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.
The oxide ion conductivity of apatite‐type lanthanum silicates has been enhanced by cation doping into both La and Si sites. The Sr and Nd fill vacancies on the La site by creating excess oxide ions and increasing the conductivity. Al and Mg, which substitute for Si at a fixed oxygen content, result in an enhancement of conductivity, accompanied by lattice enlargement. The Mg‐doped lanthanum silicate, La9.533(Si5.7Mg0.3)O26, has been shown to have the highest conductivity of 4.4 × 10−2 S/cm at 800°C. The conduction channel consists of La triangles and O triangles, and the channel sizes have been calculated from the Rietveld structure analysis of the powder X‐ray diffraction patterns. Expansions of the O triangles have been observed when introducing excess oxide ions by filling vacancies on the La site or by the substitution of Si using dopants.
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