Molecular dynamics simulations of oxide ion migration in La2Ga3O7.5 with completely ordered interstitial oxide ions

Abstract

Molecular dynamics simulations were performed on metastable La2Ga3O7.5 melilite ceramics, which were synthesized previously by direct crystallization of an under-cooled melt. La2Ga3O7.5 possesses the theoretically maximum content of interstitial oxide ions featuring complete ordering but ~3 orders of magnitude lower oxide ion conductivities compared with disordered La1·54Sr0.46Ga3O7.27 within 573–773 ​K. One dimensional diffusion paths of interstitial oxide ion was identified in La2Ga3O7.5 based on the molecular dynamics simulations, which shows that the interstitial oxide ions can migrate from the pentagonal ring containing GaO5 pyramid unit to its neighboring pentagonal ring free of interstitial oxide ion mainly along the a axis through a synergic mechanism involving continuous breaking and reformation of Ga2O8 units assisted by rotation and deformation of GaOn polyhedra and knock-on process between interstitial and framework oxygen atoms. Such constrained dimensionality on the diffusion paths arising from the confining effect for a large number of oxygen interstitials in the highly relaxed and ordered melilite superstructure accounts for the lower oxide ion conductivity in La2Ga3O7.5.