Abstract
The C3-symmetric crystal-field potential in the Fe(II)Fe(III) bimetallic oxalates splits the L = 2
Fe(II) multiplet into two doublets and one singlet. In compounds that exhibit magnetic compensation,
one of the doublets lies lowest in energy and carries an average orbital angular momentum
Lcf
z that exceeds a threshold value of roughly 0.25. In a range of Lcf
z , 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. Due to the competition with the spin-orbit
coupling, the JT distortion disappears at low temperatures in compounds that display magnetic
compensation. A comparison with recent measurements provides strong evidence for this inverse,
low-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.
Keywords: Jahn-Teller distortion; Spin-orbit interaction; Crystal fields; Bimetallic oxalates
