Interstitial Oxide Ion Migration Mechanism in Aluminate Melilite La_1+xCa_1–xAl₃O_7+0.5x Ceramics Synthesized by Glass Crystallization

Abstract

Gallate melilite materials have attracted considerable interest as new interstitial oxide ion conducting electrolytes for solid oxide fuel cells for more than a decade. However, the preparation of aluminate melilite materials as interstitial oxide ion conductors remains a challenge. Here, we show that interstitial oxide ion conducting aluminate melilite materials La1+x Ca1–x Al3O7+0.5x (x = 0–0.5) can be prepared via a full crystallization from bulk glass process. Rietveld refinements performed from combined neutron and synchrotron X-ray powder diffraction (NPD and SPD) data reveal multiple interstitial defect positions within the pentagonal ring, demonstrating the diversity of local structures around the oxygen interstitial defects in La1+x Ca1–x Al3O7+0.5x . Variable temperature solid-state 27Al nuclear magnetic resonance (NMR) spectroscopy measurements demonstrate the existence of 5-coordinated AlO5 polyhedra and dynamic exchange processes between these 5-coordinated Al sites, representing the first example of evidence for the migration mechanism of interstitial oxide ions in melilites by NMR. This latter involves framework and interstitial oxide ions, and is assisted by rotation and deformation of tetrahedra. These calculations reveal reduced mobility of interstitial oxide ions in aluminate tetrahedral network owing to its rigidity.

Development of Melilite‐Type Oxide Ion Conductors

“…Such synergistic oxide ion transport mechanism has been confirmed in La 1 + x Ca 1-x Al 3 O 7 + 0.5x aluminate melilite oxide ion conductors ( Figure 4b). [24] In this system, we confirmed the presence of interstitial oxide ions in melilite for the first time through solid-state 27 Al nuclear magnetic resonance (NMR) spectroscopy, and revealed the dynamic exchange process between the adjacent 5-coordinated Al sites through variable temperature 27 Al solid-state NMR magic angle spinning (MAS) spectroscopy. Apart from 27 Al solid-state NMR, we also attempted to perform 71 Ga solidstate NMR spectoscopy on gallate melilites oxide ion conductors.…”

“…In a second step, the glass is heated above its glass transition temperature to proceed to full crystallization, such leading to ceramic samples. This innovative bulk glass crystallization route, which can lead to fully dense ceramics with thin grain boundary, [22,24] can be considered as a soft chemistry synthesis process to prepare metastable phases which are not accessible via classic solid state reaction. Despite the fully dense feature, the thin grain boundaries and the high bulk conductivity of~0.01 S/cm at 470°C, the Ln 1 + x Sr 1-x Ga 3 O 7 + δ (Ln = Eu, Gd, Tb, x = 0.4-0.6) ceramics synthesized by glass crystallization exhibit total conductivities much worse than that of La 1.54 Sr 0.46 Ga 3 O 7.27 ( Figure 10c).…”

“…Inspired by the synthesis of transparent Ln 1 + x Sr 1-x Ga 3 O 7 + δ (Ln = Eu, Gd, Tb) melilite oxide ion conductors, we then successfully prepared the aluminate melilite oxide ion conductors La 1 + x Ca 1-x Al 3 O 7 + 0.5x (x = 0~0.5), which has low cost advantages but cannot be prepared through the traditional solid state reaction. [24] However, the interstitial oxide ion mobility in aluminate melilites is much lower than that in gallate melilites (Figure 10c), e. g. in the 600-800°C range, the conductivity of La 1.5 Ca 0.5 Al 3 O 7.25 varied within~10 À 4 -10 À 3 S/cm, which is lower than that of La 1.2 Sr 0.8 Ga 3 O 7.1 (~10 À 3 -10 À 2 S/cm) although containing smaller interstitial oxide ion content than La 1.5 Ca 0.5 Al 3 O 7.25 . Aluminate melilite oxide ion conductors also showed decomposition at temperature above 700°C.…”

“…Similar decomposition was also observed in metastable oxide ion conducting melilite phases synthesized from the glass crystallisation routes. [22,24] On the other hand, chemical compatibility with electrode materials is also of great importance for SOFC application. According to Mancini et al [41] , the La 1.…”

Local Structure and Dynamics in Methylammonium, Formamidinium, and Cesium Tin(II) Mixed-Halide Perovskites from 119Sn Solid-State NMR

“…The temperature was adjusted using diode laser heating. 85 The sample was sandwiched between two layers of ground KBr, which allowed monitoring of the effective sample temperature through the 79 Br shift of KBr. 86,87 In order to prevent any interactions between KBr and the perovskite sample, a thin layer of PTFE tape was placed between the two powders.…”