A strain-driven morphotropic phase boundary in BiFeO3 is reported, demonstrating that epitaxial strain can be used to drive the formation of a morphotropic phase boundary and create large piezoelectric responses in lead-free ferroelectric materials. The study shows that BiFeO3 (BFO) films grown on different substrates exhibit a morphotropic phase boundary between tetragonal (T) and rhombohedral (R) phases. The T phase is characterized by a tetragonal structure with a large spontaneous polarization, while the R phase is a distorted rhombohedral structure. Epitaxial strain is shown to stabilize the T phase and position BFO on a morphotropic phase transition between its T and R polymorphs. The coexistence of T and R phases in thin films leads to large piezoelectric responses, making BFO a promising candidate for probe-based data storage and actuator applications. The study also reveals that the phase transition is driven by strain, similar to the phase transitions observed in other piezoelectrics. The results suggest that strain-driven phase evolution is a generic feature, akin to chemically driven phase changes in other materials. The study provides insights into the role of subcritical nuclei in the crystallization of a glassy solid, showing that these nuclei play a critical role in the development of nuclei during phase transformation. The findings have implications for the development of advanced materials for phase-change memories and other applications.A strain-driven morphotropic phase boundary in BiFeO3 is reported, demonstrating that epitaxial strain can be used to drive the formation of a morphotropic phase boundary and create large piezoelectric responses in lead-free ferroelectric materials. The study shows that BiFeO3 (BFO) films grown on different substrates exhibit a morphotropic phase boundary between tetragonal (T) and rhombohedral (R) phases. The T phase is characterized by a tetragonal structure with a large spontaneous polarization, while the R phase is a distorted rhombohedral structure. Epitaxial strain is shown to stabilize the T phase and position BFO on a morphotropic phase transition between its T and R polymorphs. The coexistence of T and R phases in thin films leads to large piezoelectric responses, making BFO a promising candidate for probe-based data storage and actuator applications. The study also reveals that the phase transition is driven by strain, similar to the phase transitions observed in other piezoelectrics. The results suggest that strain-driven phase evolution is a generic feature, akin to chemically driven phase changes in other materials. The study provides insights into the role of subcritical nuclei in the crystallization of a glassy solid, showing that these nuclei play a critical role in the development of nuclei during phase transformation. The findings have implications for the development of advanced materials for phase-change memories and other applications.