Laser‐Driven Transient Phase Oscillations in Individual Spin Crossover Particles

Abstract

An unusual expansion dynamics of individual spin crossover nanoparticles is studied by ultrafast transmission electron microscopy. After exposure to nanosecond laser pulses, the particles exhibit considerable length oscillations during and after their expansion. The vibration period of 50–100 ns is of the same order of magnitude as the time that the particles need for a transition from the low‐spin to the high‐spin state. The observations are explained in Monte Carlo calculations using a model where elastic and thermal coupling between the molecules within a crystalline spin crossover particle govern the phase transition between the two spin states. The experimentally observed length oscillations are in agreement with the calculations, and it is shown that the system undergoes repeated transitions between the two spin states until relaxation in the high‐spin state occurs due to energy dissipation. Spin crossover particles are therefore a unique system where a resonant transition between two phases occurs in a phase transformation of first order.

Lattice Defects in Sub-Micrometer Spin-Crossover Crystals Studied by Electron Diffraction

“…Electron irradiation did not lead to any observable changes of the size or lattice parameters of the particles so that changes of the spin state by electron irradiation can be excluded. Since we have also studied the SCO under nanosecond laser pulses without additional heating of the sample, 13,14 the observed changes of the lattice parameter are not due to thermal expansion, which is small in this material in comparison to the expansion during the phase transition. 24 The measurable expansion originates from only the spin conversion.…”

“…23 A previous study by TEM imaging has already shown the expansion of similar SCO particles during the LS → HS transformation. 13,14 Figure 3 shows the anisotropic expansion of the lattice by imaging and electron diffraction (in reciprocal space, the expansion appears as a contraction of the pattern). During the transformation, the lattice parameter in the b-direction ((020)reflection) increases by 5.7% (error ±0.3%) which is roughly in accordance with the expansion of 6.3% as measured previously in [Fe(Htrz) 2 trz](BF 4 ) particles by X-ray diffraction.…”

“…Figure 4 shows the evolution of diffraction patterns when pristine (not previously heated) SCO particles at room temperature were irradiated with a sequence of laser pulses. In previous experiments by ultrafast TEM, 13,14 it was shown that the LS → HS transformation occurs within some tens of nanoseconds whereas the transformation HS → LS occurs within the cooling time at the microsecond scale (depending on the power of the laser pulses and on the thermal conductivity of the substrate). Since the present experimental conditions are similar (laser power on the sample, wavelength, and duration of laser pulses), we can assume that the transformation between the spin states happened after each laser pulse.…”

“…By contrast, diffraction and imaging by transmission electron microscopy (TEM) techniques allow studies on the microscopic scale with higher spatial resolution. The first TEM studies by ultrafast transmission electron microscopy (UTEM) have already revealed time-resolved information about the expansion dynamics of individual SCO particles under laser pulses. − …”

“…These heating–cooling cycles were repeated several times to observe a possible influence of single or multiple phase transformations on the defect structure as it has already been indicated in X-ray studies . A previous study by TEM imaging has already shown the expansion of similar SCO particles during the LS → HS transformation. , …”