  The evolution of the bismuth crystal structure upon excitation of its
A$_{1g}$ phonon has been intensely studied with short pulse optical lasers.
Here we present the first-time observation of a hard x-ray induced ultrafast
phase transition in a bismuth single crystal, at high intensities (~$10^{14}$
W/cm$^2$). The lattice evolution was followed using a recently demonstrated
x-ray single-shot probing setup. The time evolution of the (111) Bragg peak
intensity showed strong dependence on the excitation fluence. After exposure to
a sufficiently intense x-ray pulse, the peak intensity dropped to zero within
300fs, i.e. faster than one oscillation period of the A1g mode at room
temperature. Our analysis indicates a nonthermal origin of a lattice
disordering process, and excludes interpretations based on electron-ion
equilibration process, or on thermodynamic heating process leading to a plasma
formation.
