Spin-crossover materials: Getting the most from x-ray crystallography

Abstract

The physical phenomenon of spin crossover in molecular crystals is a multiscale process whose properties rely on the supramolecular organization of the spin crossover active elements, their interactions within the crystal packing and their dynamics. The delicate balance between short-range and long-range structural reorganization upon the spin transition is at the origin of remarkable and fascinating physical phenomena such as thermal, light induced and pressure induced hysteresis, multistep transitions and multimetastablility. A complete understanding of the various phenomena associated to spin crossover requires a comprehensive and thorough characterization of the overall structural architecture at all scales which goes beyond the average static crystal structure. This tutorial surveys the practical use of X-ray crystallography notably in non-ambient conditions to provide a direct view of the physical processes operating in spin crossover molecular solids from bulk single crystals to nano-crystalline powder. Advanced X-ray crystallography methods are reviewed and illustrated with a series of model examples.

Dynamical Mechanical Analysis and Micromechanics Simulations of Spin-Crossover@Polymer Particulate Composites: Toward Soft Actuator Devices

“…The elasticity law for the nontransforming polymer matrix is where σ, e, and C are stress, strain, and the stiffness matrices, respectively. The elasticity law for the transforming SCO particles before and after transformation is = C e p p p (8) = C e e ( ) p p p p T (9) where e p T is the homogeneous particle transformation strain. The subscripts m and p denote matrix and particles, respectively.…”

Hidden ordered structure in the archetypical Fe(pyrazine)[Pt(CN)4] spin-crossover porous coordination compound

“…The configuration of the pyrazine rings has important consequences for the most characteristic feature of this material: the spin-crossover transition. It is not unusual to observe order-disorder processes coupled to SCO, 39,40 in most cases related to counterions or solvent molecules present in the structure, 41,42 and more rarely to ligands. 43,44 An order-disorder process can affect the spin transition in several ways, including the temperature of the transition and its abruptness (and hysteresis), and this has been taken into account in theoretical models of the spin transition.…”

Spin Crossover Nanoparticles

“…Spin crossover (SCO) complexes present different magnetic, optical, electrical, and structural properties depending on the electronic configuration that can switch from the low spin (LS) to the high spin (HS) state and vice versa by changes of the temperature, pressure, , and magnetic fields, and also by light irradiation , and hard X-ray excitation . This spin transition can therefore be followed by different experimental techniques according to the changes that occur: magnetic susceptibility measurements, Mössbauer spectroscopy, UV–vis–NIR absorption and vibrational spectroscopy, X-ray diffraction, synchrotron radiation studies, heat capacity measurements, nuclear magnetic resonance, etc. As shown in Figure , the electronic configuration change between the LS and the HS state is for instance generally associated with a visually detectable color change.…”

Pressure Tuning of Coupled Structural and Spin State Transitions in the Molecular Complex [Fe(H2B(pz)2)2(phen)]

“…), are widespread in crystalline solids and have been investigated in various fields of physics, chemistry, mineralogy, and materials science. − Since phase transitions in crystalline materials always involve some lattice distortion, the most common physical effect that couples them is lattice strain . In this context, spin state transitions in molecular complexes of 3d 4 –3d 7 ions appear particularly interesting due to the large volume strains involved (typically 1–10%). − Thanks to this high dilation, the SCO phenomenon can strongly couple to various instabilities, leading to a great diversity of structure–property relationships. Among the numerous reported examples (see, for example, refs and and references therein), we can mention here the benchmark complexes [Fe II (ptz) 6 ]­(BF) 4 (ptz = 1- n -propyl-tetrazole), in which the SCO is coupled to a ferroelastic transition, , and [Fe II (2-pic) 3 ]­Cl 2 ·EtOH (pic = picolylamine), in which the SCO is coupled to successive order–disorder transitions …”

Preliminary observations of the interplay of radiation damage with spin crossover

“…Ndiaye et al (2021)], as by its very nature this approach neglects many intra-molecular degrees of freedom. For any real spin crossover material however, change of properties associated with the change of spin states can neither be reduced to a behaviour of an isolated individual molecule nor simplified to mechanistic models; this is why, together with theoretical modelling, the structural experiment remains the most important source of information on the microscopic processes comprising spin crossover phenomena (Pillet, 2021).…”