Direct Measurement of an Order Parameter Associated with the 110.9-K Displacive Phase Transition inK2ReCl6

The temperature dependence of an order parameter associated with the 110.9-K displacive phase transition in ${\mathrm{K}}_2$Re${\mathrm{Cl}}_6$ is measured directly in the tetragonal phase using rotation patterns obtained from nuclear Zeeman quadrupole resonance studies on single crystals. The measurements extend over the temperature range $103<T<\mathrm{}110.9\mathrm{}$ K. The order parameter follows a Landau-like behavior to within 0.4 K of the transition temperature. The zero-temperature coherence length is calculated to be 9.2 Å, which is approximately the distance between adjacent Re${\mathrm{Cl}}_6$ octahedra. Critical fluctuations in ${\mathrm{K}}_2$Re${\mathrm{Cl}}_6$ are therefore of short range.