Origin of the abnormal diffusion of transition metal in rutile

TL;DR

This study uses first-principles calculations to uncover the microscopic mechanisms behind the diverse and anisotropic diffusion behaviors of transition metals in rutile, explaining experimental observations and revealing new diffusion models.

Contribution

It introduces a novel model for large dopant diffusion in rutile, clarifying the roles of interstitial states and mechanisms along different crystallographic directions.

Findings

Large atoms like Sc and Zr have lower barriers via interstitial mechanisms.

Diffusion along [001] is limited by interstitial state formation.

Co and Ni exhibit fast, anisotropic diffusion with small barriers.

Abstract

Diffusion of dopants in rutile is the fundamental process that determines the performance of many devices in which rutile is used. The diffusion behavior is known to be highly sample-dependent, but the reasons for this are less well understood. Here, rutile is studied by using first-principles calculations, in order to unravel the microscopic origins of the diverse diffusion behaviors for different doping elements. Anomalous diffusion behavior in the open channel along [001] direction is found: larger atoms include Sc and Zr have lower energy barrier for diffusion via interstitial mechanism, apparently contradicting their known slow diffusion rate. To resolve this, we present an alternate model for the overall diffusion rate of the large-size dopants in rutile, showing that parallel to the [001] channel, it is limited by the formation of the interstitial states, whereas in the direction perpendicular to [001], it proceeds via a kick-out mechanism. By contrast, Co and Ni, prefer to stay in the interstitial site of rutile, and have conventional diffusion with a very small migration barrier in the [001] channel. This leads to highly anisotropic and fast diffusion. The diffusion mechanisms found in the present study can explain the diffusion data measured by experiments, and these findings provide novel understanding for the classic diffusion topic.