Interstitial-atom-induced phase transformation upon hydrogenation in vanadium

Abstract The effect of interstitial atoms (nitrogen, carbon) on hydrogen storage properties in vanadium was investigated. When the N concentration was below 0.4 wt%, the plateau pressures increased with increasing N concentration during absorption and desorption and vanadium samples (body-centered cubic (BCC)) transformed to VH 0.5 (body-centered tetragonal (BCT), c / a  = 1.1) and then VH 2 (face-centered cubic (FCC)). When the N concentration exceeded 0.6 wt%, a new single-phase region appeared in the pressure-composition isotherm, suggesting the formation of a new hydride phase. The X-ray diffraction data indicated that this new hydride phase was VH 1.0 with a BCT structure and c / a  = 1.24, and the phase transformation took place as V (BCC) became VH 0.5 (BCT, c / a  = 1.1), followed by VH 1.0 (BCT, c / a  = 1.24) and then VH 2 (FCC). Density functional theory calculations indicated that the BCT structure model with hydrogen atoms fully occupying the octahedral sites (denoted as the O z site) can explain the experimentally obtained crystal structure for VH 1.0 ; they also indicated that the VH 1.0 phase was stabilized by the addition of nitrogen. In addition, the N occupation site changed from the O z site in VH 0.5 and VH 1.0 to the tetrahedral site (denoted as the T site) in VH 2 in coordination with hydrogen during hydrogen absorption. A similar phenomenon was observed in carbon-containing vanadium. It can thus be concluded that the phase transformation pathway and stability of the hydride phases in the V–H system are highly sensitive to the addition of interstitial C and N atoms.