17. Jahn-Teller Transitions in the Fe(II)Fe(III) Bimetallic Oxalates

Because the orbital angular momentum L _z ^cf on the Fe(II) sites of the Fe(II)Fe(III) bimetallic oxalates is incompletely quenched by the crystal field, the spin-orbit coupling competes with the Jahn-Teller (JT) distortion energy. The value of L _z ^cf depends on the cation between the bimetallic layers. When L _z ^cf is sufficiently small, the open honeycomb lattice of each bimetallic layer is distorted at all temperatures below the JT transition temperature. But in a range of L _z ^cf , the lattice is only distorted between lower and upper JT transition temperatures, T _JT ^(l) and T _JT ^(u). For some cations, L _z ^cf may exceed the threshold required for the cancellation of the moments on the Fe(II) and Fe(III) sublattices at a temperature T _comp below the transition temperature T _c . Using elastic constants obtained from compounds that exhibit magnetic compensation, we find that T _JT ^(l) always lies between T _comp and T _c and that T _JT ^(u) always lies above T _c .