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

[EN] 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 “DevilÏs 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.
This work was supported by the Spanish Ministerio de Economia y Competitividad (MINECO), FEDER (CTQ2013-46275-P), Unidad de Excelencia Maria de Maeztu MDM-2015-0538, the Generalitat Valenciana through PROMETEO/2012/049. L.P.L. and F.J.V.M. thank the Universidad de Valencia and a MINECO for a predoctoral (FPI) grant. D.Z. thanks the Natural Science Foundation of China and China Scholarship Council. This work was supported by the Institut Universitaire de France, the National Research Agency (ANR-13-BS04-0002), Rennes Metropole and CNRS (Post-Doc funding of E.T.). E.C. and J.A.R. would like to thank G. Chastanet for meditations on the Devil's staircase.