Ordered structure of FeGe$_2$ formed during solid-phase epitaxy

Fe$_{3}$Si/Ge(Fe,Si)/Fe$_{3}$Si thin film stacks were grown by a combination of molecular beam epitaxy and solid phase epitaxy (Ge on Fe$_{3}$Si). The stacks were analyzed using electron microscopy, electron diffraction, and synchrotron X-ray diffraction. The Ge(Fe,Si) films crystallize in the well oriented, layered tetragonal structure FeGe$_{2}$ with space group P4mm. This kind of structure does not exist as a bulk material and is stabilized by solid phase epitaxy of Ge on Fe$_{3}$Si. We interpret this as an ordering phenomenon induced by minimization of the elastic energy of the epitaxial film.