M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

7 2 H I G H L I G H T S 2 0 2 3 I

Monitoring ultrafast dynamics behind non-reversible photo- induced phase transitions

A new, time-resolved X-ray diffraction-based technique that can monitor non-reversible photo- induced phase transformations on ultrashort timescales was applied to the photo-induced phase transition of a Prussian Blue Analogue, revealing that permanent and complete conversion occurs as fast as 150 ps, as a result of electronic reorganisation and cooperative elastic interactions.

In the search for new photonic devices, the use of ultrashort laser pulses as external stimuli is very promising, since it opens the way to ultrafast and contactless control in light-based technologies. This, however, necessitates photo-active materials whose physical properties can be switched very rapidly and permanently using light. In this scope, Prussian Blue Analogues (PBAs) represent very attractive multifunctional compounds, as their magnetism, ionic conduction or optical properties can be changed using ultrashort light pulses. This work studied the dynamics associated with the photo-induced phase transition in a Co-doped RbMnFe PBA, where the transformation is persistent at room temperature and is accompanied with symmetry and volume changes [1,2].

Monitoring the ultrafast dynamics associated with this non-reversible photo-transformation is, however, non- trivial. Indeed, conventional time-resolved techniques are not applicable to non-reversible phenomena as they usually consist of stroboscopic measurements performed on the same sample, assuming that the sample recovers its initial state between two consecutive measurements. A new technique was thus developed for time-resolved X-ray diffraction in close collaboration with beamline ID09 and the ESRF Sample Environment unit, where measurements are performed on crystals streaming through a liquid jet (Figure 53). In this experimental configuration, named streaming powder diffraction, each measurement is performed on a new batch of crystals, and X-ray diffraction maps the temporal reorganisation during the non-reversible phase transition.

For the studied PBA, the time-resolved measurements on ID09 revealed that the conversion from tetragonal low temperature (LT) to photo-induced cubic phase is complete within 150 ps only, which indicates an elastic- driven phase transition (Figure 54). The photo-induced cubic phase, stable at room temperature, appears above a threshold laser fluence due to the strong elastic cooperativity of PBAs. Indeed, within a crystal, a critical fraction of initially photo-converted sites, giving rise to a large volume strain, is required to destabilise the initial tetragonal phase towards the cubic phase.

Fig. 53: Experimental setup of streaming powder diffraction

at ID09. PBA crystals are dispersed in a solution flowing

through a liquid jet, where a first laser pump pulse

initiates the phase transition, probed by a delayed X-ray

pulse. Non-reversible photo- conversion from the initial low

temperature (LT) tetragonal to the photo-induced cubic

high temperature (HT) phase is directly mapped by X-ray

diffraction. Insertion of a cooling device ensures that

crystals are in the initial phase for the measurements.