Oxide ion conduction anisotropy deconvoluted in polycrystalline apatite-type lanthanum silicates

Abstract

Recent efforts to understand the anisotropic oxide ion conduction mechanism in apatite-type La_9.33+x(SiO₄)₆O₂₊₃x/2 (−0.5 ≤ x ≤ 1.25) by the sophisticated structural investigation and atomistic simulations could not definitely be corroborated nor refuted by the experimental conductivity measurements, since no measurements on single crystalline lanthanum silicates are available. In the present work the strong frequency dispersions in the bulk impedance of the polycrystalline silicates were successfully described by a hierarchical ladder network of two resistors and non-ideal capacitor elements with the smaller resistance element representing the contribution from the grains whose c-axes are oriented parallel to the transport and the larger resistance element from the grains in unfavorable orientations. All nominally lanthanum excess compositions (0.5 ≤ x ≤ 1.25) exhibited similar bulk conduction behavior which is consistent with the solubility limit at x = 0.43 ± 0.12 determined by microprobe analysis. A high-conductivity component attributable to the grains with c-axis transport exhibited an activation energy of 0.33 ± 0.02 eV. The process with a higher activation energy of 0.70 eV is ascribed to the grains oriented in the perpendicular direction with respect to transport direction. For lanthanum-deficient compositions with x < 0 the conductivity values plummeted from those of the lanthanum-excess ones and the activation energies increased up to 0.59 eV and 0.87 eV for the parallel and perpendicular direction, respectively. Oxide ion conduction anisotropy deconvoluted from the polycrystalline specimens suggests a consistent description of the conduction mechanism and defect chemistry of the lanthanum silicates. With the activation energy values of 0.33 eV for the migration of oxygen interstitials along the c-axis, the difference between the lanthanum excess and the lanthanum deficit composition can be attributed to the energy for generating oxygen interstitials which also appears to amount to ca. 0.3 eV. As the c-axis transport pathways are non-linear, the parallel and perpendicular transport are not completely separable and the migration energy for the perpendicular transport can be ascribed to the energy for the inter-channel reaction in addition to the energy for the parallel transport.