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

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.

Role of antisite disorder on preamorphization swelling in titanate pyrochlores.

Ion irradiation experiments and atomistic simulations were used to demonstrate that irradiation-induced lattice swelling in a complex oxide, Lu2Ti2O7, is due initially to the formation of cation antisite defects. X-ray diffraction revealed that cation antisite formation correlates directly with lattice swelling and indicates that the volume per antisite pair is approximately 12 Å3. First principles calculations revealed that lattice swelling is best explained by cation antisite defects. Temperature accelerated dynamics simulations indicate that cation Frenkel defects are metastable and decay to form antisite defects.