Diffusion Path of Oxide Ions in an Apatite-Type Ionic Conductor La_9.69(Si_5.70Mg_0.30)O_26.24

Abstract

Densities of coherent-scattering lengths in oxide-ion conductor La9.69(Si5.70Mg0.30)O26.24 with an apatite-type structure (space group P63/m) have been determined by a whole-pattern fitting approach based on the maximum-entropy method (MEM) using neutron powder diffraction data measured at 25 and 1558 °C. The Rietveld refinement suggested an interstitial oxygen (O5) atom site (12i; x, y, z; x ≈ 0, y ≈ 0.22, z ≈ 0.65) at the periphery of the hexagonal axis. The oxide ions located at 2a (O4) and 12i (O3) sites are largely distributed parallel and perpendicular to the c axis, respectively. The densities of coherent-scattering lengths derived from the MEM analysis revealed that the O4 oxide ions located at the 2a site (0, 0, 1/4) diffuse to nearest-neighbor 2a sites along the [001] direction. This [001] diffusion path is linearly linked with that of neighboring unit cell, which forms a long-range, one-dimensional oxide-ion diffusion pathway extending along the c axis of the hexagonal P63/m framework. The probability density of the O3 ion is connected with that of the equivalent O3 of the same tetrahedron through the interstitial O5 ion, indicating migration of the ions along a curved line connecting O3−O5−O3 atoms. This migration path is nonlinear (U-shaped) and perpendicular to the c axis. The densities of the two kinds of oxide ions (O4 and O3) migrating parallel and perpendicular to the c axis, respectively, connected with each other through the densities of interstitial oxide ion (O5). The results suggest that the O4 ion migrates via vacancy mechanism with a direct linear path along the c axis, while the O3 ion migration perpendicular to the c axis involves an interstitial mechanism.

Oxygen-17 nuclear magnetic resonance measurements on apatite-type lanthanum silicate (La9.33(SiO4)6O2)

“…The high conductivity was first observed by one of the authors [1,2]. Although the conduction mechanism has been extensively investigated [8][9][10][11][12], it has not been fully understood. Apatite has three kinds of sites for oxide ions in its structure (Fig.…”

“…Ali showed that the oxide-ion migration is mainly caused by a vacancy mechanism in the isolated (O4) oxide ion along the c-axis. However, the migration of oxide ion on theO3-O5-O4 path is significant with increasing temperature, where O5 is the interstitial oxide ion which is produced by the addition of MgO to La-apatite and is present between O3 and O4 ions [9]. Computer simulations for oxygen-excess La9.67(SiO4)6O2.5 apatite predicted that the presence of oxide ions at interstitial sites near the SiO4 tetrahedra form pseudo-"SiO5" units [13,14].…”

Apatite germanates doped with tungsten: synthesis, structure, and conductivity

“…For the silicates it has been shown that oxygen excess in the range, 0 ≤ x ≤ 0.5 is possible, while higher oxygen content, 0 ≤ x ≤ 1.0 has been achieved for the germanates [13,18,20,21]. For the silicate series, the exact location of extrastoichiometric oxygen, x, has attracted some controversy, with reports of interstitial sites close to the SiO4 tetrahedra, while in other studies, mobile oxygen at the centre of the channels has been claimed [4,8,9,10,12,13,23,27,35,36]. For the germanates, the situation appears more clearcut, with computer modeling, neutron diffraction and Raman studies indicating occupancy by oxygen sites close to the GeO4 tetrahedra leading to the formation of five coordinate Ge [10, 20-22, 37, 40].…”

Research progress in nondoped lanthanoid silicate oxyapatites as new oxygen-ion conductors

“…In 80) They refined the structure parameters using a structure model with P6 3 /m. A feature of their results was that anisotropic thermal displacements of the La1(4f ) and La2(6h) sites were expanded along the c-axis and the ab plane, respectively, at room temperature.…”