The growth of (100) diamond/iron/copper multilayer structures has been examined by reflection high energy electron diffraction, extended X-ray absorption fine structure, and scanning electron microscopy in an effort to determine the thickness limit for metastable face-centered-cubic Fe on (100) diamond. Both copper films deposited on iron layers with thicknesses below 1.4 nm and the iron layers themselves were found to be face-centered cubic single crystal, while films grown on iron that was 2.0 nm and thicker and the iron itself were found to be polycrystalline. This critical thickness range of 1.4-2.0 nm compares well with the theoretically calculated value of 1.8 nm. This value was determined using the mechanical equilibrium theories (Matthews-Blakeslee and van der Merwe) with a lattice parameter for face-centered cubic iron that was derived by estimating the functional form of the linear thermal expansion coefficient and extrapolating the Poisson's ratio for austenitic stainless steel to the temperature of interest. The shear modulus, and intrinsic stacking fault energy for fcc Fe from ̃ 1350°C down to below room temperature have also been estimated. A more likely room temperature lattice parameter for fcc Fe than is usually assumed was estimated to be 0.3579 nm. The measured in-plane lattice parameter of strained fcc Fe on diamond was 3.54 ± 0.1 Å.