Correlation between microhardness and electronic charge density of hafnium hydrides

Quantitative Vickers microhardnesses for α + δ′-, δ′-, δ-, δ + ɛ- and ɛ-phase hafnium hydrides (HfHx; 1.46 ≤ x ≤ 2.02) and deuterides (HfDx; 1.55 ≤ x ≤ 1.94) at room temperature have been measured using a Vickers hardness tester. The Vickers microhardnesses of the HfHx and HfDx gradually decreased with an increase of the hydrogen concentration, and those for 1.46 ≤ x ≤ 1.70 were higher than that of α-phase metallic Hf. It was revealed by a first-principles calculation as well as an X-ray photoelectron spectroscopy (XPS) measurement that the hydrogen concentration dependence of the microhardnesses for HfHx and HfDx was ascribed mainly in terms of its influence on charge transfer from Hf 5d to H 1s and reduction of cohesive energy.