Oxygen Vacancy Migration in Ca2Ga2+xGe1-xO7-0.5x Melilite

Compared with interstitial defects, vacancy-mediated oxide ion conduction in melilite family received little attention. Herein, melilite-type oxide ion conductors of Ga-excess compositions Ca2Ga2+xGe1-xO7-0.5x (0 < x < 0.15) demonstrate remarkable oxygen vacancy migration by experimental and computational verifications. Neutron powder diffraction data and defect formation energies of oxygen vacancy revealed that oxygen vacancies favor occurring at the bridging O sites shared by four-linked (Ga/Ge)O4 tetrahedra, opening the 5-fold rings coupled by the local structure relaxation around the oxygen vacancies. Molecular dynamics simulations revealed that the oxygen vacancies migration involves the cooperative rotation and deformation of adjacent tetrahedra, allowing the opening and re-forming of 5-fold rings in melilite structure. This study provides structural chemistry mechanisms on oxygen vacancy conduction in melilite structure.