Effect of substitutional Cr on hydrogen diffusion and thermal stability for the BCT monohydride phase of the V–H system studied by 1H NMR

The diffusion of hydrogen in the monohydride phase (β phase) of the V1−xCrx–H (x ≤ 0.1) system was studied by means of 1H nuclear magnetic resonance (NMR). Hydrogen atoms in the hydrides occupied the octahedral (O) sites in a body centered tetragonal (BCT) structure. The BCT lattice was contracted by addition of Cr because Cr has a smaller atomic radius than V. The activation energy for hydrogen diffusion, EH, and the Korringa constant, T1eT, were estimated from the temperature and frequency dependence of 1H spin–lattice relaxation times, T1. The value of EH increased with increase in the Cr content. The contraction of the BCT lattice by addition of Cr suppressed the diffusion of hydrogen which occupied the interstitial sites. Addition of Cr also increased the T1eT, which was accompanied by reducing in the stability of the monohydride phase of the V–H system.

► Activation energy for hydrogen diffusion for the octahedral sites in the monohydride phase of V1−xCrxH0.68 (x ≤ 0.1) has been determined.
► Smaller metal elements than V decrease the unit cell volume of the monohydride phase of the V–H system and suppress the mobility of hydrogen on the octahedral sites, and larger elements provide the opposite effects.
► The thermal stability of the hydride phase is estimated from the information related to the conduction electrons using NMR.
► Addition of Cr and Mo to V decreases and addition of Ti increases the thermal stability of the monohydride and dihydride phases of the V–H system.