First-principles analysis of oxide-ion conduction mechanism in neodymium silicate

Abstract

In order to reveal the conduction mechanisms of interstitial oxide ions (Oint ions) in neodymium silicate apatite, the low-energy sites and conduction pathways have been determined using first-principles calculations. Two inequivalent Oint sites are found close to the c axis and at the periphery of the O4 channel. The potential barriers of Oint ions are then evaluated by the nudged elastic band method along three types of conduction pathways connecting the Oint sites, i.e., two pathways along the c axis and one pathway in the ab plane. Along the c axis, the interstitialcy mechanism shows the lowest potential barrier of 0.57 eV, which dominantly contributes to fast long-range conduction along the c axis. The Oint conduction in the ab plane is also governed by the interstitialcy mechanism, whose potential barrier largely depends on the configuration of Nd vacancies in the range of 0.38–0.75 eV.