Interstitial Oxide Ion Conductivity in the Langasite Structure: Carrier Trapping by Formation of (Ga,Ge)₂O₈ Units in La₃Ga_5–xGe_1+xO_14+x/2 (0 < x ≤ 1.5)

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 La₃Ga_5–xGe_1+xO_14+x/2 langasites, attaining a higher defect content than reported in the lower dimensional oxyapatite and melilite systems in La₃Ga_3.5Ge_2.5O_14.75 (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 La₃Ga_5–xGe_1+xO_14+x/2 langasite family is lower than that of the corresponding melilites La_1+ySr_1–yGa₃O_7+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.