Phase Transition and Soft Mode in Orthorhombic K3Co(CN)6

Raman spectra of K 3 Co 1- x Fe x (CN) 6 ( x ≈0.03) were measured in the temperature range from room temperature to 22 K. The room temperature phase was the orthorhombic polytype of K 3 Co(CN) 6 . It was found that this crystal undergoes a structural phase transition at 81 K from the observation of a soft mode which decreases its frequency as the transition temperature is approached from both sides. On the basis of the polarized Raman measurements of the soft mode and symmetry consideration, the structure below 81 K was found to be monoclinic.