Chemical Bonding Effect on the Incorporation and Conduction of Interstitial Oxide Ions in Gallate Melilites

Abstract

The ability to incorporate high content of interstitial oxygen ions (O i ) in La 1+x Sr 1-x Ga 3 O 7+0.5x melilite owing to the good size match between La 3+ and Sr 2+ ions is well documented. Here, the complete substitution of Sr 2+ by Pb 2+ lone-pair cations results in a significant loss of this ability, even though Sr 2+ and Pb 2+ have almost the same effective ionic radius. To explore the fundamental mechanism underlying this result, density functional theory (DFT) calculations are performed on both the LaSrGa 3 O 7 -based and LaPbGa 3 O 7 -based materials, revealing a new chemical bonding effect on the incorporation of mobile oxygen interstitial defects in melilites. For LaSrGa 3 O 7 -based melilites, the interstitial oxygens have cooperatively weak antibonding interactions with the framework oxygen atoms (O f ). This antibonding O i -O f interaction pushes O i toward a 3-linked Ga ion, enhancing the covalent bonding between this Ga ion and O i . In addition, the antibonding O i -O f interaction makes the oxygen interstitial defects and framework atoms highly active, benefiting the migration of defects. In contrast, forLaPbGa 3 O 7 -based materials, the 6s 2 electrons of Pb 2+ point toward the c-axis and form antibonding with framework O 2− . This antibonding projects into the tunnel void, thereby directly hindering the entrance of interstitial oxygen atoms into the pentagonal rings.

Development of Melilite‐Type Oxide Ion Conductors

“…[9] Therefore, the interstitial oxygen site in the melilite oxide ion conductors is still controversial. More recently, we and Schuett et al carried out density functional theory (DFT) calculations on La 1.5 Sr 0.5 Ga 3 O 7.25, [25,26] respectively, which showed that the interstitial oxygen position obtained from geometry optimization indeed is consistent with the interstitial oxygen site close to the pentagonal ring center determined by our previous structural analysis of La 1.54 Sr 0.46 Ga 3 O 7.27 based on the NPD data. Schuett et al further studied locally various La/Sr cation environments in the melilite structure, which showed there are slightly offcentered positons depending on the local La/Sr cation environments.…”

“…The geometry optimization for indeed confirmed the relaxation of framework atoms in the defect structure solved from NPD data. [25] These electronic structure calculations provided new insight into the accommodation and migration of oxygen interstitials in La 1 + x Sr 1-x Ga 3 O 7 + 0.5x melilites. Interstitial oxide ions in the site close to the center of the pentagonal ring allows the formation of a covalent bond with one 3-connected tetrahedral Ga2a (Figure 6a-c).…”

“…The stoichiometric parent LaPbGa 3 O 7 can be synthesized but the solid solubility limit of La 1 + x Pb 1-x Ga 3 O 7 + 0.5x was found to be approximately x = 0.1. Thus, the oxygen interstitials are hardly incorporated into the structure by La 3 + substitution for Pb 2 + , [25] in great contrast with La 1 + x Sr 1-x Ga 3 O 7 + 0.5x . In the LaPbGa 3 O 7 material, the electronic structure calculation shows that the 6s 2 lone pair electrons of Pb are not inert electrons but do participate to chemical bonding.…”

“…x Ga 3 O 7 + 0.5x and Pr 1.1 Sr 0.9 Ga 3 O 7.05 cases. [20,25] As indicated by structural analysis on NPD data of CeSrGa 3 O 7.39 , unlike the interstitial oxygen in La 1 + x Sr 1-x Ga 3 O 7 + 0.5x entering the 3connected tetrahedral coordination environment (Figure 3ab), the interstitial oxygen in CeSrGa 3 O 7.39 enters the 4connected (Ga1)O 4 tetrahedron without terminal oxygen and is located in the framework oxygen level in the Ga 3 O 7 layer along the c axis ( Figure 3c). [27] Compared with the 3-connected tetrahedron, the 4-connected tetrahedron does not have flexible rotation and deformation possibility, which places great constraint to the migration of interstitial oxygen.…”

“…In summary, the DFT-based electronic structure calculations of LaSrGa 3 O 7 and LaPbGa 3 O 7 -based compositions revealed ionic and covalent bonding control of oxygen interstitial defects and cooperatively weak antibonding interaction between interstitial and framework oxygen atoms. From Coulomb interactions point of view, the open structure and the variable coordination-number polyhedral network with [25] . Copyright 2019, Wiley-VCH Verlag Gmbh.)…”