X-ray diffraction investigation of a spin crossover hysteresis loop

Abstract

et al.. X-ray diffraction investigation of a spin crossover hysteresis loop.The nature and the mechanism of the magnetic hysteresis for the thermal spin crossover exhibited by an iron(II) compound is investigated by mean of variable temperature powder and single crystals X-ray diffraction. The unit cell temperature dependence clearly evidences the amplitude of the strong structural re-arrangement that accompanied the spin crossover -corresponding to a variation of 8.6 % for a unit cell parameter -as well as the structural hysteresis width. To this regard, the present X-ray study reveals significant differences in the spin crossover features according to the nature of the sample -powder or single crystal -that should be taken into account in the physical properties analysis. Concerning the interplay between structural and magnetic transitions, quenching effects show that the structural transition and the spin crossover are indissociable. Furthermore, investigations of the mechanism itself of the thermal spin crossover confirm the presence of spin-like domains in the conversion region, either in the cooling or in the warming loops. The non-dependence with temperature of these domains inside the hysteresis loop demonstrates the stability of the microscopic and macroscopic structures in the corresponding thermodynamic conditions. This result is of interest in the context of the potential use of hysteresis loops to obtain high temperature photo-conversion.

Spin-crossover Compounds with Wide Thermal Hysteresis

“…29 Compound 3 undergoes spin crossover just above 200 K and has been crystallographically characterized in both spin states, with its high-spin and low-spin forms adopting different space groups. 30 The transition hysteresis in 3 varies strongly depending on how it is measured, from ¦T = 19 K by single crystal unit cell measurements, 31 to ¦T = 6065 K by magnetic susceptibility and EPR data from powder samples. 16,32 This discrepancy has not been explained in detail but may relate to the different patterns of structural defects and domain sizes in these types of sample.…”

“…16,32 This discrepancy has not been explained in detail but may relate to the different patterns of structural defects and domain sizes in these types of sample. 31 Comparison with related compounds implies that a short intermolecular CH£S contact is the structural origin of the transition cooperativity ( Figure 2). This type of contact is shown by several complexes related to 3 with different pendant substituents, which are all effectively isostructural.…”

Intermolecular control of thermoswitching and photoswitching phenomena in two spin-crossover polymorphs

“…6(a)] and the light-induced hysteresis reported in different system. 31,32,34,41,54 In BIA II, a gradual and homogeneous variation of the spin state is associated with the progressive motion of the Bragg reflections from a purely HS lattice to a purely LS one [see Fig. 6(d)]; such a homogeneous relaxation was observed in another low cooperative spin crossover complex by neutron diffraction.…”

“…It may give rise to hysteretic response under the effect of control parameter such as temperature (LITH, light-induced thermal hysteresis), 28 pressure (LIPH, light-induced pressure hysteresis), 29 or light excitation density (LIOH, light-induced optical hysteresis). 30,31 There are few SCO crystals for which the spin crossover is associated with symmetry breaking at thermal equilibrium [32][33][34][35] or in the photoinduced state ( [36][37][38] and references therein). For most of the SCO systems, the spin-state switching occurs in one step without any symmetry change, as it is the case for the two polymorphs studied here.…”

Complete and incomplete spin transitions in 1D chain iron(ii) compounds

“…In contrast, the CCI scale could be used for each structure. The concept of the CCI was tested on four SCO complexes that belong to the series [FeL 2 (NCS) 2 ] with L = btz 37 (2,2 0 -bis-4,5-dihydrothiazin, gradual spin transition), phen 38 (1,10-phenanthrolin, abrupt spin transition), dpp (dipyrido[3,2-a:2 0 3 0 -c]phenazine, hysteresis 40 K) and pm-pea 36 (N-2 0 -pyridylmethylene-4-phenylethynyl, hysteresis 40 K). For this series the increasing cooperative interactions are correlated with an increasing number of intermolecular contacts.…”