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

(NaCN)1-x(KCN)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 Na + and K+ ions.

Consequently the low-temperature state for x=0.59 is characterized by quenched quadrupolar disorder, i.e., independently frozen-in single CN- ions. 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 70K