  We present results from the SPring-8 Angstrom Compact free electron LAser
(SACLA) XFEL facility, using a high intensity ($\sim\!10^{20}\,$W/cm$^2$) X-ray
pump X-ray probe scheme to observe changes in the ionic structure of silicon
induced by X-ray heating of the electrons. By avoiding Laue spots in the
scattering signal from a single crystalline sample, we observe a rapid rise in
diffuse scattering, which we attribute to a loss of lattice order and a
transition to a liquid state within 100 fs of irradiation, a timescale which
agrees well with first principles simulations, but is faster than that
predicted by purely inertial behavior. This method is capable of observing
liquid scattering without masking or filtering of signal from the ambient
solid, allowing the liquid structure to be measured throughout and beyond the
phase change.
