Mechanism of the structural phase transformations in epitaxial YHx switchable mirrors

The detailed mechanisms of the structural phase transformations that occur in epitaxial Y–hydride switchable mirrors are revealed with high resolution transmission electron microscopy (both cross sectional and plan view). The triangular ridge network that develops in Y prior to the α–β transformation is a result of {101̄2} deformation twinning. The basal plane that is originally parallel to the film/substrate interface is rotated by twinning over 5.6° and transformed into a prismatic plane and similarly the prismatic plane is transformed into a basal plane giving a final crystal reorientation for the ridge of 95.6°. After transformation to β, nearly vertical Σ3{111} twin boundaries arise in the ridges. In contrast, horizontal twin boundaries develop in the β domains to prevent macroscopic shape changes. Inbetween the two twin variants within the domains, Shockley partial dislocations are persistently present, which enable efficient reversible β–γ switching of the mirror.