C. Chong

Electronic and structural aspects of spin transitions observed by optical microscopy. The case of [Fe(ptz)6](BF4)2.

The colorimetric analysis of images recorded with an optical microscope during the onset of the spin crossover transformation allows monitoring separately the involved electronic and structural aspects, through the separation of resonant absorption and scattering effects. Complementary information can also be obtained by using the polarized modes of the microscope. These potentialities are illustrated by the observation of [Fe(ptz)(6)](BF(4))(2) single crystals during the onset of the thermal transitions in the 110-140 K range. We characterized the interplay between the electronic (HS <--> LS) and structural (order <--> disorder) transformations. Elastic stresses and mechanical effects (hopping, self-cleavage) generated by the volume change upon electronic transition are also illustrated, with their impact on the photoswitching properties of the crystals.

Light-induced phase separation in the [Fe(ptz)6] (BF4)2 spin-crossover single crystal

We present novel insight on like-spin domains (LSD) in cooperative spin transition solids by following the photo-transformation and the subsequent relaxation of a [ Fe(ptz)6] (BF4)2 single crystal in the vicinity of the light-induced instability. Self-organization under light is observed, accompanied by Barkhausen-like noise and jumps which reveal the presence of elastic interactions between LSDs. The light-induced phase separation process is discussed in terms of a dynamic potential providing spinodal instability in the corresponding temperature range. This useful concept is applicable to all types of switchable molecular solids.

The propagation of the thermal spin transition of [Fe(btr)2(NCS)2]·H2O single crystals, observed by optical microscopy

We observed by optical microscopy the propagation of the thermal spin transition in single crystals of the archetype compound [Fe(btr)2(NCS)2]·H2O (btr = 4,4′-bis-1,2,4-triazole). In fresh crystals we could follow well-defined transformation fronts of various shapes. The propagation conditions revealed to depend very sensitively on the structural quality of the crystals. The best observations were obtained on cooling selected fresh crystals, rather thin, protected by an oil layer, under isothermal conditions. They evidenced the progression of a well-defined transformation front through the entire crystal. This is typical for a nucleation and growth process governed by inhomogeneous nucleation. The front shape was simple (circular or linear) and progression was regular in the best cases. Crystals of lesser quality (or already cycled through the transition) exhibited a more complex behaviour, assigned to the impact of internal stresses, either genuine or generated by the previous transformation of the crystals. These observations provide new insights into the physics of like-spin domains, which are briefly discussed.

Theoretical investigations on an axial next nearest neighbour Ising-like model for spin crossover solids: one- and two-step spin transitions

We investigate the thermodynamical properties of an adapted ANNNI-like (axial next nearest neighbour Ising-like) model for 1D spin crossover solids. We performed an exact treatment within the framework of the well known transfer matrix method and derived the thermal evolution of the high spin fraction, the correlation function and the heat capacity. We show that a model of competition of the interactions between similar spin state ions reproduces qualitatively the main features of experimental data: gradual, abrupt and two-step spin crossover transitions. In addition, we found that the intermediate region of the twostep transition exhibits two types of spatial organization, according to the sign of the interactions. Indeed, antiferro-like and ‘antiphase’ (called the phase � 2� in the ANNNI model) structures are obtained. We also found that these two configurations can be distinguished through the thermal evolution of their corresponding correlation functions. The existence of the phase � 2� , predicted by the present model, is in good agreement with recent observations on a spin crossover sample of [{Fe(CNBH3)(4phpy)}2(μ-bpypz)2] for which this phase was evidenced for the first time, in these materials. (Some figures in this article are in colour only in the electronic version)

Light-induced phase separation (LIPS) in [Fe(ptz)6](BF4)2 spin-crossover single crystals: Experimental data revisited through optical microscope investigation

We discuss the available experimental data for light-induced phase separation (LIPS) in the spin-crossover crystal [Fe(ptz)6](BF4)2. They are found in qualitative agreement with a spinodal instability process decribed by a macroscopic mean-field master equation. Sizable discrepancies with the model are discussed in terms of diffusion of light due to structural transformations of the crystal.

Like-spin domains (LSDs) in spin-crossover solids ?

Both theoretical and experimental clarification of the occurrence of like-spin domains (LSD) in spin-crossover solids is reported. In theory, nucleation and growth of LSD requires both a driving force and diffusion. While experimentally, a further evidence of the onset of LSDs in the vicinity of the light-induced instability of a single crystal of [Fe(ptz)6](BF4)2, which is based on the observation of the isothermal relaxation curves recorded after the light is switched off.