Aliovalent-substitution defect chemistry, crystalline-glassy phase separation and ionic conductivity in fresnoite Ba2TiSi2O8-based materials

Ba2TiSi2O8 fresnoite, containing a layered mixed TiO5 and SiO4 polyhedral network with pentagonal tunnels similar to melilite, displayed extremely limited solubility for both K+ and La3+ in Ba2+ sites. Atomistic static lattice simulations revealed high energetic costs for K+ and La3+ substitutions for Ba2+ in Ba2TiSi2O8. These results emphasize the rigidity of the mixed layered polyhedral network of Ba2TiSi2O8 fresnoite, which is not suitable for stabilizing the oxygen vacancies and interstitials. In contrast with the La-substituted compositions forming crystalline mixtures, the Ba2 - xKxTiSi2O8 - 0.5x compositions showed phase separation into crystalline Ba2TiSi2O8 and amorphous K2TiSi2O7. However, the addition of potassium enhanced the ionic conductivity of Ba2 - xKxTiSi2O8 - 0.5x compositions, which mainly arises from the potassium conduction in the glass component K2TiSi2O7 as the oxide ion conduction was found to be limited. Measurements on the pure K2TiSi2O7 glass entirely reproduced the electrical and crystallization behaviors observed in Ba2 - xKxTiSi2O8 - 0.5x composites, confirming experimentally the responsibility of the glassy material for the enhanced potassium ionic conductivity in Ba2 - xKxTiSi2O8 - 0.5x composites. This study contributes to the further understanding of oxide ionic conductivity of SrSiO3-based materials, which is currently under debate. (C) 2015 Elsevier B.V. All rights reserved.