Piezo‐ and Photo‐Crystallography Applied to Spin‐Crossover Materials

Abstract

International audienceThis chapter reviews recent piezo- and photo-crystallographic approaches to spin-crossover (SCO) materials highlighting the role of structural features in the SCO process. The SCO phenomenon is highly pressure-sensitive. The chapter emphasizes the need for crystallography under high pressure, that is piezo-crystallography, in the field of SCO. It illustrates that photo-crystallography experiments are not merely data collections under laser irradiation. Accordingly, it is shown that through varying different external parameters (such as temperature, laser irradiation wavelength or power) and careful investigation of the structural signatures, key information on the different processes can be obtained. For instance, it has been observed that for some systems the nature of photoinduced state can be triggered by choosing appropriate excitation wavelength

A critical review of the T(LIESST) temperature in spin crossover materials − What it is and what it is not

“…An example of intercept between the two regimes for non-cooperative SCO materials was also reported for the metal diluted [Fe x Mn 1-x (3-bpp) 2 ](NCSe) 2 series [45]. Another example was encountered for the pure iron(II) SCO chain [Fe(abpt) 2 …”

“…This change is associated with drastic modifications of the ligand field strength, molecular and crystal structures, microscopic and macroscopic volumes, magnetic and electronic properties such as colour, conductivity or dielectric characteristics. Among the different perturbations used, temperature and light are probably the most studied in comparison with other stimuli such as pressure for example [2]. While increasing the temperature can basically be viewed as a source of energy allowing electrons to be promoted from t 2g to e g orbitals, irradiation by light can act in different ways thus offering a wider range of possibilities.…”

Effects of Internal and External Pressure on the [Fe(PM-PEA)2(NCS)2] Spin-Crossover Compound (with PM-PEA = N-(2′-pyridylmethylene)-4-(phenylethynyl)aniline)

“…These features are confirmed by considerations based on thermodynamics and theoretical models [8][9][10][12][13][14] that also predict some interesting consequences such as the decrease of the hysteresis width with increasing pressure. From the experimental point of view, many counter-examples have been reported so far [15]. For example, non-expected hysteresis broadening [16] as well as irreversible behaviors were reported for some compounds [10,17].…”

“…Even in a family of very similar complexes, a variety of high pressure behaviors can be observed. This is the case of the [Fe(PM-L) 2 (NCS) 2 ] family (with PM-L = N-(2 1 -pyridylmethylene)-4-(aromatic function L)), showing that expectations can be fulfilled, or not, depending on the nature of the L part of the ligand [15,18]. Furthermore, in some cases, the application of pressure on a SCO material can even favor the HS state.…”

A High Pressure Investigation of the Order-Disorder Phase Transition and Accompanying Spin Crossover in [FeL12](ClO4)2 (L1 = 2,6-bis{3-methylpyrazol-1-yl}-pyrazine)

“…The latter point is particularly pertinent in the case of molecular SCO materials where both the chemical connectivity and the intermolecular interactions have strong influence on properties: small changes in chemistry of ligands can significantly alter switching properties, while different polymorphs of the same molecule can show vastly different SCO properties [15]. Pressure has also been used to decouple crystallographic phase transitions from SCO properties in order to probe their interrelationship [16].…”