Oxygen Defects and Novel Transport Mechanisms in Apatite Ionic Conductors: Combined ¹⁷O NMR and Modeling Studies

Abstract

Germanium‐based apatite compounds are fast oxide‐ion conductors for potential use in fuel cells. A combination of solid‐state 17O NMR spectroscopy, atomistic modeling, and DFT techniques help to elucidate oxygen defect sites and novel cooperative mechanisms of ion conduction. The picture shows oxygen diffusion in the studied apatite compound from molecular dynamics simulations.

Cooperative oxide-ion conduction in apatite-type lanthanum germanate—A first principles study

“…In another instance, Panchmatia et al proposed the cooperative migration mechanism, "S N 2-type process" in the ab plane from MD calculations using empirical interatomic potentials. 7) In combination with the formation process of O4 Frenkel defect, the S N 2-type process seems almost identical to the multi-oxide-ion cooperative mechanism via the O4 column shown in Figs. 6 and 7.…”

“…6), 7) The oxide-ion conduction behavior in La 10 (GeO 4 ) 6 O 3 was also investigated by first-principles molecular dynamics (FPMD) simulations in our recent study, 5) which suggests that a cooperative conduction mechanism involving several oxide ions along the c axis is dominant. This mechanism is quite different from the previously-proposed mechanisms such as "fan-like mechanism" and "S N 2-type process" by MD simulations using empirical and semi-empirical interatomic potentials.…”

Cooperative mechanisms of oxygen vacancy stabilization and migration in the isolated tetrahedral anion Scheelite structure

“…In order to elucidate how the oxygen vacancies migrate in the Scheelite structure, atomistic static lattice and MD simulations based on the interatomic potential method were performed, as previously successfully applied in other structure types based on tetrahedral units, for example, apatite-based 9 , melilite-based 12 , and LaBaGaO 4 -based 3 oxide ion conductors. The potential parameters listed in Supplementary Table 5 well reproduce the Scheelite BiVO 4 structure.…”

“…Recently, structure types containing tetrahedral moieties have received growing attention as new oxide ion conductor candidates, owing to the deformation and rotation flexibility of tetrahedral units which facilitates the stabilization and transportation of oxygen interstitial or vacancy defects 1,3,7 . Oxygen interstitials often appear as charge carriers inducing oxide ion conduction in tetrahedra-based structures 1,7–9 . Variable coordination geometry of the cations in the tetrahedral centers is important for stabilizing interstitial defects, allowing incorporation of extra oxygen atoms into the bonding environments of tetrahedral cations 1,6–9 .…”

“…Oxygen interstitials often appear as charge carriers inducing oxide ion conduction in tetrahedra-based structures 1,7–9 . Variable coordination geometry of the cations in the tetrahedral centers is important for stabilizing interstitial defects, allowing incorporation of extra oxygen atoms into the bonding environments of tetrahedral cations 1,6–9 . In low-dimensional structures, the ease of deformation and rotation of tetrahedral units allows for the accommodation and transportation of the interstitial defects.…”