First step towards a Devil's Staircase in Spin Crossover materials First step towards Devil's Staircase in Spin Crossover materials

This is the authors' abstract. We don't add key points for this paper.

The devil is in the detail: Periodic and aperiodic spin-state concentration waves form during "Devil's staircase"-type spincrossover in a new bimetallic 2D coordination polymer {Fe[(Hg(SCN)3)2](4,4'-bipy)2}n.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 gHS. Between the fully HS (gHS=1) and LS (gHS=0) phases, two steps associated with different ordering appear in terms of spin-state concentration waves (SSCW). On the gHS=0.5 step, a periodic SSCW forms with a HS-LS-HS-LS sequence. On the gHS=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.