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.