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Solid phase transition progresses with optical strain wave propagation Discovered in research into optically-induced phase transition of λ-Ti3O5 Results obtained by the University of Tokyo and the University of Tsukuba

2021.04.28

Professor Shin-ichi Ohkoshi, School of Science, the University of Tokyo, and Professor Hiroko Tokoro, Faculty of Pure and Applied Sciences, the University of Tsukuba, are driving research on photoinduced phase transition of lambda trititanium pentoxide (λ-Ti3O5), which is the only metal oxide to display photoinduced phase transition at room temperature, but as a result of an ultrahigh-speed X-ray powder diffraction experiment (time resolution: 500fs) using the Swiss X-ray free-electron laser (SwissFEL), they have now observed, for the first time, the structure within trititanium pentoxide (Ti3O5) crystals modified at 500fs (femtoseconds) as a result of light irradiation, and phase transition advancing as a result of “strain waves“ spreading at picosecond order within crystals from the laser-exposed Ti3O5 surface. These results were published in Nature Communications (February 23).

Research on the optical phase transition of solids is attracting attention on both the academic and industrial fronts from the perspective of optical memory and optical switching materials. In 2010, Ohkoshi and his colleagues discovered λ-Ti3O5, a new type of metal oxide capable of optical switching (writing and erasing) at room temperature. In addition to the photoinduced phase transition phenomenon, this λ-Ti3O5 was also found to have current-induced phase transition and long-term heat-storage capabilities, and in 2019, Ohkoshi and his colleagues also developed block-type λ-Ti3O5 as a heat-storage ceramic, with the goal of applying it in industrial settings.

Ohkoshi and Tokoro have now obtained results in research on the photoinduced phase transition of λ-Ti3O5, which is the only metal oxide that displays photoinduced phase transition at room temperature. They are engaging in this research along with Dr. Céline Mariette of the Institut de Physique de Rennes, Universite de Rennes, France, and others as one part of the international joint research of the International Associated Laboratory IM-LED, CNRS, France.

 

In this research project, changes to the crystalline structure of Ti3O5 immediately after being irradiated with light were measured in SwissFEL’s femtosecond X-ray powder diffraction experiment. As a result of observing that the position of the titanium ions making up the valence band changed instantaneously due to femtosecond laser illumination, and observing time changes in the lattice volume, including within the interior of the crystals that was not exposed to light, as well as in the phase rate, due to partial volume changes, the research group found that the lattice volume, small strain rate and phase rate of λ-Ti3O5 and β-Ti3O5 all increased linearly toward 0-16 ps (picoseconds).

On the basis of that outcome, it was inferred that when the lattice volume of λ-Ti3O5 increases, the transition from β-Ti3O5 to λ-Ti3O5 occurs simultaneously, and the diffusion of the strain (the strain waves) is involved in the phase transition.

 

Through analysis based on the elastic body model, it was possible to obtain results that closely recreated lattice deformation. It takes 16 ps for the wave surface of acoustic strain waves to reach 100nm of the crystal interface. The group concluded that this is the result of the waves coinciding with the minimum point of β-Ti3O5 volume, and being compressed by the portion that has changed to λ-Ti3O5, which is larger in volume.

 

Based on the above results, it was inferred that switching of Ti3O5 nanocrystals occurs at a picosecond scale at the same time as the propagating strain wave surface, and is incomparably faster than phase transition resulting from thermal diffusion (~ 100 nanoseconds).

Professor Ohkoshi and his colleagues believe this outcome suggests that with the optical phase transition of λ-Ti3O5, the transition is propagated in a coherent state, in contrast to the random growth that is induced by heat. In the conventional nucleation and nuclear growth models, in many cases the time dependency of the phase rate of new phases changes in a sigmoid curve shape. By comparison, the changes that the research group observed were linear and extremely unique.

In addition, based on the results of measurements taken at the European Synchrotron Radiation Facility (ESRF), it was found that the phase transition resulting from thermal diffusion occurs on a time scale of around 100 nanoseconds, which is clearly slower than the transition resulting from strain waves.

This research was carried out by the IM-LED research team at the launch of the first time-resolved X-ray diffraction measurements at the large SwissFEL facility, and it also verified that if the latest XFEL light source is used, it is possible to study interatomic movement and lattice strain at the femtosecond scale in real time.

This article has been translated by JST with permission from The Science News Ltd.(https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.

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