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

TL;DR

This study observes multi-step spin-state transitions in a novel bimetallic 2D coordination polymer, revealing both periodic and aperiodic spin-state concentration waves associated with Devil's staircase phenomena.

Contribution

It provides the first detailed structural and magnetic analysis of Devil's staircase behavior in a spin crossover material, highlighting the coexistence of periodic and aperiodic spin-state waves.

Findings

Identification of multi-step spin crossover with distinct spin-state waves

Observation of incommensurate aperiodic spin-state ordering

Structural refinement confirming complex spin-state arrangements

Abstract

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.