First-Principles Analysis of the Oxide-Ion Conduction Mechanism in Si-Deficient Lanthanum Silicate Apatite

Abstract

Si-deficient lanthanum silicate apatites (La9.33+0.67x Si6–0.5x O26, LSOs) have attracted attention for their high oxide-ion conductivities. However, the carrier species and the conduction pathways responsible for the oxide-ion conduction are still unclear. In this study, the conduction mechanism of oxide ions in Si-deficient LSO was studied by first-principles calculations. It was found that Si-deficient LSO can include a small amount of excess oxide ions, which are located at the interstitial site along the c axis. The excess oxide ions in Si-deficient LSO can rapidly diffuse along the c axis by the push–pull mechanism with a potential barrier of 0.35 eV, which involves cooperative movements of two neighboring oxide ions, O4 ions, and the excess oxide ions. The potential barrier height was in good agreement with the experimentally reported activation energy. It is shown that a small amount of excess oxygen contributes critically to oxide-ion conduction in Si-deficient LSO, as was also discussed in stoichiometric La9.33Si6O26. These findings indicate the existence of a mechanism whereby a small amount of excess carriers in the ion-conduction pathways can significantly change the energy profiles for ionic conduction and enhance ion conductivity.

Recent developments in oxide ion conductors: focusing on Dion–Jacobson phases

“…Due to their high conductivities and the flexibility of the apatite structure, apatite-type oxide-ion conductors have garnered considerable interest. [65][66][67][68] Lanthanum silicates and germanates are the most extensively studied apatite-type oxide-ion conductors, with high conductivities of B3 Â 10 À3 S cm À1 at 700 1C for La 9.33 (SiO 4 ) 6 O 2 and B0.14 S cm À1 at 950 1C for La 9 Sr-Ge 6 O 26.5 . 65,68,69 The majority of apatite-type oxide-ion conductors have hexagonal symmetry, and their structures are composed of isolated SiO 4 or GeO 4 tetrahedra and lanthanide cations located at 9-and 7-fold coordinated sites.…”

“…[65][66][67][68] Lanthanum silicates and germanates are the most extensively studied apatite-type oxide-ion conductors, with high conductivities of B3 Â 10 À3 S cm À1 at 700 1C for La 9.33 (SiO 4 ) 6 O 2 and B0.14 S cm À1 at 950 1C for La 9 Sr-Ge 6 O 26.5 . 65,68,69 The majority of apatite-type oxide-ion conductors have hexagonal symmetry, and their structures are composed of isolated SiO 4 or GeO 4 tetrahedra and lanthanide cations located at 9-and 7-fold coordinated sites. Using hightemperature neutron diffraction and MEM, Yashima et al discovered two migration pathways in the apatite-type oxideion conductor La 9.69 (Si 5.70 Mg 0.30 )O 26.24 .…”

Overlapping large polaron tunnelling in lanthanum silicate oxyapatite

“…La 3+ 10−α (SiO 4 ) 4− 6 O 2− 2+δ (LSO) is one of the most interesting materials [15,16]. In polycrystalline samples, grain alignment and doping are the two common methods to get the desired properties in the sample, the same has been employed by various research groups in the last few years [2,7,8,[10][11][12][13][16][17][18][19][20][21]. Various doping such as Al, Mg at B-site and alkaline earth and rare Earth substitutions at A-site have been done in order to get conductivities in the range of 1 S cm −1 but unfortunately, conductivity is achieved in the range of 10-50 mS cm −1 [7-11, 13, 14, 16, 19, 20, 22].…”

“…Various doping such as Al, Mg at B-site and alkaline earth and rare Earth substitutions at A-site have been done in order to get conductivities in the range of 1 S cm −1 but unfortunately, conductivity is achieved in the range of 10-50 mS cm −1 [7-11, 13, 14, 16, 19, 20, 22]. To explain this the two kind of conduction mechanism, namely, push-pull [17] and interstitial [9,18] are well described for the oxyapatites. In the push-pull conduction mechanism, interstitial oxygen cooperatively migrates with O4 to the most stable interstitial O site with the lowest potential barrier of 0.01 eV with site energies in the range of 0-0.25 eV.…”

“…It leads to strong anisotropy in the oxygen ion conduction being reported by Nakayama in 2013 [24]. Various experiments [9] and theoretical studies [17] predict that O int (interstitial oxygen) promotes cooperative movements between the O4 ion and excess oxygen is introduced as interstitial charge carriers to maintain electrical neutrality with La ions. There are four channels proposed for the conduction.…”