Ultrasonic study of short-range hydrogen ordering in Pd-hydride

Abstract

Abstract We used resonant ultrasound spectroscopy to measure the three independent elastic constants of PdH0.71 and PdH0.65 in the temperature range 1.4 K < T < 296 K. Measurements were performed as the crystals were rapidly cooled (∼10 K min− 1) and then slowly heated (∼0.1 K min− 1). During slow heating, the temperature dependence of the shear modulus C′ = (C11 – C12)/2 shows a small peak at ∼55 K, but this peak is absent during fast cooling. We associate the peak with the so-called “50 K anomaly” in palladium hydride. In contrast, the other shear modulus, C44, shows no peak during slow heating. We propose that the 50 K anomaly observed in the elastic constants, and in other thermodynamic and transport properties of Pd-hydride, is a kinetic effect arising from changes in the hydrogen atom short-range order as the alloy is heated.