Conductuion Pathway of Oxide Ions in Rare-Earth Silicate Oxyapatites

Abstract

Apatite-type RE_9.33(SiO₄)₆O₂ rare-earth silicate compounds are of great interest for their oxide ion conductivity at moderate temperatures. Crystallographic space group and structural parameters of La/Nd_9.33(SiO₄)₆O₂ were investigated based on single-crystal X-ray diffraction data at room temperature for La_9.33(SiO₄)₆O₂ and at various temperatures from 150 to 900 K for Nd_9.33(SiO₄)₆O₂. Based on the most accurate diffraction data to date, these compounds have an apatite prototype structure (space group P6₃/m) with no symmetry lowering, site splitting nor interstitial oxide ion in the structure. Most striking feature of the structure is large anisotropy in thermal motions of the oxide ion at the anion channel site. Even at room temperature, the mean-square displacement of the channel oxide ion in La_9.33(SiO₄)₆O₂ is 16 times larger in [001] than those normal to the direction. Not striking but important point is that there is no sign of interstitial oxide ion which had been proposed as a charge carrier in La_9.33(SiO₄)₆O₂ and related compounds. These findings imply straight migration of oxide ion inside the channel as a primary mechanism of oxide-ion conductivity in these compounds. Size and shape of the anion channel might be a key to achieve high oxide-ion conductivity in these compounds.