La_1+<i>x</i>Ba_1-<i>x</i>Ga₃O_{7+0.5<i>x</i>} Oxide Ion Conductor: Cationic Size Effect on the Interstitial Oxide Ion Conductivity in Gallate Melilites

Xu, Jungu, Wang, Jiehua, Tang, Xin, Kuang, Xiaojun and Rosseinsky, Matthew J ORCID: 0000-0002-1910-2483 (2017) La_1+<i>x</i>Ba_1-<i>x</i>Ga₃O_{7+0.5<i>x</i>} Oxide Ion Conductor: Cationic Size Effect on the Interstitial Oxide Ion Conductivity in Gallate Melilites. INORGANIC CHEMISTRY, 56 (12). pp. 6897-6905.

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Abstract

Substitution of La³⁺ for Ba²⁺ in LaBaGa₃O₇ melilite yields a new interstitial-oxide-ion conducting La_1+xBa_1-xGa₃O_7+0.5x solid solution, which only extends up to x = 0.35, giving a maximum interstitial oxygen content allowed in La_1+xBa_1-xGa₃O_7+0.5x as about half of those allowed in La_1+x(Sr/Ca)_1-xGa₃O_7+0.5x. La_1.35Ba_0.65Ga₃O_7.175 ceramic displays bulk conductivity ∼1.9 × 10^-3 S/cm at 600 °C, which is lower than those of La_1.35(Sr/Ca)_0.65Ga₃O_7.175, showing the reduced mobility for the oxygen interstitials in La_1+xBa_1-xGa₃O_7+0.5x than in La_1+x(Sr/Ca)_1-xGa₃O_7+0.5x. Rietveld analysis of neutron powder diffraction data reveals that the oxygen interstitials in La_1.35Ba_0.65Ga₃O_7.175 are located within the pentagonal tunnels at the Ga level between two La/Ba cations along the c-axis and stabilized via incorporating into the bonding environment of a three-linked GaO₄ among the five GaO₄ tetrahedra forming the pentagonal tunnels, similar to the Sr and Ca counterparts. Both static lattice atomistic simulation and density functional theory calculation show that LaBaGa₃O₇ has the largest formation energy for oxygen interstitial defects among La_1+xM_1-xGa₃O_7+0.5x (M = Ba, Sr, Ca), consistent with the large Ba²⁺ cations favoring interstitial oxygen defects in melilite less than the small cations Sr²⁺ and Ca²⁺. The cationic-size control of the ability to accommodate the oxygen interstitials and maintain high mobility for the oxygen interstitials in La_1+xM_1-xGa₃O_7+0.5x (M = Ba, Sr, Ca) gallate melilites is understood in terms of local structural relaxation to accommodate and transport the oxygen interstitials. The accommodation and migration of the interstitials in the melilite structure require the tunnel-cations being able to adapt to the synergic size expansion for the interstitial-containing tunnel and contraction for the tunnels neighboring the interstitial-containing tunnel and continuous tunnel-size expansion and contraction. However, the large oxygen bonding separation requirement of the large Ba²⁺ along the tunnel not only suppresses the ability to accommodate the interstitials in the tunnels neighboring the Ba²⁺-containing tunnel but also reduces the mobility of the oxygen interstitials among the pentagonal tunnels.

Item Type:	Article
Depositing User:	Symplectic Admin
Date Deposited:	27 Nov 2017 15:49
Last Modified:	12 Oct 2023 18:55
DOI:	10.1021/acs.inorgchem.7b00295
Related URLs:	Author Publisher
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3012759