Hydrogen depth-profiles in Nb hydrides formed in supercritical water

The hydrogen content x of Nb hydrides, NbH_x and/or NbD_x, formed in supercritical water under extremely high pressures and temperatures were studied as a function of the depth from the hydride surface via ion-beam analyses (elastic recoil detection analysis with Rutherford backscattering spectrometry, ERDA/RBS). The hydrogen-rich α'-NbH (x approx. = 0.9) formed a surface layer approximately 200-nm thick with a 4-25 nm Nb oxide overlayer, which was invisible or only barely visible in the X-ray diffraction (XRD) analysis, whereas the hydrogen-poor α-NbH (x < 0.2) was visible to a depth of 1 μm in the bulk. The oxide overlayer thickened with further hydrogenation. The exchange of D with H was suggested in the surface region for the hydride specimen formed with D₂O. The chemical states of the studied specimens were also analyzed via the angle-dependent hard X-ray photoelectron spectroscopy, HAXPES. The formation mechanism of Nb hydrides is discussed based on the depth profiles of the hydrogen content x and the chemical states, as well as XRD results of the Nb hydrides. (author)