Raman scattering and the electronically-induced phase transition in K ₃Fe(CN)₆ at 130K

Raman scattering has been used to investigate the phonon spectrum of monoclinic K₃Fe(CN)₆ (KFC) and its isostructural analogue K₃Co(CN)₆ (KCoC), in the 12 to 295K temperature range, with the intention of understanding the nature of the (possibly) electronically induced phase transition at 130K, which occurs only in KFC. The quasi-soft mode behaviour of the energy and integrated intensity of the approximately 53 cm ^-1 rotational phonon of largely B_g symmetry, suggests a re-orientational transition in both KFC and KCoC at about 120 and 60K respectively. In KFC, however, the anomalously temperature dependent intensity and linewidth associated with the Fe-CN asymmetric stretching and bending modes, which grow in intensity below 200K, are suggestive of a (possibly coupled) distortional process involving the Fe(CN)₆^3- octahedra. A Jahn-Teller mechanism which goes from a static to a dynamic regime in the characteristic time of the Raman experiment, at around 130K, has been tentatively invoked and shown to be essentially consistent with the Raman data and other known facts about the system. The Raman data clearly do not indicate a distortional process in KCoC, consistent with its quasi-molecular non-degenerate groundstate.