Elastic moduli of single crystal palladium hydride near the tri-critical point.

Abstract

Palladium has long been studied as a model metal hydride system because of the high absorption rates and levels of hydrogen. Even moderately high pressures and temperatures can result in almost one to one hydrogen to palladium atom ratios. Depending on the ambient temperature and hydrogen pressure, the structural phase of hydrogen atoms in the palladium lattice can have three different forms: alpha, beta, and a mixed phase (alpha + beta). There exists a triple point in the pressure versus temperature phase diagram near 290° C and 400 psi. A recent model for absorption/desorption hysteresis in metal hydride systems predicts a sharp drop in the shear modulus near this triple point. Results from recent elastic constants and acoustic attenuation measurements as a function of both temperature and pressure near the triple point will be shown [R. B. Schwarz and A. G. Khachaturyan, Acta Mater. 54, 313 (2006)]. The experimental procedures for resonant ultrasound spectroscopy measurements at high temperatures and pressures will be also discussed.