self-consistent filed (CASSCF) combined with the restricted active space spin interaction with spin-orbit coupling (RASSI-SO) were employed to probe why the single-ion magnets (SIMs) composed by “prolate” lanthanide ion ErIII cannot possess huge energy barriers. According to the proposal by Long etc., equatorially-coordinated ligand environments are preferable to “prolate” lanthanide ions to have large energy barriers. However, our calculations show that the larger gx,y values in the first excited Kramers doublets (KDs) induced by the surrounding equatorially-coordinated ligands lead to their larger transversal magnetic moments so as to fast quantum tunneling of magnetizations (QTMs) in their first excited states. And so, the spin-phonon transitions can only proceed from the ground to the first excited KDs for our studied three compounds and all models. On the other hand, the effective energy barriers Ueff of three compounds are smaller than the calculated energy gaps between the lowest two KDs due to their more flexible molecular structures. For the above reason, the energy barriers didn’t increase continuously as we expected when we decreased the Er-L bond lengths. We deduced that mononuclear ErIII compounds cannot easily possess huge energy barriers through enhancing the surrounding equatorially-coordinated ligand field.