Abstract
In this work we present a detailed study showing the importance of the Kubelka-Munk (KM) correction in the analysis of diffuse reflectivity measurements to characterize spin crossover compounds. Combined reflectance and magnetic susceptibility measurements are carried out as a function of temperature or time to highlight the conditions under which this correction becomes critical. In particular, we investigate the influence of the color contrast between the two spin states on the reflectance measurements. Interestingly, the samples’ contrast seems to play an important role on the spin-like domain structure as suggested by the symmetry of the FORC diagrams. These latest results are discussed within the framework of Classical Preisach model (CPM).
Highlights
Reflectance measurements have been widely used in the past to characterize the switching properties of Spin crossover (SCO) compounds, assuming a linear relationship between the remittance, R and the fraction of SCO complexes in a given spin state
Suggests a different dependence, but the associated shift of the critical temperatures for the thermal hysteresis loop is not significant after Kubelka-Munk correction or is lower than the possible kinetic effects). This correction becomes very important in the case of more complex studies, like First Order Reversal Curves (FORC), higher order hysteresis curves (MORC, minor cycles), or relaxation process, significantly modifying the interpretation of the measured data
The HS-LS conversions were simultaneously recorded from magnetic susceptibility and reflectance measurements
Summary
Due to the color change that usually accompanies the spin transition (e.g., from colorless at room temperature to dark purple at low temperature for some Fe(II) SCO complexes), the fraction of complexes in the high-spin state has frequently been monitored by optical measurements using diffuse reflectivity (reflectance) as many of these compounds are obtained in the form of powders. From Equation (2), one sees well that the Kubelka-Munk function gives a relation between the volume concentration of the optical centers and the reflectance which is obtained starting from a measurement at the surface. When reflectance measurements are carried out at a wavelength characteristic of the absorption of the LS species, the concentration of the optical centers is equal to the low spin fraction.
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