Diffusion Path and Conduction Mechanism of Oxide Ions in Apatite-Type Lanthanum Silicates

Apatite-type lanthanum silicates of general formula La9.33+2x/3(SiO4)(6)O2+x have appeared recently as a new promising class of oxide ion conductors with potential applications as electrolytes for solid oxide fuel cells (SOFCs). They have been shown to demonstrate relatively high oxide ion conductivity at moderate temperatures as well as at low oxygen partial pressures. In this paper, the diffusion pathways and the conduction mechanism of oxide ions in these phases are reinvestigated. This is done by means of atomic scale computer modeling techniques with both semiempirical and bond valence methods. Our results support that oxide ion conduction along the c-axis proceeds by an interstitial mechanism. They also support the presence of interstitial sites located within the conduction channel. However, contrarily to recent research, it is shown that the channel oxide ions are involved in the conduction process by a push-pull type mechanism. This mechanism brings into play a cooperative movement of both two adjacent interstitial oxide ions forming a complex defect and the channel oxide ions. This complex defect is shown to move along the c-axis via a nonlinear pathway different from the conduction path proposed in literature to date. The calculated migration energy of this mechanism is found to be equal to 0.32 eV, which compares well with activation energy measured along the c-axis for Nd-9.33(SiO4)(6)O-2 single crystals.