Jahn-Teller Distortion in Bimetallic Oxalates
Abstract
A C3-symmetric crystal-field potential in the Fe(II)Fe(III) bimetallic oxalates splits the L=2 Fe(II) multiplet into two doublets and a singlet. In compounds that exhibit magnetic compensation, one of the doublets was predicted to lie lowest in energy and carry a non-quenched orbital angular momentum , where exceeds a threshold value. In a range of , a Jahn-Teller (JT) distortion increases the energy splitting of the low-lying doublet and breaks the C3 symmetry of the bimetallic planes around the ferrimagnetic transition temperature. At low temperatures, the JT distortion disappears in compounds that display magnetic compensation due to the competition with the spin-orbit coupling. A comparison with recent measurements provides strong evidence for this re-entrant, low-temperature JT transition and a prediction for the normal, high-temperature JT transition. The size of the JT distortion is estimated using first-principles calculations, which suggest that the long-range ordering of smaller, non-C3-symmetric organic cations can eliminate magnetic compensation.