We show that the anharmonic decay of large-amplitude coherent phonons in a solid generates
strongly enhanced squeezing of the phonon modes near points of the Brillouin zone where energy
conservation in the three-phonon decay process is satisfied. The squeezing process leads to temporal
oscillations of the mean-square displacement of target modes in resonance with the coherent phonon,
which are characteristic of coherent phonon decay and do not occur in the decay of a phonon in a
well-defined number state. For realistic material parameters of optically excited group-V semimetals,
we predict that this squeezing results in strongly enhanced oscillations of the x-ray diffuse scattering
intensity at sharply-defined values of the x-ray momentum transfer. Numerical simulations of
the phonon dynamics and x-ray diffuse scattering in optically-excited bismuth, using harmonic and
anharmonic force parameters calculated with constrained density functional theory, demonstrate
oscillations of the diffuse scattering intensity of magnitude 10-20% of the thermal background at
points of the Brillouin zone, where resonance occurs. Such oscillations should be observable using
time-resolved optical-pump/x-ray-probe facilities available at current x-ray free-electron laser
sources.