In oxide epitaxy, the growth temperature and background oxygen partial pressure are considered as the most critical factors that control the phase stability of an oxide thin film. Here, we report an unusual case wherein diffusion of oxygen vacancies from the substrate overpowers the growth temperature and oxygen partial pressure to deterministically influence the phase stability of $Bi_{2}WO_{6}$ thin film grown by the pulsed laser deposition technique. We show that when grown on an oxygen-deficient $SrTiO_{3}$ substrate, the $Bi_{2}WO_{6}$ film exhibits a mixture of (001) and (100)/(010)-oriented domains alongside (001)-oriented impurity $WO_{3}$ phases. The (100)/(010)-oriented $Bi_{2}WO_{6}$ phases form a self-organized 3D nanopillar-structure, yielding a very rough film surface morphology. Oxygen annealing of the substrate or using a few monolayer-thick $SrRuO_{3}$ as the blocking layer for oxygen vacancy diffusion enables growing high-quality single-crystalline $Bi_{2}WO_{6}$ (001) thin film exhibiting an atomically smooth film surface with step-terrace structure. We propose that the large oxide-ion conductivity of $Bi_{2}WO_{6}$ facilitates diffusion of oxygen vacancies from the substrate during the film growth, accelerating the evaporation of volatile Bismuth (Bi), which hinders the epitaxial growth. Our work provides a general guideline for high-quality thin film growth of Aurivillius compounds and other oxide-ion conductors containing volatile elements.
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