B-site mixed cationic tetrahedral layer confined the concentration and mobility of interstitial oxygen in mellite family

Abstract

Melilite-type oxide ion conductors usually were observed in pure gallate compounds. Here a new mellite interstitial oxide ion conductor Ca_2−xLa_xGa₂GeO_7+x/2 (0 ≤ x ≤ 0.15) was developed by the substitution of La³⁺ for Ca²⁺ in Ca₂Ga₂GeO₇ containing mixed GaO₄/GeO₄ tetrahedral layers. Neutron powder diffraction data indicated that the introduced interstitial oxide ions were accommodated in the distorted pentagonal rings, close to the coordination environment of (Ga2/Ge)O₄ containing terminal oxygen. Defect formation energy calculations further reveal that the interstitial oxygen not only favors to approach the GaO₄ tetrahedra, in agreement with studies of other pure gallate systems, but also is confined to the GeO₄ tetrahedra. Bond-valence-energy landscape calculations and molecular dynamics simulations revealed that the interstitial oxide ions were transported among the pentagonal rings within the tetrahedral layers through a cooperative mechanism involving the synergistic knock-on motion between interstitial and framework oxygens. In Ca₂Ga₂GeO₇, the La³⁺ substitution for Ca²⁺ only extended up to x = 0.15, which is much lower than those in the La–(Ca/Sr/Ba) based gallate mellites. The x = 0.15 composition resulted in oxide ion conductivities ∼2.79 × 10⁻⁵ S cm⁻¹ to 1.68 × 10⁻³ S cm⁻¹, with oxygen transport numbers of ∼92.65%, two orders of magnitude higher than that of the parent material. However, compared with pure gallate melilite interstitial oxide ion conductors with the same interstitial oxide ion concentrations, Ca_1.85La_0.15Ga₂GeO_7.075 has about one order of magnitude lower conductivity. The lower interstitial oxide ion concentration and conductivity in Ca_2−xLa_xGa₂GeO_7+x/2 reveal that although Ge⁴⁺ has similar chemical properties to the adjacent Ga³⁺, the subtle differences in size and the resultant covalent bonding ability make the mixed GaO₄ and GeO₄ tetrahedral layers in mellite unfavorable for accommodation and transportation of interstitial oxygens.