Preferred hydride growth orientations on oxide-coated gadolinium surfaces

The initial development of hydrides on polycrystalline gadolinium (Gd), as on some other hydride forming metals, is characterized by two sequential steps. The first step involves the rapid formation of a dense pattern of small hydride spots (referred to as the "small family" of hydrides) below the native oxidation layer. The second stage takes place when some of the "small family" nucleants (referred to as "growth centers", GCs) break the oxide layer, leading to their rapid growth and finally to the massive hydriding of the sample. In the present study, the texture of the two hydride families was studied, by combining X-ray diffraction (XRD) analysis with a microscopic analysis of the hydride, using scanning electron microscopy (SEM) and atomic force microscopy (AFM). It has been observed that for the "small family", a preferred growth of the (1 0 0) _h plane of the cubic GdH ₂ takes place, whereas for the GCs, a change to the (1 1 1) _h plane of the cubic hydride dominates. These preferred growth orientations were analyzed by their structure relation with the (0 0.1) _m basal plane of the Gd metal. It has been concluded that the above texture change is due to the surface normal compressive stress component exerted by the oxidation overlayer on the developing hydride, preventing the (0 0.1) _m(1 1 1) _h growth orientation. This stress is relieved upon the rupture of that overlayer and the development of the GCs, leading to the energetically favorable mode of growth.