We utilize near-infrared femtosecond pulses to investigate coherent phonon oscillations of ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$. The coherent $A_g$ 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 $A_g$ 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.

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• Phonons
• Antiferromagnets
• Mott insulators
• DFT+U
• Ultrafast pump-probe spectroscopy