Dielectric studies of CN^- dipolar reorientation and order-disorder behavior in RbCN_1-x: KCN_x and KCN_1-x: NaCN_x

The evolution of phase transitions and CN^- dipolar reorientation are studied in two mixed alkalicyanides RbCN_1-x:KCN_x and KCN_1-x:NaCN_x for 0 ≤ x ≤ 1 with dielectric constant and loss measurements in the frequency range 10¹ to 10⁵ Hz from room temperature to 4 K. For RbCN_1-x:KCN_x, transition into an CN^- elastically ordered state occurs under cooling for all concentrations x, while CN^- antiferroelectric ordering takes place only in the 0.50 ≤ x ≤ 1 range. For KCN_1-x: NaCN_x, CN^- elastic ordering takes place only close to the pure compounds (0 ≤ x ≤ 0.10 and 0.86 ≤ x ≤ 1), while in the remaining concentration range (0.10 ≤ x ≤ 0.86), CN^- orientations freeze into a glass-like disordered state at low temperatures. Antiferroelectric ordering is detected in this case only in the 0 ≤ x ≤ 0.06 and 0.86 ≤ x ≤ 1 ranges. Multiple dielectric relaxation phenomena are observed in the elastically ordered concentration regimes of KCN_1-x: NaCN_x due to CN^- reorientation occurring in different coexisting structural phases. Dielectric relaxation data for both mixed systems are modeled either with a single or a superposition of two or three Cole-Cole type distributions of relaxation times. The obtained phase diagram for KCNi_1-x: NaCN_x disagrees with the one constructed from neutron powder diffraction because in that technique residual stresses introduced by sample grinding can modify the structure of phases.