  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.
