We present a detailed investigation of the chemistry at the growth interface between the bottom electrode and ferroelectric film in (001)-oriented epitaxial ferroelectric thin-film heterostructures. Three different ferroelectric systems, namely PbZr 0.2 Ti 0.8 O 3 , PbZr 0.52 Ti 0.48 O 3 , and BaTiO 3 deposited on SrRuO 3 /SrTiO 3 , were investigated to compare and contrast the role of lattice mismatch versus the volatility of the deposited cation species. A combination of transmission electron microscopy-based imaging and spectroscopy reveals distinct correlations among the ferroelectric thin-film composition, the deposition process, and chemical gradients observed across the ferroelectric-electrode interface. Sr diffusion from the electrode into the ferroelectric film was found to be dominant in PbZr 0.2 Ti 0.8 O 3 /SrRuO 3 /SrTiO 3 thin films. Conversely, Pb diffusion was found to be prevalent in PbZr 0.52 Ti 0.48 O 3 /SrRuO 3 /SrTiO 3 thin films. The BaTiO 3 / SrRuO 3 /SrTiO 3 heterostructure was found to have atomically sharp interfaces with no signature of any interdiffusion. We show that controlling the volatility of the cation species is as crucial as lattice mismatch in the fabrication of defect-free ferroelectric thin-film devices.