Zr/Fe100-xZrx/Zr layered structures with different concentrations x and variable Fe100-xZrx layer thicknesses were prepared in an UHV system by electron beam evaporation. The structure of the Fe100-xZrx layers was monitored in-situ by reflection high-energy electron diffraction (RHEED ). The RHEED patterns show a change from diffuse scattering to a marked intensity distribution at critical layer thicknesses t(crit.) (x). This observation is interpreted as an amorphous-ta-crystalline transition of the Fe100-xZrx layers. X-ray diffraction and magnetization measurements on the same samples clearly support this interpretation. We observe an increase of t(crit.) (x) with an increasing Zr concentration x and explain this by a lattice instability induced by the interface mismatch.