Dielectric Studies on the Dipolar Reorientation and Order-Disorder Behavior of Pure and Mixed Alkali Cyanides.

Measurements of the real and imaginary parts of the dielectric constant are applied to the study of the reorientation dynamics and collective ordering behavior of the CN('-) molecule in four pure alkali-cyanides (NaCN, KCN, RbCN and CsCN), the dipole-diluted KBr(,1-x):KCN(,x) system and the mixed alkali-cyanides KCN(,1-x):NaCN(,x) for 0 (LESSTHEQ) x (LESSTHEQ) 1. Additional information on the molecular ordering is obtained by monitoring the onset of light scattering from domains that occurs at the critical temperature of the order-disorder phase transition. Dielectric data of better accuracy are obtained on the previously investigated pure alkali-cyanides and a mean-field theoretical model is proposed to account for the absence of any paraelectric 1/T behavior in the dielectric response of KCN. Preliminary studies on the electrical conductivity of the pure alkali-cyanides to temperatures near their melting points are reported. The evolution of molecular reorientation and collective ordering in the mixed systems is discussed. In all these systems, the measured mean reorientation rates of the CN(' -) molecules imply a classical temperature-activated Arrhenius behavior and a spread of relaxation rates that increase with increasing disorder of the pure alkali-cyanide lattice. While for the RbCN:KCN system the order-disorder phase transition is observed under cooling throughout the whole mixture, for the KBr:KCN and KCN:NaCN systems, this transition disappears for a certain range of the concentration parameter x. In this range of the mixture the dipole system freezes gradually under cooling into a glass-like state of electric and elastic disorder. Very low temperature dielectric measurements on crystals with low CN('-) concentrations are reported for the KBr:KCN mixture. An empirical relation is found between the critical temperature of the ferroelastic order-disorder transition T(,c) and the Arrhenius dipolar reorientation rate (tau)(' -1)(T) of the CN('-) molecules by the concept of a critical reorientation rate (tau)('-1)(,c)(T(,c)). The observed abrupt disappearance of the order-disorder transition and change into a behavior of gradually freezing disorder at a certain x value (x = x(,c)) occurs in all mixed systems, when the spread of relaxation rates exceeds a particular value.