Experimental protocol for the determination of elastic properties at ambient and high pressure by picosecond acoustics technique

This thesis work focused on establishing an experimental protocol for the determination of the materials’ elastic constants at ambient pressure and at high pressure by picosecond acoustics. Picosecond acoustics is a pump-probe experimental technique that allows to measure the travel times of the acoustic waves that propagate across the sample. The acoustic waves’ velocities are derived from the travel times and from the sample thickness. An inversion program developed within the research group allows to extract the elastic constants from the measured sound velocities by inverting the Christoffel equation that relates them to the elastic tensor. The high pressure is generated by the diamond anvil cell (DAC) technology. The investigated samples are magnesium oxide (MgO), a transparent insulator, oriented along the [001] direction, and ruthenium (Ru), an opaque metal, oriented along the [0001] direction. The first was studied at ambient pressure, while the second was studied at both ambient and high pressure. The elastic constants of MgO were extracted with the inversion program and the results obtained agree with the values found in the literature. The values obtained are: C11= (296.96± 0.19) GPa, C44= (153.60 ± 0.10) GPa, C12= (98.78 ± 0.11) Gpa. The elastic constants at ambient pressure of Ru agree with the data in the literature and are: C11= (564± 6) GPa, C33= (626 ± 8) GPa, C44= (182.2 ± 0.5) GPa and C12= (175 ±7) Gpa. The measurements at high pressure on ruthenium did not allow to derive the elastic constants as from the data analysis it emerged that the sample had been damaged during the pressure increase. Probably this was due to the selected pressure transmitting medium of the DAC, that did not guarantee completely the pressure hydrostaticity.