Deep investigation of antiphase‐boundaries defects in rare‐earth nickelates

Abstract

Transition metal oxides (TMO's) are highly sensitive systems to external fields due to strong electron correlations and high polarizability of the metal‐oxygen bond. This makes it possible to tune their macroscopic properties by inducing slight distortions at their unit cell structure. A paradigmatic example is the rare earth (RE) nickelates, which present a tunable metal‐to‐insulator transition (MIT) and resistive switching (RS) effect, placing them as a new plausible alternative for current non‐volatile memories. Here, we use aberration‐corrected Scanning Transmission Electron Microscopy (STEM) combined with Electron Energy Loss Spectroscopy (EELS) to correlate both structure and electrical properties as a function of the RE cation specie (La, Sm, or Nd), substrate mismatch and film thickness. Nanoscale investigations show that Nikelate thin films require chemical and structural reconstructions in the form of antiphase boundaries (APBs) to compensate the mismatch with the substrate. This 2D defect suppresses one Ni‐O plane either in the in‐plane or in the out‐of‐plane direction, changing locally the lattice spacing. The APBs landscape is evaluated on RE‐nickelate epitaxial thin films grown by chemical solution deposition onto LaAl(LaAlO 3 ) 0.3 ‐(Sr 2 AlTaO6) 0.7 (LSAT), SrTiO 3 (STO) or LaAlO 3 (LAO) substrates with thicknesses that range from 6nm to 50nm, as shown in Figure 1 and 2. This structural study is also complemented with electrical and advanced XRD measurements that reveal a sharp MIT transition which is shifted when substrate, thickness or RE ionic radius is modified.