title: First Step Towards a Devil's Staircase in Spin-Crossover Materials.
abstract: 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.
authors.affiliation: Institut de Physique de Rennes, Université de Rennes 1, UMR UR1-CNRS 6251, 35000, Rennes, France.
authors.forename: Elzbieta
authors.initials: E
authors.lastname: Trzop
authors.affiliation: Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, C/ Catedrático José Beltrán Martínez 2, 46980 Paterna, Valencia, Spain.|College of Chemical Engineering, Shandong University of Technology, Zibo, 255049, China.
authors.forename: Daopeng
authors.identifier: 0000-0003-1998-1825
authors.initials: D
authors.lastname: Zhang
authors.affiliation: Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, C/ Catedrático José Beltrán Martínez 2, 46980 Paterna, Valencia, Spain.
authors.forename: Lucia
authors.initials: L
authors.lastname: Piñeiro-Lopez
authors.affiliation: Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, C/ Catedrático José Beltrán Martínez 2, 46980 Paterna, Valencia, Spain.
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Valverde-Muñoz
authors.affiliation: Departamento de Física Aplicada, Universitat Politècnica de València, Camino de Vera s/n, 46022, Valencia, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Carmen Muñoz
authors.affiliation: Department of Structure Analysis, Institute of Physics of Academy of Sciences of Czech Republic, 18221, Prague, Czech Republic.
authors.forename: Lukas
authors.identifier: 0000-0002-8987-8164
authors.initials: L
authors.lastname: Palatinus
authors.affiliation: Institut de Physique de Rennes, Université de Rennes 1, UMR UR1-CNRS 6251, 35000, Rennes, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Guerin
authors.affiliation: Institut de Physique de Rennes, Université de Rennes 1, UMR UR1-CNRS 6251, 35000, Rennes, France.
authors.forename: Hervé
authors.initials: H
authors.lastname: Cailleau
authors.affiliation: Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, C/ Catedrático José Beltrán Martínez 2, 46980 Paterna, Valencia, Spain. Jose.A.Real@uv.es.
authors.forename: Jose Antonio
authors.initials: JA
authors.lastname: Real
authors.affiliation: Institut de Physique de Rennes, Université de Rennes 1, UMR UR1-CNRS 6251, 35000, Rennes, France. eric.collet@univ-rennes1.fr.
authors.forename: Eric
authors.identifier: 0000-0003-0810-7411
authors.initials: E
authors.lastname: Collet
keywords: Devil's staircase; aperiodicity; coordination polymers; phase transitions; spin-crossover