First Step Towards a Devil's Staircase in Spin-Crossover Materials

The unprecedented bimetallic 2D coordination polymer {Fe[(Hg(SCN)(3))(2)](4,4'-bipy)(2)}(n) exhibits a thermal high-spin (HS)<-> low-spin (LS) staircase-like conversion characterized by a multi-step dependence of the HS molar fraction gamma(HS). Between the fully HS (gamma(HS)= 1) and LS (gamma(HS)= 0) phases, two steps associated with different ordering appear in terms of spin-state concentration waves (SSCW). On the gamma(HS). 0.5 step, a periodic SSCW forms with a HS-LS-HS-LS sequence. On the gamma(HS) approximate to 0.34 step, the 4D superspace crystallography structural refinement reveals an aperiodic SSCW, with a HS-LS sequence incommensurate with the molecular lattice. The formation of these different long-range spatially ordered structures of LS and HS states during the multi-step spin-crossover is discussed within the framework of Devil's staircase-type transitions. Spatially modulated phases are known in various types of materials but are uniquely related to molecular HS/LS bistability in this case.