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 view͒. The triangular ridge network that develops in Y prior to the ␣-␤ transformation is a result of ͕101 2͖ 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

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... milarly the prismatic plane is transformed into a basal plane giving a final crystal reorientation for the ridge of 95.6°. After transformation to ␤, nearly vertical ⌺3͕111͖ twin boundaries arise in the ridges. In contrast, horizontal twin boundaries develop in the ␤ domains to prevent macroscopic shape changes. Inbetween the two twin variants within the domains, Shockley partial dislocations are persistently present, which enable efficient reversible ␤-␥ switching of the mirror.