Ultrasound studies of MgB2 superconductor under hydrostatic pressure

The pressure dependence of sound velocities of longitudinal and shear wave were measured up to hydrostatic pressure of 0.5 GPa using a pulse-echo-overlap method on a polycrystalline single-phase MgB2 superconductor which was high-pressure synthesized and with a T-c of 39 K. The large hysteresis of sound velocities was observed in the experiment. Because of weaker metal-boron bonding in MgB2, it is speculated that the large hysteresis is due to relaxation between Mg and B layers under hydrostatic pressure. The pressure dependence of elastic moduli, Debye temperature, and specific heat were calculated based on sound velocity data. The results indicate that all elastic moduli and thermodynamic parameter increase with the applied pressures except specific heat. The isothermal equation of state of MgB2 is established in terms of the Murnaghan form. From the sound velocities, the pressure coefficient of the average phonon frequency was obtained.