Flatband λ-Ti3O5 towards extraordinary solar steam generation.

Time-resolved transmission electron microscopy for nanoscale chemical dynamics

Abstract Time-resolved studies with temporal resolution that separate molecular level dynamics from macroscopic changes, allow clear distinction between the time scales of the different degrees of freedom involved. Cooperative molecular switching in the solid state is exemplified by spin crossover phenomenon in crystals of transition metal complexes. Here we show the existence of a delay between the crystalline volume increase, and the cooperative macroscopic switching of molecular state. Using 100 ps X-ray diffraction, we track the molecular spin state and the structure of the lattice during the photoinduced low spin to high spin transition in microcrystals of [Fe III (3-MeO-SalEen) 2 ]PF 6 . Model simulations explain the phenomenon with thermally activated kinetics governed by local energy barriers separating the molecular states. Such behaviour is different from that encountered in materials with no local energy barriers, where phase transformation can occur simultaneously with propagation of strain. Broadly, this motivates an optimised material design, scalable with size and intrinsic energetics. 

/pdf/dynamical-limits-for-the-molecular-switching-in-a-1yrfcsar.pdf

Dynamical limits for the molecular switching in a photoexcited material revealed by X-ray diffraction

Photo-induced structural dynamics in molecular materials often involve two main components; local molecular distortions induced by the self-trapping of electronic excited state and incoherent lattice dynamics due to energy dissipation. The latter component frequently hinders clear manifestation at macroscopic scale of local self-trapped entities and their coupling to the lattice. The studied photoactive MnFe Prussian blue analogue exhibits no thermal expansion, which allows observing long-range lattice dynamics induced by the structural trapping of photo-induced charge-transfer (CT) states. The self-trapping of the photo-induced MnIIIFeII → MnIIFeIII is responsible for the emergence of various physical properties. At equilibrium, the lattice volume expansion and the ferroelastic Jahn–Teller distortion associated with the charge-transfer-based phase transition, evolve in a coupled way. Here, we use time resolved X-ray diffraction to investigate the anisotropic out-of-equilibrium lattice dynamics triggered by the self-trapping of photo-induced MnIIIFeII → MnIIFeIII CT. The observed dynamics evidence a decoupling of both types of lattice deformations in the time domain, with an impulsive lattice response driven by the local Jahn–Teller reorganization and a displacive volume expansion controlled by the long-lived photo-induced CT small-polarons, due to the population of antibonding orbitals centered on Mn–N bonds in the (a,b) lattice plane.

Out-of-equilibrium lattice response to photo-induced charge-transfer in a MnFe Prussian blue analogue

Heat storage materials preserving large amounts of thermal energy over long time frames enable the storage of waste energy from thermal energy generation or excess energy generated by intermittent and renewable sources. Trititanium pentoxide, Ti3O5, has emerged as a promising material with a high energy storage density and the capability of releasing the stored energy on demand, for example, by the application of pressure. During the release, the higher energy λ polymorph undergoes an isostructural phase transition to the lower energy β polymorph. Accurate predictions of the heat storage properties of the two polymorphs using first-principles methods are not yet available. Using hybrid density functionals and the recently proposed r2SCAN meta-generalized gradient approximation functional augmented by the D3 dispersion correction, we offer a comprehensive description of the electronic ground state of this material. We find that r2SCAN-D3 provides a similar accuracy for structural and thermodynamic properties compared to hybrid functionals at an order of magnitude reduced computational cost. Inclusion of the dispersion interaction is essential to obtain correct energy differences between the two phases. The r2SCAN-D3 method is applied to the investigation of the reaction mechanism of the phase transition, resulting in full characterization of the transition state. We highlight the good performance of the r2SCAN-D3 functional by predicting the phase transition temperature and phase transition enthalpy of the heat storage system and recommend this method for high-throughput investigations of heat storage materials. 

First-Principles Investigation of Electronic Properties and Phase Transition of Ti3O5

One of the main challenges in ultrafast material science is to trigger phase transitions with short pulses of light. Here we show how strain waves, launched by electronic and structural precursor phenomena, determine a coherent macroscopic transformation pathway for the semiconducting-to-metal transition in bistable Ti3O5 nanocrystals. Employing femtosecond powder X-ray diffraction, we measure the lattice deformation in the phase transition as a function of time. We monitor the early intra-cell distortion around the light absorbing metal dimer and the long range deformations governed by acoustic waves propagating from the laser-exposed Ti3O5 surface. We developed a simplified elastic model demonstrating that picosecond switching in nanocrystals happens concomitantly with the propagating acoustic wavefront, several decades faster than thermal processes governed by heat diffusion. Ultrafast control of materials draws interest. Here, the authors extend X-ray powder diffraction to the femtosecond timescale to follow the photo-induced semiconductor to metal transition in titanium pentaoxide, observing a phase front that moves at the speed of sound and proposing a little explored mechanism.

/pdf/strain-wave-pathway-to-semiconductor-to-metal-transition-2854umnvkj.pdf

Strain wave pathway to semiconductor-to-metal transition revealed by time-resolved X-ray powder diffraction

Thanks to the remarkable developments of ultrafast science, one of today's challenges is to modify material state by controlling with a light pulse the coherent motions that connect two different phases. Here we show how strain waves, launched by electronic and structural precursor phenomena, determine a macroscopic transformation pathway for the semiconducting-to-metal transition with large volume change in bistable Ti$_3$O$_5$ nanocrystals. Femtosecond powder X-ray diffraction allowed us to quantify the structural deformations associated with the photoinduced phase transition on relevant time scales. We monitored the early intra-cell distortions around absorbing metal dimers, but also long range crystalline deformations dynamically governed by acoustic waves launched at the laser-exposed Ti$_3$O$_5$ surface. We rationalize these observations with a simplified elastic model, demonstrating that a macroscopic transformation occurs concomitantly with the propagating acoustic wavefront on the picosecond timescale, several decades earlier than the subsequent thermal processes governed by heat diffusion.

/pdf/strain-wave-pathway-to-semiconductor-to-metal-transition-1b850bub18.pdf

Strain Wave Pathway to Semiconductor-to-Metal Transition revealed by time resolved X-ray powder diffraction

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Strain wave pathway to semiconductor-to-metal transition revealed by time-resolved X-ray powder diffraction

Strain Wave Pathway to Semiconductor-to-Metal Transition revealed by time resolved X-ray powder diffraction