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 γHS. Between the fully HS (γHS=1) and LS (γHS=0) phases, two steps associated with different ordering appear in terms of spin‐state concentration waves (SSCW). On the γHS≈0.5 step, a periodic SSCW forms with a HS‐LS‐HS‐LS sequence. On the γHS≈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. Der Teufel steckt im Detail: Periodische und aperiodische Spinzustands‐Konzentrationswellen bilden sich während eines der Cantorfunktion folgenden („Teufelstreppen”‐artigen) Spin‐Crossovers in dem 2D‐Koordinationspolymer {Fe[(Hg(SCN)3)2](4,4′‐bipy)2}n.