On the Origin of Precipitation of Transition Metals Implanted in MgO

TL;DR

This study uses first-principles calculations to analyze the stability and diffusion of transition metals in MgO, explaining why Ni precipitates out while Fe can remain embedded depending on charge state and temperature.

Contribution

It provides a computational framework to predict the precipitation behavior of implanted transition metals in oxide materials.

Findings

Ni ions readily precipitate out at 600 K.

Charged Fe ions are immobile over microsecond timescales.

Neutral Fe can migrate significantly at annealing temperatures.

Abstract

Transition metals implanted in single crystal MgO can precipitate out at grain boundaries or remain embedded in bulk. Using first-principles calculations based on density functional theory we have calculated the thermodynamic stability and diffusion coefficients of the implanted ions to explain Fe and Ni precipitation in MgO. Experimentally it has been observed that some of the Fe atoms precipitate out, while few Fe atoms in 2+ and 3+ charge states remain embedded in the lattice. Our simulation shows that at 600 K (typical annealing temperature) while neutral iron in MgO would migrate 1 $\mu$m in few microseconds, it takes several years for the charged Fe ions to migrate the same distance. On the other hand, Ni ions in all its charge states (neutral, 1+, 2+, and 3+) would migrate 1 $\mu$m in just few microseconds, at 600 K. This explains the experimental observation that implanted Ni always precipitates out. Our study paves a way forward to predict if ions implanted in stable oxide will be stable or will precipitate out.