Intramolecular exchange interaction between two rare-earth ions

Abstract

A theoretical interpretation is made of the magnetic susceptibilities of the dinuclear rare-earth complexes of phthalocyanine [RE₂(pc)(dpm)₄][where RE = Sm³⁺–Yb³⁺ and Y³⁺, pc = phthalocyaninato(2-) and dpm = 2,2,6,6-tetramethylheptane-3,5-dionato(1-)] measured from 77 to 300 K. The tendencies of the magnetic moments to decrease towards lower temperatures observed in Sm³⁺ and Eu³⁺ complexes are explained quantitatively by spin–orbit calculations in the intermediate coupling scheme. The spin–orbit coupling constants obtained are 1138 and 973 cm^–1 for the Eu³⁺ and Sm³⁺ complexes, respectively. Similar behaviour of the Yb³⁺ complex, however, is explicable in terms of the crystal-field effect on the ground-state J-levels. On the other hand, Er³⁺ and Tm³⁺ complexes showed the opposite tendency; the magnetic moments increased more than the free-ion values as the temperature decreased. This anomalous behaviour could be simulated quantitatively by a calculation based on the anisotropic spin–spin exchange interaction. The spin–spin coupling constants obtained are 0.114 and –0.553 cm^–1 for the Er³⁺ complex, and 0.273 and –1.550 cm^–1 for the Tm³⁺ complex. The mononuclear Er³⁺ and Tm³⁺ complexes did not show dependence on temperature. Er³⁺ and Tm³⁺ dinuclear complexes are considered to be the first examples having an observable intramolecular spin–spin exchange interaction between tripositive rare-earth ions.