The Langasite Family for the Development of Oxygen-Vacancy-Mediated Oxide Ion Conductors

Melilite-type oxide ion conductors have promising advantages in lowering the operating temperature of SOFCs due to the open structural framework with the deformation and rotation flexibility of the two-dimensional (2D) tetrahedral layers. Here, we have demonstrated the ability of a melilite-like langasite family (A3BC3D2O14) with a three-dimensional (3D) open structural framework with more structural and compositional flexibility to accommodate and transport oxygen vacancies in La3Ga5+xGe1–xO14–x/2 by Ga3+ substitution for Ge4+. Unlike the high oxygen vacancy formation energies in the melilite structure, the existence of octahedral units makes oxygen vacancy formation in langasite more energy-favorable, and the oxygen vacancies favor the bridging O3 site between the octahedral and tetrahedral units. Instead of the transformation of octahedral and tetrahedral units into five- and three-coordinated units, respectively, the oxygen vacancies are stabilized by the local structural relaxation transforming both the Ga1O6 and Ga3O4 units around the O3 vacancy into five-coordinated GaO5 units in the langasite La3Ga5+xGe1–xO14–x/2, as evidenced by neutron diffraction data, atomistic static lattice simulations, the atomic pair distribution function, and reverse Monte Carlo simulations. Moreover, the combined molecular dynamics simulations and BVSE calculations give the 2D long-range transport of oxygen vacancy within the ab layer of langasite through the oxygen exchanges at all the crystallographic distinct O1, O2, and O3 sites and the continuous opening and reformation of the 6-fold rings assisted by the polyhedral rotation and deformation. The langasite La3Ga5+xGe1–xO14–x/2 with a 3D melilite structure exhibits superior thermo-mechanical stability compared to the 2D melilite structure, which is potentially advantageous for SOFC applications. This study provides the langasite structure as a new structure type for developing vacancy-mediated oxide ion conductors with structural flexibility.