Diffusion Path of Oxide Ions in an Apatite-Type Ionic Conductor La9.69(Si5.70Mg0.30)O26.24

Densities of coherent-scattering lengths in oxide-ion conductor La-9.69(Si5.70Mg0.30)O-26.24 with an apatite-type structure (space group P6(3)/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 degrees C. The Rietveld refinement Suggested an interstitial oxygen (O5) atom site (12i; x, y, z; x approximate to 0, y approximate to 0.22, z approximate to 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 04 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 P6(3)/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.