Structural aspects that enhance oxygen mobility in La9-2 x/3Mn0.5 REx□0.5-x/3(SiO4)6O 2 with RE = Ca, Sr and Ba

Oxyapatite-type silicates, La9-2x/3Mn 0.5 REx□0.5-x /3(SiO4)6O2 (x = 0.5; RE = Ca, Sr and Ba), were prepared by a high temperature solid-state reaction. Structure and unit-cell parameters were deduced from the analysis of X-ray powder diffraction data. Le Bail refinement of the X-ray powder diffraction data showed that the compounds have a hexagonal cell (P 63/m space group). The electrical properties of the materials were studied using the ac impedance spectroscopy technique. The extra oxygen O(4) atoms in site 2a (0, 0, 0.25) occupy channels running through the structure that are responsible for the high oxygen ion conduction. The presence of cation vacancies should enhance oxygen hopping along the c-axis; however, the analysis of the frequency dependence of ac conductivity suggests that oxygen motions are produced along three axes. The n-factor value of the dimensionality of the oxide ions increases with the conductivity when the size of RE cation decreases. © 2014 Elsevier B.V. All rights reserved.Peer Reviewe