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

Abstract

Ionic conduction mechanisms in apatite-type lanthanum silicate and germanate studied by first-principles calculations were reviewed, along with the ones previously proposed by experiments and atomistic simulations. It was found that the most stable interstitial oxygen sites in La 10 (SiO 4 ) 6 O 3 are located at the vicinity of the O4 column and the metastable sites are present between SiO 4 tetrahedra. In contrast, La 10 (GeO 4 ) 6 O 3 showed the most stable sites between GeO 4 tetrahedra, forming Ge 2 O 9 units. These can reasonably explain interstitial sites reported experimentally. The fundamental mechanism for ionic conduction in both systems is the interstitialcy mechanism, where interstitial oxygen ions and oxygen ions at regular lattice sites undergo cooperative motion through the lattices. In particular, the interstitialcy mechanism via Si/GeO 4 tetrahedra takes place by sequential bondforming and bond-breaking events between Si/Ge and oxygen ions, and play an important role to realize rapid ionic conduction.

Oxide Ion-Conducting Materials Containing Tetrahedral Moieties: Structures and Conduction Mechanisms

“…These investigations have clearly evidenced that both silicate and germanate apatites behave as good oxide ion conductors, reaching optimal oxide ion conductivities of 10 –4 and 10 –3 S·cm –1 at 500 °C, respectively, making them promising electrolyte candidates for the intermediate-temperature SOFCs. ,, Overall, compared with the silicate apatites, the germanate counterparts display similar conduction behavior and better oxide ion conductivities but poorer stabilities and higher cost. In 2004 and 2007, Slater et al published two review papers exclusively on the apatite-type oxide ion conductors. , In the following 15 years, no further review was dedicated to the apatite-type oxide ion conductors, except for one focused on the theoretical investigation of their oxide ion migration published in 2017 . Here we emphasize the new understanding of the defect structures and conduction mechanisms of apatite-type oxide ion conductors.…”

“…It should be noted that SiO 5 units could appear as an intermediate state during the oxide ion transport in the silicate apatites, given there is no strong chemical interaction between the interstitial oxide ions and SiO 4 , as indicated by the experimental and theoretical NMR studies . This contrasts with the germanate apatites, where GeO 5 units were observed as a stable configuration owing to the variable coordination number nature for Ge, generally covering from 4 to 6. ,,, …”

“…22,74 In the following 15 years, no further review was dedicated to the apatite-type oxide ion conductors, except for one focused on the theoretical investigation of their oxide ion migration published in 2017. 48 Here we emphasize the new understanding of the defect structures and conduction mechanisms of apatite-type oxide ion conductors.…”

“…However, the O int sites have unreasonable short contacts with the framework oxygen (O f ). Matsunaga et al 48,82 performed DFT calculations on the A-site fully occupied composition La 10 (SiO 4 ) 6 O 3 , which indicated that the sites in the oxygen channel periphery are more energetically favorable than the O int site close to SiO 4 within the peripheral La channels, although their energy difference is small, consistent with the interstitial site (Figure 3b) determined experimentally for La 8.65 Sr 1.35 -Si 6 O 26.32 by Leon-Reina et al 83 Given the low occupancies of these interstitial sites, it is hard to determine accurately the sites and contents of interstitial oxide ions using the NPD technique, and even to describe local structural relaxation around the interstitial oxide ions based on the average structure obtained from NPD data. 22,42,45,46,82−87 Alternatively, Zhang et al 34 in 2012 employed a high-resolution transmission electron microscopy (HRTEM) technique to probe the presence of O int in the oxygen-excess La 9 Sr(SiO 4 )O 26.5 composition.…”

“…Thus, the structural relaxation to accommodate the oxygen defects is much more complex in the tetrahedron-based structures than in the structural prototypes consisting of the 6- or 8-coordinated polyhedral units. Normally, it is hard to determine precisely the sites and occupancies of the oxygen defects and get insight into the defect structures through diffraction techniques including synchrotron powder diffraction (SPD) and neutron powder diffraction (NPD), which probe the average structures. − Multiple complementary methods have been employed to solve the defect structures and elucidate the mechanisms of oxide ion migration, including extended X-ray absorption fine structure (EXAFS), high-resolution scanning transmission electron microscopy (HR-STEM), annular bright-field scanning transmission electron microscopy (ABF-STEM), solid-state nuclear magnetic resonance spectroscopy (SS-NMR), pair distribution function (PDF) analysis, and Raman spectroscopy for the local structures − and density functional theory (DFT) calculations − as well as interatomic potentials-based atomistic modeling and molecular dynamics (MD) simulations ,− for the defect formation energies and oxide ion migration. Thanks to these advanced techniques, deep insights into defect stabilization and migration in tetrahedron-based oxide ion conductors have been achieved, correcting initial statements on the phase formations and compositions, structures, defect types, and migration mechanisms and demonstrating how technological progress over the past 40 years has advanced the development of various oxide ion conductors based on tetrahedral moieties.…”