Dipolar and quadrupolar freezing in (NaCN)1−x(KCN)x

(NaCN${)}_{1\mathrm{-}\mathrm{x}}$(KCN${)}_{\mathrm{x}}$ mixed crystals with concentrations x=0.85 and 0.59 have been investigated by elastic and inelastic neutron scattering, x-ray diffraction, and dielectric techniques. Both crystals exhibit neither quadrupolar nor dipolar long-range order down to the lowest temperatures. However, the slowing down of the dipolar and the quadrupolar relaxation behaves very differently in the two samples investigated. In (NaCN${)}_{0.15}$(KCN${)}_{0.85}$ the freezing-in of the quadrupolar degrees of freedom is a cooperative effect which is dominated by strain-mediated interactions. The resulting low-temperature state is characterized by frozen-in orientational correlations and frozen-in lattice strains. In (NaCN${)}_{0.41}$(KCN${)}_{0.59}$ the experimental results on the dynamics of the quadrupolar freezing are indicative of single-ion behavior where the slowing down of the molecular reorientations is completely due to thermal activation across the hindering barriers of the crystal field set up by the neighboring ${\mathrm{Na}}^{+}$ and ${\mathrm{K}}^{+}$ ions. Consequently the low-temperature state for x=0.59 is characterized by quenched quadrupolar disorder, i.e., independently frozen-in single ${\mathrm{CN}}^{\mathrm{-}}$ ions. An analysis of the static dielectric susceptibilities demonstrates that dipolar interaction forces are negligible in (NaCN${)}_{0.41}$(KCN${)}_{0.59}$ while significant deviations from a Curie-type behavior exist in (NaCN${)}_{0.15}$(KCN${)}_{0.85}$ indicating the onset of short-range electric order near 70 K.