Temperature dependence of the exchange interaction and applications to electron paramagnetic resonance

A previously proposed model relating the widely observed linear temperature dependence of the EPR linewidth to temperature dependence of the isotropic symmetric-exchange integral is discussed in detail and is applied to the data of several newly investigated layered structures. This model assumes an anharmonic intermolecular potential for lattice displacement and an exponential form for the exchange energy giving a temperature dependence in the paramagnetic region. Previous agreement with experimental results on ${\mathrm{K}}_2$Cu${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O and some of the layered Cu${\mathrm{Cl}}_4$ and Mn${\mathrm{Cl}}_4$ compounds is now extended to include others of similar type as well as layered compounds of Cu${\mathrm{Br}}_4$, Mn${\mathrm{Br}}_4$, and Cu${\mathrm{Cl}}_2$ ${\mathrm{Br}}_2$. As before, bond strengths are taken from known tabulated results and overlap integrals are calculated with Slater-type orbitals. Arguments are presented that indicate the effect of the antisymmetric exchange cannot be the only cause for the EPR linewidth behavior.