Controllable growth and electronic phase transitions for metastable perovskite rare-earth nickelate films
Authors: Zhou Xuan-Chi , Jiao Yong-Jie ,   Key Laboratory of Magnetic Molecules and Magnetic Information Materials of Ministry of Education, School of Chemistry and Materials Science, Shanxi Normal University, Taiyuan 030031, China,   Collaborative Innovation Center for Shanxi Advanced Permanent Magnetic Materials and Technology, Research Institute of Materials Science, Shanxi Normal University, Taiyuan 030031, China
Published: September 2024

The multiple electronic phase transition achieved in the metastable perovskite (ReNiO<sub>3</sub>, where Re denotes a lanthanide rare-earth element) by using critical temperature, hydrogenation, electrical field and interfacial strain has attracted considerable attention in condensed matter physics and materials science, making it promising applications in the critical temperature thermistor, artificial intelligence, energy conversion and weak electric field sensing. Nevertheless, the above abundant applications are still bottlenecked by the intrinsically thermodynamic metastability related to ReNiO<sub>3</sub>. Herein, we synthesize the atomic-level flat ReNiO<sub>3</sub> film material with thermodynamic metastability using laser molecular beam epitaxy (LMBE) that exhibits excellent thermally-driven electronic phase transitions. Notably, the interfacial heterogeneous nucleation of ReNiO<sub>3</sub> film can be triggered by the template effect of (001)-oriented LaAlO<sub>3</sub> substrates, owing to the similar lattice constants between LaAlO<sub>3</sub> substrate and ReNiO<sub>3</sub> film. In addition, we elucidate the key role of <i>in situ</i> annealing under oxygen-enriched atmosphere in stabilizing the distorted perovskite structure related to ReNiO<sub>3</sub>. Apart from the depositing process related to LMBE, the ReNiO<sub>3</sub> with heavy rare-earth composition exhibits a more distorted NiO<sub>6</sub> octahedron and a higher Gibbs free energy that is rather difficult to synthesize by using physical vacuum deposition. As a representative case, the <i>in situ</i> annealing-assisted LMBE process cannot be utilized to deposit the SmNiO<sub>3</sub> film, in which the impurity peaks related to Re<sub>2</sub>O<sub>3</sub> and NiO are observed in its XRD spectra. With the assistance of X-ray photoelectron spectraoscopy and near-edge X-ray absorption fine structure, the valence state of nickel for ReNiO<sub>3</sub> is found to be +3, and the <inline-formula><tex-math id="Z-20240919220616">\begin{document}$t_{2{\mathrm{g}}}^6e_{\mathrm{g}}^1 $\end{document}</tex-math></inline-formula> configuration is observed. Considering the highly tunable electronic orbital configuration of ReNiO<sub>3</sub> related to the NiO<sub>6</sub> octahedron, co-occupying the A-site of perovskite structure with Nd and Sm elements regulates the transition temperature (<i>T</i><sub>MIT</sub>) for ReNiO<sub>3</sub> within a broad temperature range. Furthermore, we demonstrate the anisotropy in the electronic phase transitions for Nd<sub>1–<i>x</i></sub>Sm<sub><i>x</i></sub>NiO<sub>3</sub>, in which case the <i>T</i><sub>MIT</sub> achieved in the Nd<sub>1–<i>x</i></sub>Sm<sub><i>x</i></sub>NiO<sub>3</sub>/LaAlO<sub>3</sub> (111) heterostructure exceeds the one deposited on the (001)-oriented LaAlO<sub>3</sub> substrate. The presently observed anisotropy in the electrical transportation for Nd<sub>1–<i>x</i></sub>Sm<sub><i>x</i></sub>NiO<sub>3</sub> film material is related to the anisotropic in-plane NiO<sub>6</sub> octahedron configuration triggered by differently oriented LaAlO<sub>3</sub> substrates. The present work is expected to introduce a new degree of freedom to regulate the electronic phase transition, explore new electronic phase in ReNiO<sub>3</sub> material system, and pave the way for growing atomic-level flat ReNiO<sub>3</sub> film materials with expected electronic phase transitions.