Thermal spin transition in[Fe(NH2−trz)3]Br2investigated by spectroscopic ellipsometry

Abstract

We investigated the thermal hysteresis of a pelleted sample of the spin-crossover compound $\left[\mathrm{Fe}{\left(\mathrm{N}{\mathrm{H}}_2\text{−}\mathrm{trz}\right)}_3\right]{\mathrm{Br}}_2$ by means of spectroscopic ellipsometry, in the temperature range $264–358\mathrm{K}$. The ellipsometric parameters $(\psi ,\Delta )$ have been recorded in the optical range $240–1000\mathrm{nm}$. The corresponding absorption and dispersion spectra show temperature-invariant isobestic points located at 240 and $291\mathrm{nm}$, respectively. We found that the high-spin-fraction data, derived from the integrated absorption curves, are in excellent agreement with the magnetic data recorded on the same sample. We also investigated the consistency of the optical data by application of the Kramers-Kronig relations, which are well obeyed above $\sim 450\mathrm{nm}$. All these results demonstrate that the spectroscopic ellipsometry is well adapted to characterize the spin-crossover transition. We also expect that this nondestructive technique will be highly relevant to investigate the physical properties of thin films of switchable molecular solids, involving a change in the electronic properties upon a thermally or photoinduced phase transition.