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 �Lcf
z , where Lcf
z exceeds a threshold value. 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. 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 excellent agreement
for the reverse, 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.
