  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.
