The topochemistry of hydride formation in rare earth metals

The topochemistry of hydride formation in the rare earth metals was investigated by metallographic examination of partially hydrogenated samples. The effects of surface overlayers ("oxides") on the development of the hydride phases are discussed and are related to measured overall reaction rates. Four different types of hydride topochemical forms are identified. The development of each of these types is accounted for by the interplay of the hydrogen diffusion rates in the metallic matrix and the nucleation and growth rates of the hydride phases. Temperature-induced changes in the type of progression of the hydride phases are discussed. The effects of higher (trihydride) phases are illustrated. A method for the estimation of the diffusion constants of hydrogen in the α phase metallic solid solutions is presented.