Elastic constants of palladium hydride at elevated temperature and pressure.

Abstract

Hydrogen atoms occupy the octahedral interstitial sites provided by metals with fcc lattices (such as palladium). Palladium hydride (PdHx) exists in two phases. At room temperature, the low concentration (PdHx=0.02) called α-phase has slightly larger lattice parameter compared to that of pure palladium. Above this concentration and up to (PdHx=0.6) the high concentration β-phase appears resulting in (α+β)-phase, a mixed phase region, and the lattice parameter becomes substantially greater than that of pure palladium. When x>0.6, α-phase disappears and the system becomes purely β-phase. The three phases coexist at the tri-critical point (temperature, pressure, and concentration). Resonant ultrasound spectroscopy (RUS) has been used to investigate the elastic constants of palladium hydride crystal and the equilibrium dynamics of the system around the tri-critical point temperature and pressure. A large decrease in shear modulus is noticed near this tri-critical point. The desire to have fundamental understanding of metal-hydrogen systems and to explore this for possible technological applications has been the motivation for research of these systems. This investigation includes the design and construction of high temperature and high pressure RUS cell.