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Fig. 55: Time-resolved powder patterns for different laser/X-ray time delays. The difference in the curves for laser ON laser OFF are shown below the powder intensities (oscillating around 0). Significant changes are observed, in particular the increase in intensity of distinct Bragg peaks of the high-volume metallic λ phase (around 1.3 and 2.3 Å-1). The λ phase fraction (right) Rietveld refinement. Different photon energy and grazing incidence angles were used to vary the excitation ratio, revealing the strain-driven transformation on ps time scale, which clearly precedes the late transformation driven by heat diffusion around 100 ns.

Sweeping semiconductor-to-metal transition following laser excitation of Ti3O5 In the quest to understand structural transformations in laser-excited solids, the semiconductor-to- metal phase transition in nanocrystals of titanium pentaoxide (Ti3O5) was studied by time-resolved powder diffraction with high spatial and temporal resolution.

Materials that can be transformed by changes in temperature, pressure or light are of great interest for technological applications such as optical and heat storage. Pulsed laser-driven transformations allow for ultrafast switching and exploring non-equilibrium states of matter.

One of the challenges in ultrafast material science is to trigger a phase transition like the semiconductor-to-metal transition studied here, with short pulses of light. For that, it is important to probe the structural changes induced by the photoexcitation of the material. This is challenging experimentally and theoretically as illustrated in this work by the semiconducting-to-metal phase transition in nanocrystals of titanium pentaoxide (Ti3O5). In this material, the absorption of photons causes the transfer of electrons into empty orbitals on the Ti-Ti dimers. In less

than one microsecond, a large fraction of the sample is converted to the metallic phase. Previous studies have described strain propagation [1-3], but were limited to the simpler case when no phase transition occurs. Earlier studies of Ti3O5 did not have the time resolution [4], or the structural sensitivity [5] to observe the propagation of the strain wave driven by changes inside the unit cell and their role in the macroscopic phase transition. In brief, the nature and the order of events in the transition was not understood until now.

A pellet of Ti3O5 semiconducting nanocrystals was excited by ultrashort 800-nm laser pulses and probed by powder X-ray diffraction (PXRD) with single pulses of X-rays. The experiments were performed on beamline ID09 (Figure 55) and at the Bernina beamline at SwissFEL (Figure 56). These measurements made it possible to probe the lattice deformation from femtoseconds to microseconds. The high-quality powder X-ray diffraction patterns allowed for full Rietveld refinement for both phases, thus providing an extremely detailed picture. The structural parameters included unit cell volume and interatomic distances as a function of time, as well as the fraction of the two phases.

The semiconducting-to-metal transition proceeds in three steps. First, on the sub picosecond time scale, before the unit cells have time to expand, local structural changes