  We utilize near-infrared femtosecond pulses to investigate coherent phonon
oscillations of Ca2RuO4. The coherent Ag phonon mode of the lowest frequency
changes abruptly not only its amplitude but also the oscillation-phase as the
spin order develops. In addition, the phonon mode shows a redshift entering the
magnetically ordered state, which indicates a spin-phonon coupling in the
system. Density functional theory calculations reveal that the Ag oscillations
result in octahedral tilting distortions, which are exactly in sync with the
lattice deformation driven by the magnetic ordering. We suggest that the
structural distortions by the spin-phonon coupling can induce the unusual
oscillation-phase shift between impulsive and displacive type oscillations.
