Temperature Dependence of the Exchange Interaction in ${\mathrm{K}}_2$ Cu${\mathrm{Cl}}_4$ · 2${\mathrm{H}}_2$O

The electron spin resonance of ${\mathrm{Cu}}^{2+}$ in ${\mathrm{K}}_2$Cu${\mathrm{Cl}}_4$ · 2${\mathrm{H}}_2$0 has been observed to change from a two-line spectrum at room temperatures to a single-line resonance at 77 °K. A detailed analysis of the Zeeman anisotropy indicates that the symmetry of the crystal field does not change with temperature. Analysis of the temperature and frequency dependence of the resonance spectrum indicates that the exchange interaction between nearest-neighbor inequivalent ions varies from $\frac{J_1}{k}$ approximately 0.27 °K at 77 °K to $\frac{J_1}{k}$ of the order of 0.05 °K at 300 °K. The anomalously large temperature dependence of the exchange cannot be ascribed to the effect of thermal expansion. Instead, there exists evidence that it may well be associated with the presence of a low-lying optical phonon. In contrast to the pronounced decrease of the nearest-neighbor exchange with increasing temperature, the exchange coupling between equivalent next-nearest-neighbor ions is observed either to be temperature independent or possibly to increase slightly with increasing temperature.