Theory of Structural Variation in a Quasi-One-Dimensional Conductor

Neutron-scattering studies of the quasi-one-dimensional conductor ${\mathrm{K}}_2$Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$ ${\mathrm{Br}}_{0.30}$·${\mathrm{H}}_2$O demonstrate not only the existence of soft phonon modes but also the coexistence of central peaks and the development of a superlattice on cooling which, however, never becomes a true structural phase transition. We suggest that the central peak is due to bromine-induced, static, platinum-chain distortion and show that it affects the electronic motion sufficiently to create a strong enough fluctuation in the three-dimensional system to remove the expected phase transition.