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

Abstract Ba 2 TiSi 2 O 8 fresnoite, containing a layered mixed TiO 5 and SiO 4 polyhedral network with pentagonal tunnels similar to melilite, displayed extremely limited solubility for both K + and La 3+ in Ba 2+ sites. Atomistic static lattice simulations revealed high energetic costs for K + and La 3+ substitutions for Ba 2+ in Ba 2 TiSi 2 O 8 . These results emphasize the rigidity of the mixed layered polyhedral network of Ba 2 TiSi 2 O 8 fresnoite, which is not suitable for stabilizing the oxygen vacancies and interstitials. In contrast with the La-substituted compositions forming crystalline mixtures, the Ba 2 −  x K x TiSi 2 O 8 − 0.5 x compositions showed phase separation into crystalline Ba 2 TiSi 2 O 8 and amorphous K 2 TiSi 2 O 7 . However, the addition of potassium enhanced the ionic conductivity of Ba 2 −  x K x TiSi 2 O 8 − 0.5 x compositions, which mainly arises from the potassium conduction in the glass component K 2 TiSi 2 O 7 as the oxide ion conduction was found to be limited. Measurements on the pure K 2 TiSi 2 O 7 glass entirely reproduced the electrical and crystallization behaviors observed in Ba 2 −  x K x TiSi 2 O 8 − 0.5 x composites, confirming experimentally the responsibility of the glassy material for the enhanced potassium ionic conductivity in Ba 2 −  x K x TiSi 2 O 8 − 0.5 x composites. This study contributes to the further understanding of oxide ionic conductivity of SrSiO 3 -based materials, which is currently under debate.