Interstitial Oxide Ion Conductivity in the Langasite Structure: Carrier Trapping by Formation of (Ga,Ge)2O8 Units in La3Ga5-x Ge1+x O14+x/2 (0 < x ≤ 1.5).

Maria Diaz-Lopez,John B Claridge,Matthew J Rosseinsky,Matthew S Dyer,Frédéric Blanc,Michael J Pitcher,Ming Li,J Felix Shin

doi:10.1021/acs.chemmater.9b01734

Maria Diaz-Lopez, John B Claridge + Show 6 more

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https://doi.org/10.1021/acs.chemmater.9b01734

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Abstract

Framework oxides with the capacity to host mobile interstitial oxide anions are of interest as electrolytes in intermediate temperature solid oxide fuel cells (SOFCs). High performance materials of this type are currently limited to the anisotropic oxyapatite and melilite structure types. The langasite structure is based on a corner-shared tetrahedral network similar to that in melilite but is three-dimensionally connected by additional octahedral sites that bridge the layers by corner sharing. Using low-temperature synthesis, we introduce interstitial oxide charge carriers into the La3Ga5–xGe1+xO14+x/2 langasites, attaining a higher defect content than reported in the lower dimensional oxyapatite and melilite systems in La3Ga3.5Ge2.5O14.75 (x = 1.5). Neutron diffraction and multinuclear solid state 17O and 71Ga NMR, supported by DFT calculations, show that the excess oxygen is accommodated by the formation of a (Ge,Ga)2O8 structural unit, formed from a pair of edge-sharing five-coordinated Ga/Ge square-based pyramidal sites bridged by the interstitial oxide and a strongly displaced framework oxide. This leads to more substantial local deformations of the structure than observed in the interstitial-doped melilite, enabled by the octahedral site whose primary coordination environment is little changed by formation of the pair of square-based pyramids from the originally tetrahedral sites. AC impedance spectroscopy on spark plasma sintered pellets showed that, despite its higher interstitial oxide content, the ionic conductivity of the La3Ga5–xGe1+xO14+x/2 langasite family is lower than that of the corresponding melilites La1+ySr1–yGa3O7+y/2. The cooperative structural relaxation that forms the interstitial-based (Ga,Ge)2O8 units stabilizes higher defect concentrations than the single-site GaO5 trigonal bipyramids found in melilite but effectively traps the charge carriers. This highlights the importance of controlling local structural relaxation in the design of new framework electrolytes and suggests that the propensity of a framework to form extended units around defects will influence its ability to generate high mobility interstitial carriers.

Highlights

Solid oxide fuel cells (SOFCs) are efficient all-solid-state energy conversion devices that facilitate the use of clean fuels such as hydrogen in place of current fossil-fuel based power generation
We have combined neutron diffraction, multinuclear nuclear magnetic resonance (NMR) spectroscopy, and density functional theory (DFT) calculations to show that the langasite structure of La3Ga5GeO14 can accommodate a substantial concentration of interstitial oxide ions by tuning the Ga/Ge ratio according to La3Ga5−xGe1+xO14+x/2, with the solid solution extending to x = 1.5
This corresponds to a 5.4 mol % excess of oxide ions with respect to the parent framework, which is higher than that of the established interstitial oxide ion conductors such as the oxyapatites, which can accommodate a theoretical maximum of 3.8 mol % excess oxide,[7] and melilites which accommodate up to 4.6 mol %

Summary

Introduction

Solid oxide fuel cells (SOFCs) are efficient all-solid-state energy conversion devices that facilitate the use of clean fuels such as hydrogen in place of current fossil-fuel based power generation. The need for sufficient ionic diffusion throughout the device means that current commercial SOFCs operate at elevated temperatures of up to 1000 °C, which creates problems with cell construction and durability Further expansion of this technology is dependent on reducing their operating temperatures to an intermediate temperature range between 600 and 800 °C, motivating the search for new electrolyte materials that conduct oxide ions efficiently under these conditions.[1−3] The most widely used solid oxide electrolytes are materials based on the fluorite or perovskite structures (e.g., Zr1−xYxO2−x/2 or YSZ,[4] Ce1−xGdxO2−δ or GDC,[5] and La1−xSrxGayMg1−yO3−δ or LSGM6) where cations formally adopt 12-, 8-, or 6-fold coordination and ionic diffusion is mediated by vacancies on the oxide sublattice.

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