  Recent progress on the signatures of pressure-induced high temperature
superconductivity in Ruddlesden Popper (RP) nickelates (Lan+1NinO3n+1) has
attracted growing interest in both theoretical calculations and experimental
efforts. The fabrication of high-quality single crystalline RP nickelate thin
films is critical for possible reducing the superconducting transition pressure
and advancing applications in microelectronics in the future. In this study, we
report the observations of an active phase transition in RP nickelate films
induced by misfit strain. We found that RP nickelate films favor the perovskite
structure (n = infinite) under tensile strains, while compressive strains
stabilize the La3Ni2O7 (n = 2) phase. The selection of distinct phases is
governed by the strain dependent formation energy and electronic configuration.
In compressively strained La3Ni2O7, we experimentally determined splitting
energy is ~0.2 eV and electrons prefer to occupy in-plane orbitals. First
principles calculations unveil a robust coupling between strain effects and the
valence state of Ni ions in RP nickelates, suggesting a dual driving force for
the inevitable phase co-existence transition in RP nickelates. Our work
underscores the sensitivity of RP nickelate formation to epitaxial strain,
presenting a significant challenge in fabricating pure-phase RP nickelate
films. Therefore, special attention to stacking defects and grain boundaries
between different RP phases is essential when discussing the pressure-induced
superconductivity in RP nickelates.
