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

Abstract

Apatite-type lanthanum silicates of general formula La9.33+2x/3(SiO4)6O2+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 Nd9.33(SiO4)6O2 single crystals.

Ionic conduction mechanisms of apatite-type lanthanum silicate and germanate from first principles

“…4(b)], the interstitial oxygen ion goes into the adjacent O4 site at a very short distance of 0.07 nm and subsequently the adjacent O4 ion is pushed into the adjacent interstitial site. This corresponds to the push-pull mechanism proposed by Béchade et al 15) It is interesting to see that the calculated potential barrier is less than 0.01 eV, which is significantly low compared with potential barriers expected for conventional ionic conductors.…”

“…Another mechanism is an interstitialcy mechanism that accompanies cooperative motion of interstitial oxygen and O4 like the push-pull mechanism. 15) In addition, some of the previous studies proposed a vacancy mechanism in the O4 channel, where an oxygen vacancy of the O4 column induces rapid ionic conduction along the O4 column. Figure 4 shows trajectories of the interstitial oxygen and the cooperating oxygen ions in La 10 (SiO 4 ) 6 O 3 .…”

“…In contrast, Béchade et al applied static lattice simulations and the bond valence method for stoichiometric La 9.33 (SiO 4 ) 6 O 2 , and proposed the push-pull mechanism, where interstitial oxygen ions do not diffuse through the O4 channel independently but undergo cooperative movement with O4 ions. 15) Regarding the oxygen-excess phase of La 9.33+x (SiO 4 ) 6 O 2+1.5x (x > 0.0), static lattice simulations by Jones et al indicated that the interstitial oxygen ions are located at the periphery of the O4 channel to form "SiO 5 " units with the adjacent SiO 4 tetrahedra and diffuse from one SiO 4 tetrahedron to another (the "handover" mechanism). 16) Ali et al showed that the interstitial oxygen ions (O5) are present at the periphery of the O4 channel.…”

Oxygen-17 nuclear magnetic resonance measurements on apatite-type lanthanum silicate (La9.33(SiO4)6O2)

“…Oxide-ion conduction is caused by a "hand-over" mechanism, where the interstitial oxide ion is transferred from a SiO4 unit to the next, momentarily forming a Si2O9 unit. A simulation study predicted the presence of interstitial oxide ions but according to this study the ions located near the O4 site diffuse along the c-axis via a vacancy diffusion mechanism [10]. Therefore from our knowledge on the conduction mechanism in apatite, we observe that there are discrepancies in the presence and locations of the interstitial oxide ions.…”

“…The high conductivity was first observed by one of the authors [1,2]. Although the conduction mechanism has been extensively investigated [8][9][10][11][12], it has not been fully understood. Apatite has three kinds of sites for oxide ions in its structure (Fig.…”