Neutron-Scattering Study of the Structural Phase Transition in the One-Dimensional Conductor ${\mathrm{K}}_2$Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$${\mathrm{Br}}_{0.3}$${\mathrm{·}3\mathrm{D}}_2$O

We report a study of the temperature dependence of the $2k_{\mathrm{F}}$ instability in the one-dimensional conductor ${\mathrm{K}}_2$Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$ ${\mathrm{Br}}_{0.30}$ ${\mathrm{·}3\mathrm{D}}_2$O using a neutron scattering technique. Whereas the extra scattering at room temperature has a one-dimensional character corresponding to a periodicity along the $C$ axis of 6.66 Pt-Pt spacings, below 220 K it indicates the development of a three-dimensional superlattice structure with a new unit cell $2a\times 2a\times 6.66c$. However, this transition seems to be incomplete, in the sense that the coupling between adjacent chains does not achieve long-range order.