s Single Laser Shot Spin State Switching of {FeII(pz)[Pt(CN)4]} Inside Thermal Hysteresis Studied by X-ray Diffraction

Background: Photoinduced phase transitions represent a new way to control physical properties of materials by light. Achieving macroscopic and complete switching with a single laser pulse is of great interest for various applications, especially in the bistable hysteresis domain. Spin-crossover materials are prototype photoactive systems and various physical properties (magnetic, chromic, dielectric...) switch by light between low spin and high spin states. Objective: It is of fundamental interest to understand how the photoswitching initiated by a single ns laser pulse occurs inside the thermal hysteresis. For this purpose, we investigate the photoresponse of the spincrossover {FeII(pz)[Pt(CN)4]} material. Method: The photoinduced transformation inside the thermal hysteresis is investigated by combining the complementary optical microscopy and x-ray diffraction techniques, which are sensitive electronic and structural reorganizations. Results: These single-crystal studies show that a complete conversion from low spin to high spin states can be reached with a single laser shot above a threshold excitation density, as previously reported by Bousseksou by Raman spectroscopy. The structural reorganization after a single laser pulse is similar to the one observed during the complete thermal conversion from low spin to high spin states. Partial conversions, obtained with weaker excitation densities, are associated with the formation of high spin domains, evidenced by x-ray diffraction. Conclusion: Our results show that the photoinduced phase transition is stabilized by an important volume change. In addition, the non-linear response to light excitation density indicates that the process is mainly driven by the temperature jump of the crystal following laser excitation.