Thermodynamic-driven selective synthesis and phase transformation of Sr-doped neodymium nickelate Ruddlesden-Popper epitaxial films

Recent discovery of the signature of 80 K superconductivity in La3Ni2O7 single crystals under high pressure sheds the light on the realization of high temperature superconductors from Ruddlesden-Popper nickelates. Here, we demonstrated the realization of selective fabrication of Nd0.8Sr0.2NiO3 perovskite films and (Nd0.8Sr0.2)(2)NiO4 Ruddlesden-Popper films from one ceramic Nd0.8Sr0.2NiO3 target simply by controlling the growth temperature. Our results further show that the as-grown Nd0.8Sr0.2NiO3 films can be transformed to (Nd0.8Sr0.2)(2)NiO4 by annealing in air at 1000 degrees C. Nd0.8Sr0.2NiO3 and (Nd0.8Sr0.2)(2)NiO4 films were found to be metallic and insulating, respectively. X-ray photoelectron spectroscopy results reveal that the as-grown (Nd0.8Sr0.2)(2)NiO4 films contained NiO impurity, which can be removed by high temperature annealing. X-ray absorption spectroscopy measurements indicate a lower Ni valence state and weakened hybridization between Ni-O in (Nd0.8Sr0.2)(2)NiO4 films. Substrate lattice strain strongly affects the stability of Nd0.8Sr0.2NiO3 and the formation of (Nd0.8Sr0.2)(2)NiO4. These results suggest an important role of thermodynamic principles in the growth and post-annealing of nickelate films. These findings provide an approach to obtain Ruddlesden-Popper series nickelate films and offer certain impetuses to the development of nickelate superconductivity.(c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/).