Ab initio calculations of mechanical properties in β-M[H.sub.2-x][He.sub.x]

The ³He (helium) accumulation due to tritium decays in rare-earth metal tritides can influence their mechanical properties and stabilities, limiting their applications as neutron generators. Ab initio calculations based on density functional theory have been performed to investigate the elastic constants, bulk modulus, shear modulus and Young's modulus of the β-M[H.sub.2-x][He.sub.x] (M = Er, Sc) systems, with the variety of the x (the ratio of He to M atoms), ranging from 0 to 0.1875. The changes in the mechanical properties of β-Er[H.sub.2-x][He.sub.x] and β-Sc[H.sub.2-x][He.sub.x] exhibit two regimes, which is in agreement with the experimental observations. It appears that the different He concentrations cause the different interactions between metal atoms and their neighbors in β-M[H.sub.2-x][He.sub.x], a key factor which gives rise to the observed two regimes. When x is about 0.13, the mechanical properties of metal hydrides degrade significantly. Furthermore, the degradation mechanisms of the mechanical properties in the β-M[H.sub.2-x][He.sub.x] systems depend mainly on the crystal structure of metal hydrides.