The Onset of Phase Separation in the Double Perovskite Oxide La$_2$MnNiO$_6$

TL;DR

This study investigates the phase separation in La$_2$MnNiO$_6$ thin films, revealing that oxygen vacancies and electrostatic potential buildup promote NiO precipitate formation beyond 1-5 nm thickness, with implications for controlling film quality.

Contribution

It uncovers the role of electrostatic potential and oxygen vacancies in phase separation of La$_2$MnNiO$_6$ films, providing strategies to extend defect-free growth.

Findings

NiO phases precipitate after 1-5 nm growth

No strain change observed away from interface

Oxygen vacancies increase with thickness due to electrostatic potential

Abstract

Identification of kinetic and thermodynamic factors that control crystal nucleation and growth represents a central challenge in materials synthesis. Here we report that apparently defect-free growth of La$_2$MnNiO$_6$ (LMNO) thin films supported on SrTiO$_3$ (STO) proceeds up to $1-5$ nm, after which it is disrupted by precipitation of NiO phases. Local geometric phase analysis and ensemble-averaged X-ray reciprocal space mapping show no change in the film strain away from the interface, indicating that mechanisms other than strain relaxation induce the formation of the NiO phases. $Ab \, initio$ simulations suggest that oxygen vacancies become more likely with increasing thickness, due to the electrostatic potential build-up associated with the polarity mismatch at the film-substrate interface, this, in turn, promotes the formation of Ni-rich regions. These results suggest that the precipitate-free region could be extended further by increasing the oxygen chemical potential through the use of an elevated oxygen pressure or by incorporating electron redistributing dopants to suppress the built-in potential.