Discovery of a Rare-Earth-Free Oxide-Ion Conductor Ca₃Ga₄O₉ by Screening through Bond Valence-Based Energy Calculations, Synthesis, and Characterization of Structural and Transport Properties

Abstract

In this work, we have discovered Ca 3 Ga 4 O 9 as a rare-earth-free oxide-ion conductor by a combined technique of bond valence (BV)-based energy calculations, synthesis, and characterization of structural and transport properties. Here, the energy barriers for oxide-ion migration (E b ) of 217 Ga-containing oxides were calculated by the BV method to screen the candidate materials of oxide-ion conductors. We chose the orthorhombic calcium gallate Ca 3 Ga 4 O 9 as a candidate of oxide-ion conductors, because Ca 3 Ga 4 O 9 had a relatively low E b . Ca 3 Ga 4 O 9 was synthesized by a solid-state-reaction method. Rietveld analyses of time-of-flight neutron and synchrotron X-ray powder diffraction data of Ca 3 Ga 4 O 9 indicated an orthorhombic Cmm2 layered crystal structure consisting of Ca 18 and (Ga 4 O 9 ) 6 units where the (Ga 4 O 9 ) 6 units form the two-dimensional (2D) corner-sharing GaO 4 tetrahedral network. The electromotive force measurements with an oxygen concentration cell showed that the transport numbers of the oxide ion were 0.69 at 1073 K and 0.84 at 973 K in Ca 3 Ga 4 O 9 , which indicates that the major carrier of Ca 3 Ga 4 O 9 is the oxide ion. The oxide-ion conductivity was estimated to be 1.03(8) × 10 −5 S cm −1 at 1073 K. The total electrical conductivity and impedance spectroscopy measurements of this Ca 3 Ga 4 O 9 sample indicated that the bulk conductivity was much higher than the grain-boundary conductivity and that the total conductivity was equivalent to the bulk conductivity. The bond valence-based energy landscape calculated using the refined crystal parameters of Ca 3 Ga 4 O 9 indicated 2D oxide-ion diffusion in the layered tetrahedral network [(Ga 4 O 9 ) 6 unit]. It was found that the structural and transport properties of Ca 3 Ga 4 O 9 are similar to those of LaSrGa 3 O 7 melilite.

The relationship between oxide-ion conductivity and cation vacancy order in the hybrid hexagonal perovskite Ba3VWO8.5

“…Bond-valence based methods have been shown to reproduce well the ionic conduction pathway in several ionic conductors 17,[45][46][47][48][49][50] and have successfully been employed for the screening and discovery of new oxide ion conducting materials. [51][52][53][54] Despite being not as accurate, BVSE barriers generally show a good agreement with results from density functional theory (DFT) calculations. 35 Therefore, BVSE analysis can provide useful general indicators on the energetics of ionic migration of inherently complex crystal structures, at a fraction of the computational cost, and the results can be used as a proxy, in relative terms, for more detailed studies.…”

Enhanced Oxygen Ion Conductivity and Mechanistic Understanding in Ba3Nb1–xVxMoO8.5

“…Bond-valence site energy (BVSE) calculations were employed to analyze the migration pathway and energy landscape of the oxide ion conduction in the average crystal structure of Ba3Nb1-xVxMoO8.5. Bond-valence based energy analysis has been extensively used for the investigation of conduction pathways in various oxide ion 10,42,43,44,45,46,47 and sodium and lithium ionic conductors. 48,49,50 The reliability of ionic conduction pathways obtained from bond-valence energy based methods has been confirmed by agreement with diffraction experiments and by results from maximum entropy studies and molecular dynamics calculations.…”

“…42,47,51,52 Furthermore, BVS methods have been effective in the screening and successful discovery of novel oxide ion conductors. 42,43,44,45 The low computational cost and reliability of the predicted relative migration energy barriers make softBV particularly useful for further investigation of the conduction pathways of inherently complex disordered crystal phases such as Ba3Nb1-xVxMoO8.5, for which density functional theory (DFT) calculations might be particularly challenging and computationally expensive. 33 While it is expected that the detailed energy barriers will be determined by the local distribution of oxygen populations and metal stacking configurations, BVSE analysis on the average structure can provide useful general indications and trends on the energetics of ionic migration in Ba3Nb1-xVx-MoO8.5, and as such can be used as proxy, in relative terms, for the activation energy determined by impedance spectroscopy measurements or more accurate computational DFT methods.…”

Expanding the family of mineral-like anhydrous alkali copper sulfate framework structures: new phases, topological analysis and evaluation of ion migration potentialities

“…Brown (2009Brown ( , 2016] is thereby supplemented by an approach which benefits from the known positive correlation between ion conduction pathways and maps of low bond-valence mismatch (Adams, 2000;Adams & Prasada Rao, 2011). The construction of BVELs served to visualize the ion diffusion networks as mappings on energy isosurfaces (Fedotov et al, 2018;Katcho et al, 2019;Yasui et al, 2019). The actual BVEL calculations employed the software 3DBVSMAPPER (Sale & Avdeev, 2012), which generates a spatial distribution of energy values to produce a 3D surface within the unit cell.…”