Localized modes and hydrogen trapping in niobium with substitutional impurities

Abstract

The trapping of hydrogen by the substitutional impurities Ti and Cr in Nb has been investigated by neutron inelastic scattering measurements of hydrogen vibration spectra as a function of temperature. In the case of Ti, the hydrogen is in a trap which is deep enough to prevent precipitation into the hydride phase at low temperatures. In the trapped state, the hydrogen occupies a tetrahedral site which is likely to be a neighbor of the Ti impurity. The higher-energy vibrational-mode peak is shifted down by ≈ 10 meV and broadened somewhat with respect to that in pure niobium. In the case of Cr impurities the trap is shallower, and precipitation to the hydride phase is not inhibited at low temperatures. By studying the detailed behavior of the temperature dependence of the vibrational line shapes for dissolved, trapped, and precipitated H, a binding energy at the Cr trap of 105±10 meV has been derived.