In the present work, coherent phonon dynamics in the phase-change materials has been studied using the SACLA free-electron laser facility. 2. Results and discussion The samples used in the experiments were 20 nm thick epitaxial films, deposited on a Si substrate, which was subsequently polished to reach 100 im thickness to enable diffraction measurements in transmission mode. Measurements were performed for the (222) and (200) crystallographical planes. The samples were irradiated by an 800 nm wavelength laser with 30 fs pulse duration, which excited coherent optical phonons in the material. The response of the crystal lattice was observed by an average signal from the diffracted X-ray beam (generated by a X-ray free electron laser at the SACLA facility) for every point from a sequence of the different time delays with sub-picosecond resolution on a multi-port charge-coupled device detector. A typical dependence of the diffracted beam signal intensity vs. time delay is presented in figure 1.